of the international earthquake engineering -...

Pjoceedings of the international , seminar on earthquake engineering

Held under the auspices of the Federal Government of Yugoslavia and of Unesco Skopje, 29 September to 2 October 1964

Actes du colloque international sur le génie paraséismique

Tenu sous les auspices du gouvernement fédéral de Yougoslavie et de l'Unesco Skopje, 29 septembre - 2 octobre 1964

Unesco

Proceedings of the international seminar on earthquake engineering

Actes du colloque international sur le génie paraséismique

Published in 1968 by the United Nations Educational, Scientific and Cultural Organization, place de Fontenoy, Paris-7e Printed by Van Buggenhoudt, Brussels

Publié en 1968 par l'organisation des Nations Unies pour l'éducation, la science et la culture, place de Fontenoy, Paris-7e Imprimé par Van Buggenhoudt, Bruxelles.

@ Unesco 1968 Printed in Belgium NS.66/D.46/AF

.

Contents Table des matières

Introduction Introduction

S.A. Bubnov General report Rapport général

A. Recent developments in earthquake engineering research Activités récentes dans le domaine du génie paraséismique

I. Alpan A. Beles

T:Hisada ,

S. V. Medvedev

I . - . I

.*<

A. A. Moinfar

J. Ferry Borges

R. W. Clough

Earthquake engineering in Israel The problem of engineering seismology in Romania

Damage to reinforced-concrete buildings in Niigata City due to the earthquake of 16 June 1964

Measurement of ground motion and structural vibrations caused by earthquakes

Report on the work undertaken in Iran on tbe problems of earthquake- resistance regulations for the Iranian building code

How to design structures to resist earthquakes

Earthquake engineering research at the University of California, Berkeley

7 9

11 15

21 25

29

35

39

41

49

M. Ipek Research activities in the Seismological 55 Institute of the Technical University of Istanbul

domaine du génie séismique

to earthquake shocks

E. Lauletta Récentes activités italiennes dans le 59

T. Hisada, M. Izumi, Structural response of a tall building 63 and M. Hirosawa

D. N. Roustanovitch Essai d’étude séismique des zones épicentrales sur l’exemple du tremblement de terre d’Ashkhabad du 6 octobre 1948

incidence of earthquakes S. V. Medvedev Seismic zoning with reference to the

B. Geological and seismological investigations in the Skopje area Études géologiques et séismologiques dans la région de Skopje

A. Zátopek The Skopje earthquake of 26 July 1963, and the seismicity of Macedonia

N. N. Ambraseys General characteristics of the Skopje earthquake-a summary

M. Arsovski, Seismological and geological N. Grujic investigations of the Skopje and D. Gojgic valley and urban area D. N. Roustanovitch, Études séismologiques instrumentales V. A. Tokmakov, de la zone épicentrale et D. Hadgievski du séisme de Skopje du 26 juillet

1963 D. A. Lilienberg Qualités géomorphologiques structu-

rales et mouvements tectoniques contemporains dans la vallée de Skopje

J. A. Mechtcherikov Mouvements actuels de l’écorce terrestre et leur étude dans la région de Skopje (résumé)

C. Earthquake engineering studies in Skopje Études séismotechniques effectuées dans la région de Skopje

J. Despeyroux

K. HololEev and Dj. Solovje,-

F. CaCoviC

S. TerEelj

A. Velkov

S. A. Bubnov

Les enseignements du séisme de Skopje du 26 juillet 1963

The influence of the earthquake of 26 July 1963 on constructions in Skopje

Preliminary results on testing of brick walls

Preliminary results on testing of reinforced-concrete columns

Analysis of damage to fourteen- storey buildings in Skopje

Problems of earthquake-resistant design and engineering

69

71

77

81

87

91

95

99

103

111

121

123

125

129

General discussion of Session C Discussion générale de la Session C

135

Introduction

Under the joint auspices of the Federal Government of Yugoslavia and Unesco, and with the financial support of the United Nations Expanded Programme of Technical Assistance, an International Seminar on Earthquake Engineering was held in Skopje (Yugoslavia), from 29 September to 2 October 1964.

The objects of the seminar were: 1. To exchange information on recent developments in earthquake engineering

and on research activities in European countries, Japan and North America. 2. To bring together the scientists and engineers who had worked in Skopje

since the earthquake of 26 July 1963, and to review the results of their work. Unesco arranged for the participation of twelve experts in seismology and earthquake engineering from the following countries : Czechoslovakia, France, Iran, Israel, Italy, Japan, Portugal, Romania, Turkey, Union of Soviet Socialist Repub- lics, United Kingdom and the United States of America. Among these participants were four who had carried out missions for Unesco in Skopje. Ali local arrangements for the seminar, including the provision of simultaneous

interpretation between Serbo-Croat and English, French and Russian, were made by the Yugoslav Government with the help of a local organizing committee presided over by Mr. K. Kitanowski, Deputy Mayor of Skopje. About fifty scientists and engineers from all parts of Yugoslavia attended the seminar. The seminar was opened on the morning of 29 September by Mr. Blagoje

Popov, Mayor of Skopje, ,in the presence of Mr. Kemal Seifula, Vice-president of the Executive Council of the Socialist Republic of Macedonia, Messrs. Grivcev and Kirijas, members of the Executive Council, and Mr. J. ZmaiE, Director- General of the Department of Technical Co-operation of the Federal Government of Yugoslavia. The meeting elected as president, Mr. K. Kitanowski and as general reporter,

Mr. S. Bubnov. Mr. Lj. Kostowski and Mr. E. M. Fournier d'Albe (Unesco) acted as joint secretaries. The meeting was divided into three sessions:

7

Session A Recent developments in earthquake engineering research Chairman: Mr. V. Ribarii: (Ljubljana) Reporters: Mr. M. Arsovski (Skopje)

Geological and seismological investigations, in Skopje Chairman: Mr. J. Miladinov (Belgrade) Reporters: Mr. S. Bubnov (Ljubljana)

Engineering investigations in Skopje Chairman: Mr. P. Serafimov (Skopje) Reporters: Mr. I. Alpan (Haifa)

Mr. A. Zátopek (Prague) Session B

Mr. S. V. Medvedev (Moscow) Session C

Mr. N. N. Ambraseys (London)

8

Introduction

U n colloque international sur le génie paraséismique s'est tenu à Skopje, en Yougoslavie, du 29 septembre au 2 octobre 1964, sous les auspices du gouvernement fédéral de Yougoslavie et de l'Unesco, et avec le concours financier du Programme élargi d'assistance technique des Nations Unies.

1. Faire le point de l'actualité dans le domaine du génie paraséismique et des travaux de recherche effectués en Europe, au Japon et en Amérique du Nord;

2. Réunir les savants et les techniciens qui avaient travaillé à Skopje depuis le séisme du 26 juillet 1963 et dresser le bilan de leurs travaux.

L'Unesco avait fait appel à douze spécialistes de la séismologie et du génie para- séismique venus des pays suivants : Etats-Unis d'Amérique, France, Iran, Israël, Italie, Japon, Portugal, Roumanie, Royaume-Uni, Tchécoslovaquie, Turquie et URSS. Quatre d'entre eux avaient accompli des missions à Skopje pour le compte de l'Unesco. Toute l'organisation matérielle du colloque, y compris l'interprétation simul-

tanée en serbo-croate, en anglais, en français et en russe a été assurée par les soins du gouvernement yougoslave, avec le concours d'un comité d'organisation local présidé par le maire adjoint de Skopje, M. Kitanowski. Une cinquantaine de savants et de techniciens de toutes les régions de Yougoslavie assistaient à la réunion. Le colloque a été ouvert dans la matinée du 29 septembre par M. Blagoje

Popov, maire de Skopje, en présence de M. Kemal Seifula, vice-président du Conseil exécutif de la République socialiste de Macédoine, de MM. Grivcev et Kirijas, membres du Conseil exécutif, et de M. J. ZmaiC, directeur général du Département de la coopération technique du gouvernement fédéral de Yougoslavie. Le bureau suivant a été élu: président, M. Kitanowski; rapporteur général,

M. S. Bubnov; secrétaires, MM. Kostowski et E. M. Fournier d'Albe (Unesco). Le colloque s'est divisé en trois groupes:

Les objectifs de ce colloque étaient les suivants:

9

Groupe A Activités récentes dans le domaine du génie paraséismique Président: M. V. RibariE (Ljubljana) Rapporteurs: M. M. Arsovski (Skopje)

Etudes géologiques et séismologiques effectuées à Skopje Président: M. J. Miladinov (Belgrade) Rapporteurs: M . S. Bubnov (Ljubljana)

M. S. V. Medvedev (Moscou) Etudes séismotechniques effectuées à Skopje Président: M . P. Serafimov (Skopje) Rapporteurs: M. I. Alpan (Haïfa)

M. A. Zátopek (Prague) Groupe B

Groupe C

M. N. N. Ambraseys (Londres)

10

S. A. Bubnov

General report

The International Seminar on Earthquake Engineering was divided into three sessions : Session A, on the general problems of seismology, engineering seismology and

Session B, on the seismological, geological, geomorphological and geotectonic

Session C, on the effects of the earthquake on buildings in Skopje and the prob-

earthquake engineering in the world.

aspects of the earthquake in Skopje of 26 July 1963.

lems of earthquake resistant design and construction.

Session A The investigation of damaging earthquake effects during recent years has demonstrated that the statical analysis of buildings and other structures must be replaced by dynamical analysis, taking into consideration the additional forces and deformations of elastic and plastic character which are due to ground motions. Examples were cited from different countries concerning the effects of strong

earthquakes on various kinds of structures, including specially designed earthquake-resistant structures, and data were presented on the corresponding studies. It was emphasized that a necessary condition for further progress in designing

earthquake-resistant structures is the acquisition of a greater volume of instrumental data provided by standardized strong-motion instruments. Unesco was asked for assistance in organizing a working group to study and propose an appropriate network of strong-motion observations and the most suitable form of standardization of instruments. Extensive research into the problems of earthquake engineering has been

started in many countries, in order to minimize loss of life and material damage Caused by earthquakes. Both theoretical and experimental studies have been undertaken on the behaviour of all kinds of structures during earthquakes, the connexion between the structure and the ground and different dynamical conditions in the ground and structure itself. Attention was paid especially to the

S. A. Bubnov General report

non-linear dynamical response of structures and its analysis by means of electronic computers. Examples were given of research programmes of this kind and certain results were reported. The discussion revealed a need for an internationally organized rapid exchange

of information. The questions concerned mainly technical details, but some of them revealed the existence of serious problems, the solution of which will require a deeper study of the fundamental dynamics of structures during earthquakes, as well as of technical problems connected with the construction of earthquake- resistant structures (elastic and plastic elements, energy-absorbers, etc.). The need for world-wide co-operation was emphasized.

Session B New analyses of the macroseismic data of past earthquakes in Macedonia, especially in the Skopje valley, were presented. On the basis of these data, formulae were given for the frequency of earthquakes of different intensity in the Skopje valley and in Macedonia. Studies of the types of damage to buildings by the earthquake in Skopje have

been made in an attempt to analyse the motion of the ground during the earthquake. There is some indication that this motion was of very short duration and had the typical nature of a shock. Emphasis was placed on the connexion between the area flooded by the Vardar

river and the area of maximal damage to buildings. Attention was drawn to the need to commence with the elaboration of a seismic

regionalization map of Macedonia. For this purpose, it is necessary to intensify instrumental observations of earthquakes of small intensity, in order to locate the maximum number of small earthquakes and determine the depth of their foci. The need for greater knowledge of the energy of the ground motion was empha-

sized because this permits the development of the design of structures taking into account non-linear deformations. The seismological parameters of the Skopje earthquake were discussed, partic-

ularly its intensity, depth of focus and magnitude. An analysis was presented of the after-shocks and the locations of their foci.

A tectonic map of the Skopje valley showing the principal directions of the faults in this region was also presented. Extensive geological, geophysical and seismic investigations have been carried

out recently with a view to the preparation of a microzoning map of the Skopje area, and to determining the geological structure. A microzoning map for Skopje was shown. A report was presented on the results of studies of microtremors and after-shocks by five seismological stations with equipment from the U.S.S.R. On the basis of geomorphological investigations, it was shown that the intensity

of the vertical tectonic movements connected with earthquakes could be determined. It appeared that the vertical movements have been more marked in the zone of the Vardar river during recent years. It was shown how geomorphological methods could be used to determine the

12

S. A. Bubnov General report

directions of the principal seismic faults: these methods were applied to the Skopje valley. Investigations of the movements of the earth’s crust show that the slow and the fast movements are connected. This implies that the study of slow movements could throw some light on the problem of forecasting earthquakes. For the study of contemporary movements of the earth’s crust a complex method must be applied. Such studies should be undertaken in the Skopje area and in Macedonia generally.

Session C The mosaic block structure of the Skopje valley determined the mechanism of the earthquake. The principal shock was a very short one, and several buildings remained on the brink of destruction. In this type of earthquake, the influence of damping is very small and therefore elastic structures stood up better to the shock, which may be considered to be characteristic of shallow earthquakes. It was noted that, in the past, the earthquakes in the Skopje area have all been shallow. A parallel was drawn between the type of earthquake and the damage caused

by the Skopje and Agadir earthquakes. An analysis was made of the behaviour of different buildings, constructed

with different materials : bricks, reinforced concrete and mixed materials. The buildings of brick with a shortperiod of oscillation were heavily damaged. The great importance of collar-beams in brick structures was emphasized. Structures of reinforced-concrete framework stood up well to the earthquake. .

Some damage suffered by this type of structure was due to errors in design and construction. Attention was drawn to the need to take into account the foundations of

buildings, since differential movements of foundations can increase the liability to damage. A report was presented on a survey, carried out by the Union of Yugoslav

Laboratories, of the damage to buildings in Skopje. Two hundred and sixty-six different constructions have been surveyed and studied. A detailed analysis of the damage to a fourteen-storey building was presented. A report was presented on tests of the response of brick walls to combined

vertical and horizontal loads. Some were tested after having been destroyed and repaired. A report was presented on the results of testing reinforced-concrete colums.

It was emphasized that steel constructions in general stood up well to the earthquake.

During the seminar the participants from the European countries unanimously agreed that: (u) a European Commission on Earthquake Engineering should be established as a branch of the International Association for Earthquake Engineer- ing; (6) the Yugoslav National Committee should establish a working group to draw up draft statutes of this commission.

13

S. A. Bubnov

Rapport général

Le colloque international sur le génie paraséismique s’est divisé en trois sessions. La session A a été consacrée aux problèmes généraux de la séismologie, de&ismo- technique et de génie paraséismique.

La session B a été consacrée aux aspects séismologiques, géologiques, géomorpho- logiques et géotectoniques du tremblement de terre qui a ravagé la ville de Skopje le 26 juillet 1963.

La session C a été consacrée aux effets du séisme sur les bâtiments de Skopje et aux problèmes que pose la construction de bâtiments capables de résister aux tremblements de terre.

Session A L’étude des dommages causés par les tremblements de terre des dernières années a montré que l’étude statique des bâtiments et autres ouvrages devrait être rem- placée par une étude dynamique qui tienne compte des poussées et des défor- mations élastiques et plastiques supplémentaires dues aux mouvements tectoniques. On a cité plusieurs exemples pris dans différents pays du comportement de

divers types de constructions, y compris celles qui ont été conçues spécialement pour résister aux tremblements de terre; des études documentées ont été présentées. Un fait a été souligné: on ne pourra mettre au point des structures offrant

une meilleure résistance aux mouvements séismiques que si l’on dispose d‘un plus grand volume de données instrumentales. Celles-ci doivent être recueillies au moyen de séismographes pour tremblements de terre intenses répondant tous aux mêmes normes. I1 a donc été demandé à l’Unesco de collaborer à la constitu- tion d’un groupe de travail qui aurait pour tâche de présenter un projet de mise en place d’un réseau de stations d’observation équipées de ces appareils et d’étudier le meilleur moyen de normaliser les séismographes. Le souci d’épargner des vies humaines et de diminuer les dégâts matériels dus

aux séismes a conduit nombre de pays à entreprendre des recherches très poussées dans le domaine du génie paraséismique. I1 s’agit d‘études tant théoriques qu’expé-

15

S. A. Bubnov Rapport général

rimentales sur le comportement de toutes sortes de structures pendant les tremblements de terre, et sur les rapports entre la structure et le terrain dans différentes conditions dynamiques. On s’est particulièrement attaché à l’étude du comportement dynamique non

linéaire de certaines structures, analysé au moyen de calculateurs. Des exemples de programmes de recherches de ce genre ont été cités et certains résultats ont été communiqués. Les débats ont fait apparaître la nécessité d’organiser un échange rapide des

informations à l’échelle internationale. Bien que les questions aient surtout porté sur les détails techniques, certaines ont révélé l’existence de graves problèmes dont la solution exige l’étude plus poussée, d’une part, de la dynamique fondamentale des constructions pendant les tremblements de terre, d‘autre part, des problèmes techniques liés à la construction d’ouvrages et de bâtiments résistant aux secousses séismiques (élasto-plasticité, coefficient d’amortissement, etc.). L’intérêt d’une coopération à l’échelle mondiale a également été mis en relief.

Session B Le colloque a pris connaissance de nouvelles analyses des données macroséis- miques relatives aux tremblements de terre enregistrés en Macédoine, et plus particulièrement dans la vallée de Skopje. A partir de ces données, on a présenté des formules indiquant la fréquence des séismes de différente intensité intéressant cette région. Des études ont été faites en vue de déterminer les mouvements du sol pendant

’ le grand séisme de Skopje d’après la nature des dégâts subis par les bâtiments. 11 semble que ce mouvement ait été de très courte durée et ait pris la forme carac- téristique d’une secousse. On a signalé une corrélation entre la zone inondée par les eaux du Vardar et

le périmètre de destruction maximale des bâtiments. La nécessité d’entreprendre l’élaboration d’une carte séismique de la Macédoine a été soulignée. 11 faudrait intensifier pour cela les observations instrumentales des séismes de faible amplitude, de façon à enregistrer le plus grand nombre possible de ces phénomènes et de déterminer la profondeur de leur foyer. Une autre question qui demande à être approfondie est celle de l’énergie des

mouvements du sol; si elle était mieux connue, on pourrait mettre au point des structures offrant une meilleure résistance aux déformations non linéaires. Le colloque a étudié les paramètres séismologiques du tremblement de terre de Skopje, et notamment l’intensité de la secousse, sa magnitude et la profondeur du foyer. Une analyse des répliques qui ont suivi le séisme, avec indication de leur foyer

présumé, a été présentée à la session B ainsi qu’une carte tectonique de la vallée de Skopje, montrant la direction des principales failles. D e vastes travaux de géologie, de géophysique et de séismologie ont récemment été effectués en vue d’établir une carte détaillée de la vallée de Skopje et de déterminer la structure géologique du terrain. Le colloque a pu voir une carte de la ville de Skopje (micro- régionalisation) ainsi qu’une étude des essaims et des répliques enregistrés par cinq stations séismologiques équipées de matériel soviétique.

16

S. A. Bubnov Rapport général

Des chercheurs ont montré, à partir d’études géomorphologiques, qu’on peut déterminer l’intensité des mouvements tectoniques verticaux qui accompagnent les tremblements de terre. Ces mouvements verticaux ont été plus marqués dans la zone du Vardar, au cours des dernières années. On a montré aussi comment les méthodes géomorphologiques pouvaient servir à déterminer la direction des principales failles séismiques : ces méthodes ont été appliquées dans la vallée de Skopje. L‘étude des mouvements de l’écorce terrestre montre qu’il existe un rapport entre les mouvements lents et les mouvements rapides. I1 est donc permis de penser que l’étude des mouvements lents facilitera les pronostics de séismicité. L’étude des mouvements actuels de l’écorce terrestre exige des méthodes complexes. I1 faudrait entreprendre cette étude en Macédoine d’une façon générale et notamment dans la région de Skopje.

Session C

C’est la structure en mosaïque du terrain de la vallée de Skopje qui a déterminé le mécanisme du tremblement de terre. La principale secousse a été de très courte durée et plusieurs bâtiments n’ont été qu’ébranlés. L‘amortissement intervient très peu dans ce genre de séisme, aussi les structures élastiques ont-elles mieux résisté; c’est d’ailleurs là une caractéristique des secousses telluriques à foyer superficiel. On a noté que tous les tremblements de terre enregistrés dans la région de Skopje avaient un foyer peu profond. Dans une étude fondée sur les tremblements de terre de Skopje et d’Agadir, un parallèle a été établi entre le type de séisme et les dommages qu’il provoque. On a d’autre part étudié le comportement de constructions en Wérents maté-

riaux : briques, béton armé et constructions mixtes. Les bâtiments en brique, qui ont une courte période d’oscillation, ont été les plus fortement endommagés. Les travaux ont fait ressortir la très grande importance des chaînages dans les constructions en brique. En revanche, les bâtiments à ossature en béton armé ont bien résisté. Les

défaillances constatées tiennent le plus souvent à des défauts de construction. On a également souligné l’importance des conditions de fondation. Les fonda-

tions subissent en effet des déplacements différentiels qui peuvent augmenter la vulnérabilité de l’immeuble. Le colloque a pris connaissance d’une étude réalisée par l’Union des labora-

toires yougoslaves sur le comportement de 266 constructions différentes au cours du séisme de Skopje. D’autres travaux ont porté sur l’observation détaillée des dégâts subis par un

immeuble de quatorze étages et sur des tests indiquant la réaction des murs de brique aux poussées s’exerçant à la fois verticalement et horizontalement. Certains de ces tests ont été effectués sur des murs endommagés, puis réparés. Enfin, une communication a rendu compte d’essais portant sur des colonnes de béton armé. Les travaux ont fait ressortir les qualités de résistance aux séismes des constructions à armature métallique.

17

S. A. Bubnov Rapport génkral

A u cours du colloque, les participants des pays d'Europe ont été unanimes à souhaiter: a) que soit créée une commission européenne du génie paraséismique, dans le cadre de l'Association internationale de génie paraséismique; b) que la Commission nationale yougoslave constitue un groupe de travail chargé de rédiger un projet de statuts pour cette commission.

18

A

Recent developments in earthquake engineering research Activités récentes dans le domaine du génie paraséismique

I. Aipan Israel Institute of Technology, Haifa

Earthquake engineering in Israel

As a blunt introductory statement : earthquake engineering, as nowadays understood, is not practised in Israel. This does not, of course, imply that our engineers are unaware of earthquake problems but these are not considered to be of an urgent nature. The main reasons for this attitude are : (u) until fairly recently we built low

(up to four floors) and quite robustly (reinforced-concrete “boxes”) ; (b) Israel is considered to be in a region of low seismic activity. However, lately our building style has been changing to taller and more flexible

buildings: twelve to fifteen floors are not uncommon and the tallest building to date numbers thirty-two floors. Consequently, the awareness of earthquake engineering problems is growing and the attached code proposal represents an attempt in the right direction regardless of its objective merits. At the same time a seismological unit, attached to the Geological Institute,

has been operating for some time. It has prepared and distributed questionnaires to a fair number of voluntary “earthquake watchers”; it is operating two seismographs and is in the process of setting up a more versatile model of weak-motion seismograph in Jerusalem. Unfortunately not enough funds are available to set up strong-motion apparatus. On the basis of past records (see bibliography) this unit is working on a tentative

intensity map to supersede the rather crude map which appears in the code proposals. The subject of structural dynamics, which should legitimately include seismic

design, does not adequately figure in the curriculum of our Institute of Technology, but here, too, changes are imminent.

PROPOSED ISRAEL STANDARD CODE. SEISMIC LOADS ON BUILDINGS (RESUME)

In Israel there is no extensive experience concerning the seismic resistance of buildings. The committee for standards on building loads has, therefore, decided

21

Recent developments in earthquake engineering research

to publish the collected material on earthquake loads as a “proposed standard” to be used, if desired, as a guide to designers.

Design principles

For the purposes of application of this proposed standard, the country has been divided into two regions : Region A-maximum expected intensity VI1 Region B-maximum expected intensity IX

on the Mercalli-Cancani scale. The influence of earthquakes will be taken into account in design computations as a horizontal force H, applied in any direction to the centre of gravity of every part of the building.

H = C(G + tcP) where : C = seismic coefficient (dependent on region, type of building and foundation

G = dead load; tc = incidence coefficient of live load; P = live load.

soil) ;

TABLE 1. The seismic coefficient C

Type of building and soil Region A Region B

1 Whole buildings 1.1 Mdtistorey buildings founded on good soil , o. 1 20.2

(allowable bearing pressure 15 t/mz and above) mi-4.5 mi-4.5 0.2 0.4 1.2 Multistorey buildings founded on bad soil

(allowable bearing pressure less than 15 t/m2) mf4.5 m4-4.5 1.3 Various structures such as reservoirs, connected buildings, 0.02 0.04

etc., founded on good soil (allowable bearing pressure 15 t/mz and above)

(allowable bearing pressure less than 15 t/ma)

~~

1.4 Various structures (as above under 1.3) founded on bad soil 0.08

1.5 Slender structures (water towers, chimneys) 0.04 0.08 1.6 Jetties 0.01 0.02

2 Structural parts 2.1 Cantilever elements (parapets, isolated walls, ornaments) 0.20 0.40

0.04

2.2 Interior walls, columns, beams, internal partitions, wall panels 0.05 0.10 2.3 Various installation in buildings: pumps, machines, tanks, 0.05 0.10

pipes

rn = number of storeys above the one under consideration and assuming that the floor panels or other structural stiffness elements transmit the forces in their plane.

22

I. Alpan Earthquake engineering in Israel

TABLE 2. The incidence coefficient

Type of structure U Type of structure U

1 Residential and public buildings 0.0 3 Stores 0.5 2 Shops, workshops, garages, etc. 0.3 4 Liquid reservoirs, soils, etc. 1 .o

Relaxing provi&ons with respect to loads

It is not required to add wind loads to earthquake loads (superposition). Simi- larly, in earthquake-resistant design, allowable stress requirements may be relaxed (with concrete an increase of one-third in allowable stress is permitted).

BIBLIOGRAPHY

KALNER-AMLRAN. 1950-1951. A revised earthquake catalogue of Palestine. Israel Explo- ration journal, vol. 1, no. 4.

L~SCHNER, S. S. 1956. Influence of earthquakes on structures andprinciples of computation of structures with regard to earthquake dangers. Jerusalem, Water Planning for IsraelLtd.

NEEV, D. 1963. The Mediterranean coastline of Israel: a fault line of recent activity. NEUMANN, H. 1955. Earthquakes and their effect on buildings. In the field of building. Technion-Tsrael, Institute of Technology. (Bull. no. 29.)

PLASSARD, J.; KOGOL, B. 1962. Catalogue des sdismes ressentis au Liban. Publication of the Observatoire de Ksara (Liban).

REINER, M . 1955. Earthquake consideration in the design of buildings. In the field of building. Technion-Israel, Institute of Technology. (Bull. no. 24.)

SIEBERG, A. 1932. Untersuchungen uber Erdbeden und Bruchschollenbau im östlichen Mittelmeergebiet. Denkschr. Mediz. Naturwiss. Geol. Jena, vol. 18, p. 159-273.

SHALEM, N. 1951. Seismicity in Palestine and in the neighbouring areas (macroseismical investigations). A report submitted to the Israel Water Planning Service. - . 1951. The tremor of September loth, 1953. Bull. Research Council of Israel, vol. 3, no. 3, p. 266. - . 1953. O n the tremor of 28.12.52. in the Galilee and Carmel areas. Bull. Research Council of Israel, vol. 2, no. 4, p. 428-439. - . 1954. Seismicity and Erythrean disturbances in the Levant. Publication of the Bur. Central Internat. Seismol., ser. A., fasc. 19, p. 267-275. - . 1954. The Red Sea and the Erythrean disturbances. Proc. 19e Cong. Geol. Internat., sect. 15, fasc. 17, p. 226.

-. 1955. [Seismic tidal waves (tsunamis) in the eastern Mediterranean.] Bull. Geol. Soc Israel, (Jerusalem), vol. 3. (In Hebrew.) - . 1955. The tremor of 13th September, 1954 and the instrumental research from January 1954-June 1955; Tahal. Jerusalem, Water Planning for Israel Ltd. and Geol. Survey of Israel, July.

*

23

A. Beles Academy of Sciences of the Romanian People’s Republic, Bucharest

The problem of engineering seismology in Romania

Until 10 November 1940, the last earthquake mentioned in the history of Romania as having produced important damage, occurred at noon on 26 October 1802 and was known as the “great earthquake”. As the other earthquakes that have occurred since then have caused generally

little damage, seismicity has not been taken into account in the design of structures in our country. Therefore, when at 4.39 a.m. on 10 November 1940 a very strong earthquake

ravaged an important area of our country and especially Bucharest, there was generally great surprise. Until 1900 most of the buildings in Romania were of traditional shape, with

load-bearing brick walls and wood floors, without any allowance for horizontal forces, except for the usual wall-ties and floor-joist anchors. From 1910 the use of reinforced concrete in floors and frames assured a better

resistance against horizontal forces but only the effect of wind was taken into consideration. Generally the presence of interior brick walls was considered as providing sufficient lateral bracing. It was, therefore, a great shock when on 10 November 1940, the most important

building in Bucharest, just completed with reinforced-concrete frames, collapsed completely causing the death of more than 100 people. Many other buildings with reinforced-concrete frames were also damaged. It should be noted that all the reinforced concrete was poured in place and that no prefabricated reinforced- concrete elements were used either for floors or for frames. Soon after the earthquake, official instructions were elaborated based on the

“static method of design”. A horizontal uniform acceleration of 0.1 g for the regions in the vicinity of the epicentre and of 0.05 g for the more distant regions was recommended. This acceleration was considered to be uniform with height in the structure. After 1945, the economic structure of the country being transformed on new

bases, the development of industry received a rapid advance, new factories were built, new towns were founded and new dwelling-houses erected, and the problem

25


of more adequate earthquake-resistant design was therefore again raised. Meanwhile new methods of design had been invented and perfected. The

elastic properties of the building materials and of the structures themselves were considered, the movement of the ground during earthquake could be recorded by means of strong-motion seismometers and, as a consequence, a new “dynamic method” was introduced in the design of earthquake-resistant structures. For a rational design it was necessary first to draw a map with isoseismics for

the whole country. Investigations were made, in order to establish the principal hypocentres of

Romania, by the Institute of Seismology, working together with the Institute of Geology. The most important was found in the region of Vrancea, on the outer curve of the Carpathian mountains. It was established that on the occasion of the earthquake of November 1940 the depth of the hypocentre was 170 km. The other hypocentres are more superficial and consequently have more localized influence. The isoseisms were established by the same institutes and were connected with

those of the surrounding countries. ,

New regulations were established, taking into consideration the different codes used in other countries, especially the Union of Soviet Socialist Republics and the United States. These new Romanian regulations for aseismic design are based on the dynamic method. According to the principles adopted in the dynamic method, the base shear

force is established as a lateral force transmitted into the structure from the foundation, and has a horizontal direction. The shear force is obtained by multiplying the total weight Q of the structure by a coefficient of seismicity c:

S = c.Q. The coefficient c is at least 0.02 and is the product of five parameters:

c = k,.n.P.&’F. The influence of the seismicity of the region is represented by k,. Three regions are considered according to the Mercalli-Cancani modified scale : Scale of intensity VII: k, = 0.025. Scale of intensity VIII: k, = 0.05. Scale of intensity IX: k, = 0.10.

The influence of the ground is represented by n and has been established on the following considerations : Soils with a resistance of more than 2 kg/cm2: Soils with a resistance up to 2 kg/cm2: Soils with very low resistance, saturated with

n = 1. = 1.25.

water at the level of the foundations: n = 1.5. ß is a dynamic coefficient representing the corresponding value of the accelerogram response spectrum obtained from the hyperbola :

0.9 ß=-

T ’ where Tis the fundamental period of the structure.

26

A. Beles The problem of engineering seismology in Romania

This hyperbola is limited to a highest value of 3, corresponding to a fundamental period of 0.3 sec and to a lowest value 0.6, corresponding to a period of 1.5 sec. The hyperbola being obtained as a “standard acceleration spectrum” for a

system with one degree of freedom, and as the usual structures are very often systems with many degrees of freedom, a correction is necessary to transform the real system into a single mass system. The correction coefficient is 6, defined by the formula :

where u k is the deflection of the multidegree of freedom system for the considered mode of vibration, and Q k is the mass at the considered point of deflection. The coefficient varies for the usual cases from 1 to 0.8. As an average, it is

recommended to take 1. The coefficient Y represents theinfluence of the building material and of the structure on the damping of the vibration produced by the seismic forces. This coefficient has the following values : For low, rigid structures with high damping, Y = 1. For semi-flexible structures as those with reinforced-concrete

For very flexible structures where the effect of damping is very small, frames, Y = 1.2.

Y = 1.5. By means of the formula given at the beginning, the shear force at the base of the structure can be determined. The distribution of this shear force along the structure is given by the formula:

where uk is the deflection of the point k under the Qk loadings. For simplification u,+ can be taken as a parabola. Besides horizontal forces, supplementary vertical forces are also taken into account by increasing the vertical forces from 25 to 100 per cent according to the degree of intensity. The regulations are supplemented by simplified methods for an easier deter-

mination of the different parameters adopted in the general shear force formula. Thus, for instance, for usual structures the fundamental period can be determined by the formula:

H T = 0.07 -- for low rigid structures, J B

and H T = 0.09 I__ for semi-flexible structures.

V B H is the height in metres and B the breadth of the structure.

27


For usual dwelling-houses with self-bearing brick walls one can take : T = 0.3 + 0.05 12,

where n is the number of storeys up to six storeys, and

for buildings with reinforced-concrete frames having more than six storeys. For different industrial structures simplified methods are given to calculate

the fundamental period. The torsional effect is taken into consideration even for symmetrical structures. For elements where concentration of stresses can occur, as for instance at the

passage from a flexible to a rigid part of the structure, for anchor-bolts, etc., the stresses are multipIied by values ranging from 1.25 to 2.

The regulations provide ample indications for an aseismic conformation of the structures and details for different elements. Special indications are given in connexion with the defects observed on the occasion of the earthquake of 10 November 1940. The Romanian earthquake code is at the first draft stage and its application

is in course of trial.

T = 0.10 y1

Discussion

R. W. Clough The seismic coefficient that has been proposed for Romania includes the product of five coefficients. If the maximum value of each of these is taken, the seismic coefficient is 67.5 per cent. D o you consider this coefficient to be realistic?

This is known, and we wanted to see if such cases can occur and what are their eventual implications. The study is in course.

N. N. Ambraseys Est-ce que votre code prévoit des déformations plastiques (permanentes) ?

A. Beles Account is taken in the code of such deformations, but no values are given for their limitation.

A. Beles

28

T. Hisada Head, Structural Division, Building Research Institute, Tokyo

Damage to reinforced-concrete buildings in Niigata City due to the earthquake of 16 June 1964

An earthquake occurred off the coast of Niigata Prefecture, Honshu Island (Japan), at 1.02 p.m. on 16 June 1964. Areas extending for about 200 k m along the coast suffered significantly from the shock. An investigation team from the Building Research Institute was sent to Niigata

City, which suffered most from the earthquake. An extensive survey of the damage to reinforced-concrete buildings in the city was carried out by the team, and a report thereon is to be published in the near future. In this report, the writer, representing the Building Research Institute team,

presents a summary of the main results obtained from the observations together with some seismological and geological data concerned.

EARTHQUAKE

The Japan Meteorological Agency reported that the location of the epicentre was at 380 4' N. 1390 2' E., which is about 55 k m NNE. of Niigata City, and the depth of focus was about 40 k m below the earth's surface. The magnitude of the earthquake determined by Matsushiro Seismological

Observatory was reported as 7.7. The maximum ground accelerations recorded by the strong-motion accelero-

meters which compose the Japanese network were 160 gal on an area of reclaimed land in Niigata City, and 100 gal on a site of sand-clay layer in Akita City at about 170 k m NNE. of the epicentre.

GROUND DAMAGE CONSEQUENT TO EARTHQUAKE

In the affected area of Niigata City, there could be seen ground subsidence, fissures and horizontal displacements in many places. Sand boiling and expulsion of ground water were also observed in many such

areas. Landslips were observed in some areas such as riversides and sand-dunes.

29


EXTENT OF DAMAGE

It was reported that 2,130 houses were destroyed, 6,240 moderately or severely damaged and 31,240 slightly damaged. In the affected areas, civil engineering structures such as bridges, roads, railways and quay walls were severely damaged, and water works were entirely disrupted. About 15,000 houses were inundated by the collapse of the protective embank-

ments along the Shinano river. The damage due to flood was increased by the onset of tsunami and subsidence of ground. Fire broke out in oil tanks located near the river mouth and about 300 neigh-

bouring houses were burnt down by the spreading fire. It should be noted that the number of deaths following the earthquake was

only twenty-six.

LOCATION AND SUBSOIL CONDITION OF NIIGATA CITY

The city of Niigata is situated on the northern coast of Honshu, about 250 k m N N W . of Tokyo. The city straddles the Shinano river at its mouth, extending upstream for about 8 k m on sand deposits.

1. Very loose sand layer (N e 5) (N = number of blows per 30 cm in standard penetration test) extends from the ground surface to about 5 m in depth, including lenses of silt in some places.

2. Loose sand layer (5 < N e 10) lies below the above layer in a thickness of about 5 m.

3. Medium sand layer (10 c N c 30) lies, in general, at a depth of 10 to 15 m below the ground surface.

4. Thick layers of dense sand and hard clay (N > 30) extend below the medium sand layer.

On the contrary, in the less affected areas, the dense sand layers are much closer to the ground surface than in the more affected areas. Mechanical analyses of the upper soft sand indicate an average composition

of 95 per cent sand and 5 per cent silt, and 26 per cent water content (ratio to dry weight). The sand is composed mostly of grains of about 0.3 mm diameter and has high granular uniformity. Comparisons of data obtained from borings made both before and after the

earthquake show that, in general, the loose sand layers were compacted by the earthquake and that the dense sand layers were loosened; furthermore, that the critical values of compactness of sand (i.e., stable density) are dependent on depth below ground surface.

General features of subsoil in the affected areas are as follows:

THE AFFECTED AREAS AND CAUSE OF DAMAGE

Damage in the city, due to the earthquake, occurred in specific areas which are generally located along both sides of the Shinano river.

30

T. Hisada Damage to reinforced-concrete buildings in Niigata City

Such affected areas, where many buildings tilted, subsided and suffered structural damage due to foundation failures, almost coincide with the old course of the river. It is observed that in the severely damaged areas medium and dense sand layers (N > IO) do not exist within 8 m of the ground surface. The causes of damage to buildings were the subsidence, fissures and displace-

ment of the supporting ground and the failing of bearing power in pile foundations. Little or no structural damage to buildings occurred as the direct result of the

earthquake. It is considered that the ground damage mentioned above was caused by the

liquefaction of loose sand layers. This is the first time that “quicksand” effects have occurred in such an extensive area as in Niigata City, and this fact has given an important warning to reconsider some of our present methods of designing earthquake-resistant structures.

TILTINGS AND SUBSIDENCES OF BUILDINGS

At the time of the earthquake, there were 1,530 reinforced-concrete buildings in Niigata City. Of those buildings, about 3 10 tilted and/or subsided, principally in the areas of severe damage. The extent of tilting angles (e) of buildings was distributed as follows: O < 8 < lo

(1/57 rad.), 54 per cent; 1 < 8 c 2S0, 27 per cent; 2.50, 19 per cent. A four-storey reinforced-concrete apartment-house located in reclaimed land along the Shinano river overturned. Remarkable tilts of buildings occurred in the direction of short span, and the

angles of tilt generally were proportional to the slenderness ratios (ratio of height to width) of the buildings. Subsidences of buildings were mostly less than 1.5 m; the maximum reached

3 m. The buildings with a basement storey suffered less subsidence and tilt than

buildings without basements. ‘

INFLUENCE OF FOUNDATIONS ON DAMAGE

Buildings with piles suffered less damage than those without piles. Buildings on soft sand of considerable thickness generally suffered damage,

particularly where piles were absent or, to a lesser extent, where friction piles only were used. Buildings having foundations supported at a depth of more than 10 m from

the ground surface suffered less damage. Significantly less damage was observed where the buildings were supported by

sand layers having values of N above 15. Buildings supported by concrete piles driven into compact sand layers (N > 25) suffered little or no damage. The use of vibro-flotation in improving compactness of sand soil at construc-

tion sites, was observed to have been of considerable effectiveness.

31


STRUCTURAL DAMAGE

Out of 310 reinforced-concrete buildings, 110 suffered more or less structural damage. Buildings of longer length in plan were more liable to suffer structural damage. Principal structural damage to reinforced-concrete buildings was as follows :

cracking or separation at the irregular parts or at the expansion joints in the buildings, cracking of shear walls or partition walls, cracking of floor slabs, cracking around the openings, bending cracks in columns or beams, shearing cracks in columns or beams. No building with a basement storey suffered significant structural damage.

PROPOSAL FOR REPAIR A N D RECONSTRUCTION WORKS

1.

2.

3.

4.

As the result of investigations, the Building Research Institute has proposed measures for repair and reconstruction works of buildings in Niigata City. The following technical criteria were adopted : Based on the subsoil conditions and the distribution of damage to structures, Niigata City should be classified into three zones: A (firm and good), B (inter- mediate) and C (soft and bad). Foundations of all buildings in zone C and of important buildings in zone B should be designed and reconstructed in accordance with the recommendations. The recommendations include restrictions upon the use of footing and raft foundations on loose sand layers (exception where the soil is improved), and also requirements for extending piles to the dense sand layers. Methods of repair works were suggested which include necessary precautions, and involve the requirement that the safety of the restored buildings be confirmed by appropriate methods.

ACKNOWLEDGEMENT

Appreciation is expressed to the local authority, the Building Constructors’ Society and many private firms, all of whom kindly co-operated with the investigation of the Building Research Institute. The writer wishes to express his appreciation to members of the team for

the compilation of the results of analyses and also to Mr. B. H. Falconer, Unesco expert to IISEE, for his kind comments.

32

Discussion Damage to reinforced-concrete buildings in Niigata City

S. V. Medvedev T. Hisada

J. Petrovski

T. Hisada

N. Radojkovic

T. Hisada

M. Janic

T. Hisada

Discussion

N. N. Ambraseys

T. Hisada

N. N. Ambraseys

T. Hisada

In what way has the Niigata earthquake affected your building code?

The Japanese seismic code will be revised by taking into consideration the damage to buildings in Niigata City.

Was the spectrum obtained from corrected records (for relative movement between buildings and ground, tilting, etc.)?

The spectrum was made from the record obtained on the basement floor of a four-storey reinforced-concrete building. It is not clear whether there was relative movement between building and ground, or not.

What is the cause of such severe damage to buildings by earthquakes in Japan?

In past earthquakes, the main cause of severe damage to buildings has generally been structural distortions. In the case of the Niigata earthquake, the causes of damage are ground fissures, subsidence and horizontal displacements.

Will the Japanese seismic code be revised? The Japanese code will be revised in the future, taking into consideration the results of studies of the Niigata earthquake.

Comment se sont comportées les constructions en béton pré- contraint à Niigata?

There was no building of prestressed concrete construction in the Niigata area.

Vous avez montré une photo d'un pont détruit. Pouvez-vous en expliquer les causes?

I a m not in a position to answer your question, but I think the main cause might be the liquefaction of loose sand layers supporting the steel pile piers.

What was the level of the ground-water table in Niigata? Was any study made of influence of the water table on damage to buildings?

The water table is about 50 c m to 100'cm below the ground surface. Studies of this latter question are under way.

S. V. Medvedev

T. Hisada

33

S. V. Medvedev Institute of Earth Physics, Moscow

Measurement of ground motion and structural vibrations caused by earthquakes

Accurate quantitative information on ground motion caused by destructive earthquakes is essential for the improvement and development of calculation methods and building and construction design in seismic areas. However, seismological equipment for measuring strong ground motion and structural vibrations is used in only very few such areas, and the observational programme must be extended considerably. For this purpose Unesco organized a working group on the measurement of

strong ground motion; it was composed of Professors D. Hudson (United States), S. Medvedev (U.S.S.R.), and H. Umemura (Japan). It met in January 1964 to collect and examine basic specifications of instruments used in various countries to record strong motion. It was decided that existing instruments had much in common and that, therefore, there was no need to choose an instrument or instruments for standard use. A list of specifications was drawn up to guide development of work on the measurement of ground motion. Such specifications are necessary both for the operation of existing instruments and for the production of new models. Strong-motion accelerographs should satisfy the following basic requirements :

1. The desirable natural period of vibration of the accelerograph element is about 0.05 sec, but periods of not more than 0.10 sec are acceptable.

2. The instrument should be able to record maximum acceleration of 1.0 g (i.e., equal to gravitational acceleration). The sensitivity should be such that earthquakes causing only very slight damage can be measured.

3. The damping of the accelerograph element should be not less than 60 per cent of critical.

4. The recording speed should be not less than 10 mm per sec. 5. The instrument should be triggered off by the earthquake itself. 6. The recording time should be at least 30 sec. The instrument should be capable

of making at least two recordings without re-setting. 7. The instrument should not rely on mains electricity. 8. The instrument should be easy to install and operate.

35


Strong-motion seismoscopes should satisfy the following basic requirements : 1. The seismoscope should be capable of measuring the maximum relative

horizontal displacement between the instrument mass and the ground without keeping a time log.

2. It should be a functional model of a real structure, and its natural period and damping should be made similar to the period and damping of prevalent . structures in the region in question. (Seismoscopes in use have natural periods ranging from 0.25 sec to 0.75 sec, and usual damping values are from 7 to 10 per cent of critical).

3. It is desirable to install seismoscopes in the vicinity of a recording accelero-

4. It is desirable to locate the seismoscopes in such a way as to be able to obtain information on the influence of geological conditions on the intensity of ground motion.

5. The seismoscope reading should be correlated approximately with the seismic intensity scale.

Apart from accelerographs and seismoscopes for the recording of ground motion and structural vibration caused by destructive earthquakes, another range of instruments should be used. Time measurements should be taken of displacement, velocity and distortion. A distinct possibility for the future is the measurement of ground-motion velocity. This factor is the most useful of all in making allow- ances for plastic distortion in building design. Training aids should be produced for those concerned with the installation

and operation of instruments for recording destructive earthquakes. The sole basis for the development of earthquake engineering is observation

by means of instruments. Seismological equipment capable of providing an exhaustive record of strong ground motion represents the main source of information about earthquakes.

in the U.S.S.R.: UAR accelerograph, UAR velocimeter, S B M and AIS.2 seis-

in the United States : STD and M K . 1 1 accelerographs, Wilmot seismoscope. in Japan: SMAC.B, DTS.3 and AR.240 accelerographs. W e may take, for example, a UAR type of accelerograph. This is an instrument

used for the automatic recording of severe earthquakes, designed at the Institute of Earth Physics, Moscow. It comprises three main units: a group of seismographs, the recording mechanism and a triggering seismoscope. All three units are mounted on the same plate, which should be strengthened at the base. As soon as vibration begins, the recording mechanism is automatically set

in motion by the triggering seismoscope. Recording ceases after 30 sec, though the mechanism may be set for a different period. A film, 6 cm wide, wound on to an aluminium drum, is used for recording.

A strong earthquake causes the contacts of the seismoscope to close, and they operate the relay of the instrument’s electronic system. The relay switches on the galvanometer lamp and unclamps the instrument, releasing the drum; 0.5 sec

graph.

At present the following basic instruments are used:

moscopes.

36

S. V. Medvedev Measurement of ground motion and structural vibrations caused by earthquakes

after it is switched on, the drum is revolving at a uniform rate. After 30 sec, the relay stops the mechanism. The UAR instrument has three component parts. It may be used as an accelero-

graph, a velocimeter or a seismograph. When a particular kinematographic element of motion is to be recorded, the appropriate unit of seismographs in the instrument is assembled. The principal characteristics of the accelerometer are : natural period of vibra-

tion, 0.045 sec; constant damping, 0.7; equivalent length, 1.9 cm; sensitivity, 62 gal per mm. Accelerations of seismic vibrations of period 0.1 sec or more are recorded with the same magnification. A three-component velocimeter may be mounted in the UAR instead of the

accelerometer unit, since their dimensions are the same. The technical characteristics of the velocimeter are : natural period of vibration of the pendulum, 0.4 sec; damping, 7.5; equivalent length, 1.9 cm; sensitivity, 0.6 cm per sec per mm. Seismic vibrations with periods of between 0.05 and 3 sec are recorded by the

velocimeter with the same coefficient of magnification. The construction of the seismometer is similar to that of the accelerometer,

but the pendulum is shaped so that its natural period of vibration is relatively long. UAR instruments are powered by two electric dry batteries (100 volts and

6 volts). Earthquakes are rare in any one place-which, of course, is fortunate for

the inhabitants, but at the same time it makes it difficult to collect data. A very wide network of stations should be established in order to obtain the necessary information. It is particularly important that new instrumental data should be collected,

because electronic computers are being increasingly used every year in the planning of structures and buildings with a view to their resisting seismic forces. Readings of strong ground motion provide the physical basis for formulating

codes and regulations to be observed in designing buildings in seismic regions. W e can find the necessary methods of strengthening buildings only if we have adequate quantitative information concerning the vibration of a building’s foundation. Another important point is the measurement of vibrations of the buildings

themselves when earthquakes occur. Objective data, which are of great importance in designing a building which will be subject to dynamic action, can only be obtained by measuring the vibrations of existing buildings.

Discussion

A. Beles Qu’est-ce que l’on considère comme secousse, comme grandeur mécanique, qui puisse être enregistrée? Le séisme produit un mouvement d’un point de la terre; ce mouvement est continu, il peut changer de direction et de vitesse, celle-ci de même

37


S. V. Medvedev

peut changer de direction et de valeur, etc. Quand interviennent la secousse et le choc?

Les accélérographes pour les tremblements de terre intenses entrent en fonctionnement après le passage de la première onde longitudinale, avec un décalage d’un dixième de seco-nde au maximum. Auparavant, l’appareil est immobile.

38

A. A. Moinfar Technical Bureau, Plan Organization, Teheran

Report on the work undertaken in Iran on the problems of earthquake-resistance regulations for the Iranian building code

Although Iran is a country located in the seismic zone, unfortunately up to now we have had no building code with provisions for the resistance of buildings to earthquakes. Therefore the resistance of our existing buildings is only now being investigated by designers and engineers. Most of our buildings are not constructed by qualified engineers. They are,

therefore, not adequately constructed to resist earthquake shocks, and are dangerous in time of earthquake. The reason for the lack of the earthquake-resistance regulations in Iran is

perhaps due to the fact that, from the time earthquake-resistant building methods came into use up to recent years, we have not experienced in our country any major destructive earthquake. The recent recrudescence of seismic activity in Iran made the people realize

that the country is situated in the seismic zone, and the authorities are now seriously worried about what the future may have in store. After the severe earthquake of 1 September 1962 in the Ghazvin region, which

caused extensive damage over a large area by demolishing more than 300 villages with some 12,000 fatalities, the Plan Organization of Iran, which is responsible for the development projects of the country, became interested in studies related to earthquake engineering.. The Technical Bureau of the organization formed a committee for the construction of buildings resistant to earthquake shock. The draft tentative regulations, proposed by the committee, are now prepared

and this draft is under study in the Ministry of Development and Housing with the prospect of becoming law in the near future. On this occasion, I would like to discuss in brief these tentative regulations. The materials which are used for construction of village houses in Iran are

mainly sun-dried bricks and adobe, with wooden planks or sun-dried brick arches for the roof. Brick buildings in the villages are very rare but this type of building with weak

mortar is common in the cities; neither the typical rural houses nor the brick buildings in the cities are adequate against severe or moderate earthquake shock.

39


Therefore, in the proposed regulations, the construction of adobe and sun-dried brick buildings is forbidden. As for brick buildings, some restrictions are suggested, and there are some limitations to the height of masonry buildings, the minimum wall ratio, the percentage of openings, the necessity of reinforced-concrete tie- beams, etc. With regard to typical Iranian jack arches with steel I-beams, some reinforce-

ments have been specified to increase the rigidity of the slab. It has been indicated in these regulations that all buildings which are more

than three storeys high, or higher than 11 m, must be built with reinforced concrete or a steel skeleton. These buildings shall be designed and constructed to resist earthquake forces. In order to establish the earthquake-resistance regulations for Iran, some

modern and up-to-date codes from different countries have been taken into consideration, with some changes to suit our country’s requirements. I hope that the necessary formalities for these regulations will be completed

in the near future and they will be put into force to save human lives and property.

ACKNOWLEDGEMENT

I would like to take this opportunity to express my appreciation for the valuable assistance given by foreign scientists and Unesco experts, especially those named below, in the progress of earthquake engineering in Iran, and also for their guidance in the compilation of earthquake-resistance regulations : T. Naito, Waseda University, Tokyo (Japan); T. Hagiwara, Tokyo University, Tokyo (Japan); N. N. Ambraseys, Imperial College of Science, University of London (United Kingdom); R. W. Clough, University of California, Berkeley (United States); J. Despeyroux, Bureau Securitas, Paris (France); S. Omote, Tokyo University, Tokyo (Japan); T. Kobori, Kyoto University, Kyoto (Japan).

40

J. Ferry Borges Head, Building and Bridges Department, Laboratório Nacional de Engenharia Civil, Lisboa

How to design structures to resist earthquakes

INTRODUCTION

The assessment of the safety of structures under the action of earthquakes involves the following main problems: definition of the seismic actions; study of the dynamic behaviour of the structures under the seismic actions; analysis of safety based on the behaviour of the structure. If these problems are conveniently solved, the judgement of a design or the

choice of the most convenient design are possible. The present paper discusses basic concepts and fundamental hypotheses to be

used directly in the study of a given structure or in the establishment of general design rules applicable to structures of a given type. The problems concerning reinforced-concrete buildings are given particular attention.

SEISMIC ACTIONS

The need for a convenient scientific description of earthquake actions is obvious. The simple observation of an accelerogram shows that soil vibration cannot be represented by a sinusoidkl movement. Although the convenient superposition of sinusoidal vibrations allows the reproduction of any type of accelerogram, this method involves the definition of too many constants to be truly practical. Housner was the first to call attention to the random character of earthquake

vibrations [5].l The modern development of the theory of random vibrations renders this hypothesis very fruitful. Being particularly interested in the behaviour of linear oscillators, Housner

defined earthquake actions through velocity spectra. Although convenient for the particular purpose, this definition has to be extended if the behaviour of structures of other types is to be analysed. According to the theory of random

1. The figures in brackets refer to the bibliography at the end of this article.

41


vibrations, the acceleration power spectral density is the convenient magnitude to be considered. Recent studies have shown that earthquakes can be represented by a random

vibration of constant spectral density in the range O to 5 Hz, and of zero density beyond this range. The record of the N.-S. component of the El Centro 1940 earthquake is often

taken as reference. The corresponding spectral density is 690 cm2 sec4 Hz-l. The standard time duration is taken as 30 sec [7]. Taking into account the type of the earthquake, the distance to the epicentre

and the soil conditions, it is to be expected that, instead of remaining constant for frequencies between O and 5 Hz, the spectral density changes as a function of the indicated parameters. Some preliminary results show that the spectral density can be considered to change as a function of the frequency, according to a law analogous to that of the simple oscillator transfer function. Future information based on further earthquake records may justify a more

accurate representation of earthquake actions than the one assumed above. The definition of the variation of the spectral density for frequencies above 5 Hz is particularly important for studying the behaviour of rigid structures. Even so, the assumed representation may be considered as a sound scientific basis for structural engineering studies. A quantification of the structural safety against earthquake actions requires

us not only to define the type of vibrations to be considered but also to ascribe a probability to the occurrence of an earthquake with given characteristics. In this case the information available is scarce also. In most regions seismicity has to be judged qualitatively and even for the regions about which more information is available, such as California, the probability of occurrence of earthquakes cannot be accurately defined. In zones with high seismicity, intensities such as that recorded in El Centro

seem to be a convenient hypothesis for design purposes.

STUDY OF THE DYNAMIC BEHAVIOUR OF STRUCTURES

The dynamic behaviour of structures depends on the intensity of the acting loads. For small-amplitude vibrations structures behave linearly, but as the amplitude increases behaviour of the non-linear type becomes more and more important. In general, non-linear behaviour cannot be disregarded if rupture is to be studied. In some types of structures, under increasing dynamic loads, cracks or local

ruptures can occur that completely change the behaviour of the structure. In such a case a study of final rupture requires that these changes in the structural behaviour be duly considered. The dynamic behaviour of structures can be studied both by analytical and by

experimental methods [2]. The matrix formulation of dynamic problems is very general and powerful.

The need to consider the systems defined by a finite number of degrees of freedom

42

J. Ferry Borges How to design structures to resist earthquakes

does not limit its application as, for practical purposes, continuous systems can always be replaced by lumped mass systems. The fact that the practical solution of the problem involves operations on

matrices, sometimes of a very high order, is no longer a difficulty thanks to modern electronic computers. The analytical formulation of the dynamic problems requires a knowledge of

matrices of three types: mass, stiffness and damping. The first is in general well known. The second can be determined by analytical or experimental means. The information available on the third matrix is in general insufficient, and simplifying assumptions and values based on past experience have to be used. In the usual types of structures, the analytical determination of the stiffness

matrix does not involve particular difficulties. In cases of structures whose behaviour is imperfectly known or when particular influences such as foundation conditions may be of interest, the stiffness matrix can be obtained by inversion of a deformability matrix experimentally determined by the testing of models or prototypes. For the time being, the analytical study of dynamic behaviour beyond the

linear range is only possible for very simple structures, such as a one-degree-of- freedom oscillator, and also, by considering simple force-displacement diagrams, bi-linear or elasto-plastic, linear or hysteretic structures. Experimental methods based on model tests can be of great help in studying

the dynamic behaviour of structures. The testing technique adopted at the Lisbon Civil Engineering Research Institute

is particularly useful. Models are subjected to random vibrations similar to those of earthquakes and

the parameters of interest, such as displacements and strains, are directly recorded. Even the duration of the earthquake is reduced to scale, several tests being performed for a given level of acceleration. It is thus possible to determine the maximum values of the parameters in each test and to compute the mean of these maximum values in several tests. By increasing the level of the spectral density of acceleration, non-linear pheno-

mena can also be detected and studied. A diagram of the testing set-up is presented in Figure 1. Both sinusoidal and

random vibrations can be induced in the model by an electromagnetic vibrator and, in the latter case, it is possible to adjust the convenient values of the acceleration spectral density for the different frequencies, by acting on a spectrum equalizer. The quantities of interest such as accelerations, displacements and strains can be directly recorded on paper or on magnetic tape. The maximum values can be directly determined from the paper records. Tape records are used in an electronic spectrum analyser to determine spectral density diagrams.

BASIC RESULTS CONCERNING THE DYNAMIC BEHAVIOUR OF SIMPLE STRUCTURES

It is particularly important to analyse the behaviour of one-degree-of-freedom oscillators under the'action of seismic loadings.

43


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Considering a linear oscillator with damping 0.05 of critical, acted upon by the random vibration defined above. The mean maximum values of the displacements are related to the natural frequency by curve 1 of Figure 2. For an elasto- plastic oscillator with a yielding factor qv = 0.10 and the fraction of critical damping indicated above, this relation is represented by curve 2. These results obtained by J. Pereira [6] confirm the well-known conclusion that maximum displacements in elastic and elasto-plastic oscillators with the same natural frequency are not much different. In fact, Figure 2 also indicates the mean values of the maximum displacements

undergone by oscillators with fractions of critical damping 0.03 and 0.10 and yielding factors 0.03, 0.06, 0.12 and 00, as computed by Berg and Thomaides for the El Centro, N.-S., 1940 earthquake [il. Also represented in the same figure are the maximum displacements computed by the Japanese committee SERAC [8] for a linear oscillator with damping 0.05 of critical, for the same component of the El Centro earthquake. The theoretical and experimental results can be roughly represented by the

simple expression 6 8=- f

where 8, expressed in centimetres, is the mean of the maximum relative displacements of a simple oscillator under the earthquake considered andf, expressed in Hz, is the natural frequency of the oscillator. The fact that the maximum displacements of linear and elasto-plastic oscillators

are approximately equal (in the practical range of the parameters of interest) is of paramount importance for the design of earthquake-resistant structures [3]. This result derived for one-degree-of-freedom systems can be extended and the

following two main conclusions drawn: 1. The magnitude of the displacements of a structure under an earthquake does

not depend on its strength, but only on the stiffness of the structure. 2. To analyse the safety of a structure it is necessary to verify whether it can

withstand the displacements due to earthquakes without collapsing. The above conclusions concern structures with an elastic or an elasto-plastic behaviour.

DESIGN OF SIMPLE REINFORCED-CONCRETE STRUCTURES

Let us consider the usual case of buildings with reinforced-concrete structure and masonry walls. According to the experience gained in several earthquakes, particularly that of Agadir in 1960 [4], walls collapse at one of the lowest floors (where the acting forces are most intense) and the strength has to be guaranteed by the reinforced-concrete structure alone. At this stage the structure can be represented by a simple oscillator. The relative displacements at the floor in question are then given by expression (I)

45


4

2 I 1.5 L

i A-

\ 1 \ \ \ \ \ \ \ \

O 1 2 3 4

6 f

- 6 = - @ Elastic behaviour @ Elasto-plastic behaviour, qy = 0.1

Berg and Thomaides

0 SERAC Fig. 2

46


and to guarantee the safety of the structure it is necessary that it be able to withstand these displacements. Comparing the allowable deformability of the columns with the displacements

due to the earthquakes, a simple condition is obtained that in the usual cases simply limits the mean compressive stress due to permanent loads to 0.19-0.22 of the ultimate compressive stress (determined by testing cubes) of the concrete used. This condition has to be complemented by a second one that limitSAhe ratio

of plastic to elastic deformation and so requires a minimum strength of the structure. Using some simpiifying hypotheses, this condition can be expressed by the value

of the seismic coefficient for ultimate design c, given by the expression

where fis the natural frequency of the structure. In the usual cases of frequencies between 2 and 5 Hz, this amounts to using seismic coefficients between 0.14 and 0.35. Finally it is necessary to set a limit to the flexibility of the structure in order

to limit the relative displacements between floors. This condition can be expressed in terms of minimum depth of the columns. The above criterion of design of reinforced-concrete structures was presented

in a recent paper [3]. This is an instance of the way modern concepts of earthquake engineering can be expressed by simple design rules.

c = 0.07f (2)

CONCLUSIONS

The main conclusions that can be derived are the following : 1. The actual development of earthquake engineering makes it possible to study,

on a scientific basis, the behaviour of structures during an earthquake. 2. By means of model tests or analytical studies, it is possible conveniently to

analyse the safety of important structures such as dams, bridges and buildings. 3. By means of simple design rules, it is possible to guarantee the economy and

safety of structures of common type such as buildings.

BIBLIOGRAPHY

BERG, G. V.; THOMAIDES. Energy absorption by structures and in earthquakes. Paper presented at the second World conference on Earthquake Engineering, Tokyo, 1960.

BORGES, J. Ferry. Dynamic structural studies on models. Paper presented at the seventh Congress of the International Association for Bridge and Structural Engineering, Rio de Janeiro, August 1964.

-. Seismic design criteria for reinforced concrete buildings. Paper to be presented at the third World Conference on Earthquake Engineering, New Zealand, 1965.

-; COSTA, J. M. Madeira. O estudo do comportamento das construçóes quando do sismo de Agadir. Reunião sobre Construção Anti-sísmica, Laboratório Nacional

de Engenharia Civil, Lisboa, 1961.

47


HOUSNER, G. W. Characteristics of strong-motion earthquakes. Bulletin of the Seis- mological Society of America, vol. 37, 1947.

PEREIRA, J. Jervis. Behaviour of an elasto-plastic oscillator acted by random vibration. Paper to be presented at the third World Conference on Earthquake Engineering, N e w Zealand, 1965.

RAVARA, A. Spectral analysis of seismic actions. Paper to be presented at the third World Conference on Earthquake Engineering, N e w Zealand, 1965.

SERAC. Non-linear response analysis of tall buildings to strong earthquakes and its application to dynamic design. Tokyo, 1964. (Report no. 4.)

Discussion

A. Zatopek

I. Borges

Were your results, as they are presented here, obtained with respect to the frequency of vibration or for a frequency range?

It was considered that earthquake actions are represented by a constant spectral density of acceleration between O and 5Hz, with the value of 600 cm2 sec-l Hz-, and zero outside this range.

48

R. W. Clough Department of Civil Engineering, University of California, Berkeley (United States)

Earthquake engineering research at the University of California, Berkeley

INTRODUCTION

The ultimate objective of earthquake engineering is the reduction of loss of life and property due to earthquakes. To achieve this objective, it is necessary that knowledge concerning all aspects of the earthquake problem be extended. For the purposes of this discussion, it is convenient to divide earthquake engineering research into the following categories : 1. Study of the fund.amenta1 characteristics of earthquake ground motions.

This includes the recording of earthquake motions by means of strong-motion seismographs located in many different regions, the use of seismoscopes and similar devices to evaluate local variations of intensity due to ground conditions and computer analysis of the records to establish the essential properties of the seismograms.

2. Study of the significant earthquake-resistant properties of engineering structures. Structural performance during earthquakes depends upon the mass and stiffness distribution within the structure, the strength and ductility of the materials and the energy absorption capability due both to internal friction and to elastic deformation effects. In general, the foundation system forms an integral part of the structure, and the behaviour of the foundation structure and the soil must be considered in the investigation.

3. Study of structural, response to earthquake excitation. For elastic structures having well-defined mass, damping and stiffness properties, the analysis of the response to any specified ground motion may be carried out by well-known procedures. Many digital computer programmes are available that will perform this task automatically. However, it must be recognized that severe earthquakes may produce inelastic deformations, even in well-designed structures ; thus, techniques must be developed for computing the inelastic earthquake response of all types of structures and for assessing their ductility or yield amplitude requirements.

4. Development of-improved building codes and design techniques. Most modern

49


code requirements and design techniques are based upon a pseudo-static force concept. Forces are prescribed by seismic coefficients which generally provide adequate strength in traditional forms of building construction, but which may be grossly inadequate in proportioning the members and connexions of modern or unusual structures. More rational design concepts must be developed which will adequately recognize the energy absorption (as well as strength) requirements of the structure.

CURRENT RESEARCH PROGRAMMES AT BERKELEY

The preceding rather detailed summary of the basic research objectives in earthquake engineering has been presented in order to provide a framework into which may be fitted the research problems currently under investigation at Berkeley. The specific objectives, methods and (where available) results of each of these problems will be described in the sequence of the above description.

Characteristics of strong ground motions

This project has been active for over two years, and is now nearly complete. Its purpose is to evaluate the characteristics of a number of strong-motion records which have been obtained in the United States. Included among these are most of the important earthquake records obtained from various parts of the country, as well as a number of strong-motion seismograph records obtained during underground tests of nuclear explosives. The accelerogram for each case was converted to punched card form and the

properties of the motion evaluated by digital computer. Initially it was believed that the power-spectral-density of the motion might be its most significant measure but, after evaluation of many different properties, it was concluded that the elastic velocity response spectrum (plotted against frequency) gave the most meaningful indication of the motion. Velocity spectrum plots of this type have now been constructed for all the records under consideration, and the special characteristics of each plot are being evaluated. The most prominent relationship to be observed is the decrease of dominant frequency with increasing epicentral distance. A summary of this work is to be published in the proceedings of the third World Congress on Earthquake Engineering.

Dynamic properties of buildings

The most effective way of evaluating the actual dynamic properties of buildings and other structures is by field vibration studies. To facilitate such research, the State of California provided funds to the Earthquake Engineering Research Institute for the design and construction of four shaking machines, each capable of generating a 1,000 Ib force at 1 cps. The machines were designed to be used in pairs, each pair being operated from a single speed controller, either in phase

50

R. W. Clough Earthquake engineering research at the University of California

or out of phase. It is also possible to operate all four machines from a single controller. At present, one pair of these machines is being used by the California Institute

of Technology, and the other by the University of California at Berkeley. The Berkeley investigation is on the largest building yet studied in this way-a seventeen-storey steel-framed structure which is part of the University of California Medical Centre in San Francisco. The building is very simple in form, a rectangular plan with all of the columns located around the edges and clear spans within. Elevator shafts, stairways and services such as water, heat, etc., are contained in independent, free-standing towers, external to the main building. The test programme involves placing the shaking machines in the building at

various stages of construction, and exciting the principal modes of vibration by operating at the appropriate frequencies. Measurements of vibration amplitude are made at several points in the structure so that the mode shapes and frequencies, damping coefficients and possible non-linearities of structural response may be determined. Up to now, only the first set of vibration data has been obtained, but three additional test runs are planned. It should be noted that similar tests performed by the California Institute of Technology have yielded very valuable information on smaller concrete-frame and steel-frame buildings in Los Angeles. It is expected that the results of the present test will provide the first such data on a tall steel-frame structure.

Yield behaviour of steel frames and joints under earthquake conditions

Since field tests of actual structures must be limited to stress levels within the elastic range, it is necessary to evaluate the ultimate strength and energy absorption capacity of buildings by testing component parts in the laboratory. A programme of such research is now being formulated at Berkeley, in which typical joints and members of multistorey steel-frame construction will be tested in a specially designed low-cycle fatigue machine. Using amplitude of deformation and number of cycles of loading as the principal parameters, the energy absorption capacity of each component will be evaluated.

Non-linear earthquake response of multistorey buildings

The preceding projects are aimed at defining the ground motions and structural properties which may be involved in the earthquake response problem. Another project, which has been active for about two years, has as its objective the evaluation of the expected earthquake response of multistorey buildings, taking account of bi-linear hysteretic behaviour in the members. The first phase of this project was the development of a digital computer programme capable of evaluating the response to arbitrary dynamic loadings of buildings up to thirty storeys in height. Arbitrary bi-linear moment curvature properties may be assigned to each member, and the programme determines the maximum inelastic deformation developed in each member during the course of the earthquake.

51


The computer programme was completed about a year ago. To date it has been used in a series of preliminary studies which have demonstrated its power and value. These results are to be published in the proceedings of the third World Congress on Earthquake Engineering. A comprehensive sequence of computations is now being planned in order to establish the significant parameters of inelastic response behaviour and to evaluate ductile yield requirements in typical buildings. It may be noted that the preliminary studies have demonstrated that buildings can be designed so that the columns remain elastic while the girders deform plas- tically to absorb the earthquake energy (thus minimizing danger of collapse). Also, it has been found that maximum yield amplitudes of twelve to sixteen times the elastic limit deformation may be developed in normal structural designs, with the probability that much greater yield amplitudes may be indicated in special configurations.

Earthquake effects on earth dams

In connexion with its large water resources development plan, the State of Cali- fornia is sponsoring a number of special research programmes. The University of California at Berkeley has been asked to undertake one part of the state’s earthquake research programme-investigations on the earthquake resistance of earth dams. This study has been divided into three separate, interrelated phases : (a) field measurements to determine actual elasticity and damping properties of earth dams ; (b) laboratory measurements to evaluate the stress-strain properties of earth dam materials, including non-linear “yield” characteristics ; (c) analytical research to predict the linear and non-linear response of earth dams to specified ground motion. In the field investigation, the same vibration generators mentioned previously

are being used to produce harmonic oscillations of selected earth dams. By mounting the vibrators at the crest of the dam, operating at various frequencies, and by measuring the vibratory motion at a number of locations, it is possible to establish the natural frequencies and mode shapes, and to estimate the internal damping of the material. Measurements have been made on two dams, to date. In the laboratory investigation, undisturbed samples of earth dam materials

are being subjected to cyclical strain amplitudes, using a specially constructed dynamic tri-axial test apparatus. With this device it is possible to vary the amplitude and frequency of the imposed strain, and to obtain corresponding stress-strain graphs. The shape of the hysteresis loops which are recorded gives information on the linearity and internal damping capacity of the material. The analytical studies are being performed with the digital computer, using the

basic assumption that the dam may be treated as a plane strain problem. A standard programme for the analysis of arbitrary plane strain systems is used to evaluate the stiffness matrix of the cross-section (the physical properties of the soil may be assumed to vary arbitrarily over the section). Then, using these stiff- nesses, and any specified mass distribution, the vibration mode shapes and frequencies of the section are computed. Finally, standard mode-superposition procedures

52

R. W. Clough Earthquake engineering research at the University of California

are employed to determine the response to any specified ground-motion excitation. The resulting stress histories at any number of specified points in the cross-section are plotted automatically by Fe computer. In the initial studies it has been assumed that the dam rests on a rigid base;

the programme is now being modified, however, to account for an elastic foundation of any specified stiffness. The final development with this programme will be the introduction of non-linear stress-strain properties for the soil. These non- linear analyses will be performed by assuming linear action during short time intervals, but changing the properties from one interval to the next as indicated by the state of deformation which exists.

Earthquake effects on bridges supported by long piles

The last investigation to be mentioned concerns the earthquake behaviour of bridges supported by piles extending through a deep layer of mud or soft clay. In such a situation, it is evident that the clay layer as well as the bridge will respond to the earthquake excitation. This problem also is being investigated analytically by means of the digital computer, a lumped mass mathematical model being used to represent the clay layer and the bridge structure penetrating through it. Non- linear stress-strain properties have been assumed for the clay, but the structure is assumed to remain elastic. Preliminary studies have evaluated the dynamic response of the clay layer

alone (without the piles); they have shown that if the material exhibits significant hysteretic energy loss, the clay layer will serve to filter out high-frequency excita- tions and to reduce effectively the response amplitudes at all frequencies. Linearly elastic clay properties, on the other hand, cause significant amplification of the low-frequency ground-motion components. Studies on the behaviour of the system with piles penetrating through the clay

layer are not yet complete. Preliminary results indicate that the most effective design is provided by slender flexible piles which can move easily with the clay layer. Stiff piles cannot resist the clay motion, nor can they accommodate the required deformations without overstress.

CONCLUSION

Two remarks should be made in concluding this survey, in order to put it in the proper perspective : 1. Only research work being conducted at the University of California, Berkeley,

has been mentioned. It should be noted that a great amount of research on earthquake engineering problems is also being done at other laboratories in the United States, notably at the California Institute of Technology.

2. The research programmes currently under way are not directed specifically toward developing improved design techniques and code requirements. How- ever, this is the ultimate objective of the work, and when sufficient knowledge

53


has been gathered it is hoped that definite suggestions for improvements, both in design and in codes, can be made.

Discussion

F. Borges

R. W. Clough

N. N. Ambraseys

R. W. Clough

N. A. Meshcherikov

R. W. Clough

As there is a correspondence between velocity spectra and acceleration-spectral density diagrams and as the latter are conveniently dealt with by random vibration theory, I would ask why Professor Clough thinks the former are more repre sentative.

Our analyses showed great similarities between the power- spectral-density of the accelerogram and its velocity-response spectrum. However, the velocity spectrum seemed to be more sensitive in distinguishing between different earthquake records, and thus seemed to give a more characteristic measurement of the motion.

D o spectra from records obtained near the epicentre of an earthquake show the same characteristics as those obtained at some distance (i.e., wave movements and block or mass movements)? .

Epicentral distance definitely affects the predominate period of the ground motion; response spectra show a shift toward lower frequencies with increasing epicentral distances. Presu- mably this tendency would be accentuated in accelerograms recorded in the region of a block movement.

Is it possible to use the results of geodetic measurements for the study of ground motions connected with earthquakes?

The relationship between distortions of the earth’s crust, as indicated by geodetic measurements, and earthquake activity is being studied by the United States Coast and Geodetic Survey along the San Andreas fault in California. Deñnite measurable displacements have been observed as a result of small earthquakes which have been recorded along the fault. To date, it has not been possible to relate the ground-motion intensities to the measured ground displacements.

54

Muzaffer Ipek Technical University, Istanbul

Research activities in the Seismological Institute ’

of the Technical University of Istanbul

The Seismological Institute of the Technical University of Istanbul was established in 1952 for the purpose of studying the seismic character of Turkey, developing earthquake-resistant construction and teaching on related subjects. Year by year the institute is acquiring more qualified personnel and better equipment and, as a result, performing its duties more satisfactorily. Recent activities in the institute can be summarized mainly under three headings:

seismological activities, establishment of an earthquake engineering laboratory and studies on earthquake engineering.

SEISMOLOGICAL ACTIVITIES

In parallel with routine seismological observations and bulletin publication, since the establishment of the institute, data on past and recent earthquakes have been accumulated. Unfortunately data on historical earthquakes are inadequate and unreliable. A seismicity map which takes into account all available data will not show the true earthquake danger in the country. For this reason, our study of the seismicity of Turkey is based on the more accurate data acquired during the last hundred years. Strong earthquakes are not rare in Turkey, and from time to time such shocks

affect different parts of the country and cause minor damage. The effects of these earthquakes are studied through questionnaire cards sent to the earthquake area. The percentage of answers is about 40-50, and this method helps in the construction of isoseismal maps. At present seventeen of these maps with maximum epicentral intensities ranging from VI to IX are available in the institute. These maps will be helpful in future seismicity studies. A list of notable earthquakes, studied through questionnaire cards, is shown

in the table below.

55


TABLE 1. List of some notable earthquakes

Earthquake

Hasankale Misis Gönen-Yenice Karabunin Edirne Eskisehir Fethiye Abant Köycegiz Rodos-Marmaris Igdir Balikesir Donizli Lapseki- Ayvaeik Gema Doh-Marmara Tefenni

Date

3 January 1952 22 October 1952 18 March 1953 2 May 1953 18 June 1953 20 February 1956 24-25 April 1957 26 June 1957 25 April 1959 23 May 1961 4 September 1962 14 September 1962 1 March 1963

29 March 1963 14 June 1963 18 September 1963 22 November 1963

- Epicentre Maximum intensity

400 N, 410 6’ E 37O 4’ N, 35O 47‘ E 39O 55‘ N, 270 18’ E 380 6’ N, 260 3’ E 410 40‘ N, 26O 34’ E 390 49’ N, 300 21’ E 360 47‘ N, 280 56‘ E 40° 60’ N, 31° 20’ E 360 57’ N, 280 41‘ E 360 7’ N, 28O 5‘ E 390 5’ N, 430 55’ E 39O 6’ N, 280 3’ E 370 84‘ N, 29O E 400 N, 260 6’ E 400 1‘ N, 290 2‘ E 400 75‘ N, 290 E 370 20’ N, 290 49’ E

VI VI IX-x VI1 VI VI11 VI11 IX VI VI1 VI1 VI1 VI1 VI VI VI11 VI1

ESTABLISHMENT OF AN EARTHQUAKE ENGINEERING LABORATORY

Until recently, the studies in the institute were concentrated on seismology rather than earthquake engineering. In 1963 a project was undertaken to establish an earthquake engineering laboratory in the institute. As a first step this laboratory acquired a shaking table, measuring instruments and other indispensable equipment. With the limited funds of the university, it was necessary to design and construct the shaking table in Turkey with domestic materials and workmanship. Only a hydraulic oil drive imported from Germany will be used to change the frequency of the table continuously. The amplitude of the table is also variable from O to 20 m. The vibration of the table will be in a horizontal direction. Measuring instruments and other equipment were imported from Japan. The

principal instruments are a set of dynamic strain amplifiers (Shinkoh, type DSó/RX, six channels) and a set of pen-writing oscillographs (Watanabe, type EO-6, six channels). For vibration tests in actual structures, a hand-operated vibration generator

was manufactured.

STUDIES ON EARTHQUAKE ENGINEERING

At present, the members of the institute who work in the field of earthquake engineering are few: Miss Silva Aynacyan, an electrical engineer and the author.

56

Muzaffer Ipek Research activities in the Seismological Institute of the Technical University of Istanbul

Miss S. Aynacyan is working on her doctoral thesis, under the guidance of Professor Kazim Ergin, on the subject of “Vibrational Characteristics of Surface Layers with Non-uniform Thickness”. She began this research on model studies during her stay in the Earthquake Research Institute, University of Tokyo. Mr. Ipek is working on “The Effect of Masonry Filler Walls; on Vibration

Characteristics of Framed Structures”, his doctoral thesis under the guidance of Professor Rifat Yarar, and on “Non-linear Vibrations of Building Structures”, a research project supported by an OECD fellowship. In 1963, the Technical University of Istanbul opened its Computation Centre

equipped with an IBM 1620 data-processing system, which introduced a new way of thinking to the university. Speciñc problems of seismology and earthquake engineering which require

lengthy numerical computation are no longer unthinkable for the seismologists and engineers of the seismological institute. W e hope that with the powerful aid of the electronic computer, we shall be able to attack more problems efficiently.

RECENT PUBLICATIONS OF THE INSTITUTE

IPEK, M. A graphical method to obtain phase plane curves of non-linear free vibration governed by the equation mx + f(x) = O. - . [A study on seismic code.] (In Turkish.) - . Observation and analyses on earthquake problems. - . On the effective width of plate in #at-slab structures subjected to lateral forces.

OMOTE, S. Seismology. (Turkish translation by S. Aynacyan.) - ; IPEK, M. [Seismicity of Turkey.] (In Turkish.) UMEMLJRE, H. Engineering seismology. (Turkish translation by M. Ipek.)

57

E. Lauietta Istituto Spenmentale Modeiii e Strutture, Bergamo

Récentes activités italiennes dans le domaine du génie séismique

1. Le problème posé par les constructions devant résister aux mouvements séis- miques a suscité ces dernières années un intérêt croissant auprès des chercheurs et des constructeurs italiens. Aux études et aux initiatives disparates (signalons les travaux de Danusso, Bertolini, Krall, Priolo) succèdent actuellement des recherches systématiques menées par des équipes dépendant d’instituts universitaires et de laboratoires privés. L’origine de ces recherches se trouve dans les problèmes soulevés par la réalisa-

tion de grandes constructions en terrain séismique (pylônes du détroit de Messine, barrage d’Ambiesta) et dans la nécessité désormais évidente de réformer à la lumière d’une mûre réflexion les anciennes normes actuellement en usage. Les contacts qu’ont eus les chercheurs italiens avec leurs collègues japonais et

californiens, leurs voyages d’étude en Amérique et au Japon, leur ont permis de se mettre au diapason des écoles les plus évoluées en cette matière. La création de l’Associazione Italiana di Ingegneria Sismica (AIDIS) a officielle-

ment marqué la reprise générale de cette activité.

2. Dans ce cadre, réconfortant dans l’ensemble, je ne puis malheureusement m’empêcher de constater combien le relevé des séismogrammes relatifs aux tremblements de terre italiens et enregistrés à proximité des épicentres, présente aujourd’hui encore de larges déficiences; toutefois on prévoit l’installation d’un réseau d’une cinquantaine de stations de mesures équipées de l’accélérographe, bien connu, à trois composantes, pour tremblement de terre intense, depuis long- temps utilisé en Californie; le Ministère des travaux publics, sous l’influence de I’AIDIS, a promis d’en assurer la réalisation. Quant au bilan des récentes recherches italiennes dans le domaine de la construc-

tion, il s’avère être véritablement positif. 3. Les tendances diverses des chercheurs et des écoles, plus que la différence d’outillage et de personnel, ont fait que dans les instituts universitaires des recherches essentiellement théoriques et analytiques se développent, alors qu’on insiste surtout sur les recherches expérimentales dans les laboratoires privés.

59

Activités récentes dans le domaine du génie paraséismique

Les recherches concernant la statique séismique proprement dite ont été nombreuses. Je rappelle, en particulier, les études expérimentales sur modèle dynamique pour les grands pylônes traversant le détroit de Messine (SAE - Lecco), les études théorico-expérimentales de Finzi (Milano), Berio (Cagliari), Castiglioni (Milano) sur le comportement de pendules simples dans le domaine élasto-plasto-visqueux, les études expérimentales de Pozzo (Cagliari) sur le coefficient d’amortissement du béton. Entrant davantage dans le détail des problèmes de statique séismique, je signale avant tout les recherches de l’Istituto di Scienza delle Costruzioni del Politecnico di Milano, sur le comportement des structures élastiques à plusieurs degrés de liberté et douées d’amortissement visqueux. On y a étudié les conditions de validité de la technique de l’analyse “modale”

confirmant que les types d’amortissement visqueux connus sous le nom de “mass damping” et “viscous damping” sont les seuls rigoureusement valables : l’intuition qui la voudrait valable au moins dans les limites qui intéressent les applications pratiques, même pour d’autres types d’amortissement, pour si petits qu’ils soient, se verra confirmée ou démentie par une abondante recherche numérique déjà préparée et en voie de proche exécution. Dans le même institut une étude est actuellement en cours, qui parait devoir

être particulièrement intéressante sur le mode de superposition des effets dus aux modes simples de vibration, étude basée sur le calcul des probabilités. Dans le secteur des études théoriques, je sais que sont également intéressés par

ce sujet l’ïstituto di Scienza delle Costruzioni del Politecnico di Torino et 1’Istituto di Costruzioni e Ponti dell’ Università di Napoli, ce dernier travaillant à l’aide d‘un calculateur analogique; mais je ne possède actuellement pas de plus amples détails sur leurs activités.

4. Enfin, dans le secteur des recherches expérimentales sur modèles, se distingue I’Istituto Sperimentale Modelli e Strutture (Ismes) di Bergamo, en relation avec l’Istituto di Costruzioni e Ponti del Politecnico di Torino; je le connais particulière- ment bien et, par conséquent, je m’étendrai davantage sur ses réalisations. Ses travaux ont commencé en 1956, à l’occasion du projet du barrage à coupole

de I’Ambiesta (Italie): on effectua des calculs dynamiques, on construisit et mit au point un outillage d’essai (voir 8 3); c’est ainsi que furent effectuées les pre- mières expériences sur modèles dynamiques. Au cours des années suivantes on fit l’étude sur modèle du comportement

dynamique des barrages suivants, à la demande de sociétés étrangères: Kurobe IV (Japon), Grancarevo (Yougoslavie), El Novillo (Mexique), Soledad (Mexique), Santarosa (Mexique), Rapel (Chili). Pour chacun des ouvrages en question on réalisa trois modèles étudiés jusqu’à

rupture, chacun d’eux ayant été soumis à oscillations verticales ou horizontales dans la direction parallèle ou orthogonale à la corde du barrage. Les modèles des différents barrages - étant donné la variété des dimensions

et des caractères mécaniques de la roche de fondation - ont généralement demandé la solution de problèmes bien particuliers, liés surtout à la mise au point de matériaux adéquats.

60

E. Lauletta Rhntes activités italiennes dani le domaine du génie séismique

En dehors des barrages, signalons également l’étude sur modèles d’un gratte- ciel en béton armé de 200 mètres de hauteur et actuellement en voie de réalisation à Montréal (Canada). L’outillage dont dispose actuellement 1’Ismes est constitué d’une grande table

de 5 x 4 mètres, actionnée par voie mécanique ou électromagnétique (force d’excitation de 10 t et champ de fréquence 5 - 30 Hz) et capable d’oscillations sinusoïdales aussi bien stationnaires qu’à nombre limité de cycles. Pour les essais in situ ou sur éléments singuliers, elle dispose de deux vibrodynes

de masse excentrique: une de 10 t et champ de fréquence 5 - 25 Hz et l‘autre de 0,4 t et champ de fréquence 5 - 75 Hz. Les modèles destinés à être posés sur la table vibrante sont fabriqués en résines

synthétiques pour les essais dans le domaine élastique ou en matériaux du genre béton à densité élevée, de faible module d’élasticité et de faible charge de rupture, particulièrement bien indiqués pour les essais à outrance. La technique suivie pour ces essais est généralement la suivante: en premier lieu,

étude du modèle dans le domaine élastique et emploi des critères de l’analyse “modale” pour effectuer le passage de la “réponse” du modèle aux oscillations sinusoïdales à la “réponse” de la structure réelle aux tremblements de terre. Ensuite, on provoque la rupture du modèle en augmentant l’amplitude des oscillations en correspondance avec les fréquences les plus significatives ; bien entendu, ce système ne permet pas de définir le coefficient de sécurité de l’ouvrage face au tremblement de terre, mais il définit plus simplement un certain coefficient d’effica- cité dynamique de l’ouvrage qui a un caractère comparatif; en outre, il permet d’obtenir des indications qualitatives pour un examen critique de la structure qui peut conduire à la correction ou i la confirmation du projet. L’institut met également au point une table vibrante de dimension moyenne

commandée électromagnétiquement, capable d’osciller sur programmation (valeur de pointe de la force produite 2,2 t sur une large gamme de fréquences) et repro- duisant au pied du modèle aussi bien des tremblements de terre réels que des tremblements résultant d’oscillations avec distributions accidentelles de vitesses ou d’accélérations. Nous étudions également la construction d’une table vibrante de très grandes

dimensions (force produite de 25 ou 50 t), mise en oscillation par programmation sous l’action de vérins hydrauliques Outre les travaux de caractère privé (l’étude de la cathédrale de San Francisco

est actuellement en cours), nous effectuons également des recherches de caractère général parmi lesquelles celles concernant le coefficient d’amortissement du béton. Au cours de l’année prochaine nous prévoyons l’étude du comportement

séismique des grands viaducs.

commande électronique.

Discussion F. Borges Je connais et j’appuie les essais faits à Bergamo et surtout l’essai

du gratte-ciel dont il a été question. Les essais sur modèle dans le domaine élastique sont intéressants pour déterminer

61


E. Lauletta

Serafimov

E. Lauletta

les modes de vibration et les coefficients d’amortissement. Mon doute provient de la difficulté de faire le transfert des coefficients d’amortissement du modèle au prototype, surtout si on utilise pour ces modèles des matériaux mérents de ceux du prototype.

Au sujet des coefficients d’amortissement employés dans l’étude sur le modèle du gratte-ciel de Montréal, le coefficient d’amortissement du modèle pour chaque mode i était connu(9i). Le coefficient d’amortissement réel a été supposé dans un

premier cas égal aux coefficients du modèle, et dans le deuxième cas ~i = const = 0,1, tel qu’on le suppose dans les calculs. Pour les deux cas, les réponses Rm sur le modèle ont été reportées au. réel tout d’abord par les échelles mécaniques ordinaires (déplacements, contraintes, etc.) et après par le rapport

Dans l’idée de spectrum il y a: soit le tremblement de terre, soit l’amortissement de la structure. Les déficiences de cette méthode sont évidentes (ce sont les déficiences de l’analyse de modèle, du fait de travailler seulement en domaine élas- tique, de l’imparfaite connaissance du coefficient d’amortissement du béton réel). Elles ont, cependant, les mêmes défi- ciences qu’ont toutes les méthodes de calcul qu’on peut employer. Le modèle a, néanmoins, l’avantage de ne pas avoir schématisé la structure.

Avez-vous fait des recherches sur les modèles de barrage du type allégé, plus particulièrement du type Noëzli?

Je connais un seul cas de structures que nous avons étudié sur modèles dans ces dernières années après que la construction eut supporté des tremblements de terre: c’est le barrage de 1’Ambiesta (Alpes orientales). Le barrage a bien tenu, sans se fissurer: la période fondamentale enregistrée sur l’arc du sommet au moyen d’un séismographe est en accord avec la période obtenue sur le modèle pour la même direction des oscillations.

62

T. Hisada Building Research Institute, M. Izumi Tokyo M. Hirosawa

Structural response of a tall building to earthquake shocks

EARTHQUAKE RESPONSE OF TALL BUILDINGS

In consequence of recent development of electronic computation techniques, research work on the analysis of structural response to strong earthquakes is being carried out in many institutes interested in earthquake problems. A research project now in progress in the Building Research Institute, Tokyo,

is the systematic analysis of the earthquake response of tall buildings in elastic and elasto-plastic ranges, using electronic computers. The method of analysis is summarized below. 1. In the analysis, a tall building is represented by a serial mass-spring system

of shear type having many degrees of freedom. 2. The linear and non-linear responses of a structure are calculated. In the latter,

an elasto-plastic system of bi-linear restoring force-displacement characteristics is assumed.

3. Earthquake motions as input data can be of any type. The acceleration records obtained by strong-motion seismographs are often used with modification of amplitudes.

4. For explanation, we use the following notation: mi = mass of ith floor; iKi = spring constant in elastic range at ith floor; zKi = spring constant in plastic range at ith floor; Ci = damping coefficient at ith floor; ,T = undamped natural period of sth mode; Qi = restoring force at ith floor; Qvi = yield shearing force at ith floor; di = relative displacement of ith floor to (i-1)th floor; dyi = yield relative displacement of ith floor to (i-1)th floor; pi = ductility factor at ith floor (pi = di max/dyi); Mi = overturning moment al the foot of ith floor;

63


n

(t) = ground acceleration. 2

S.C. = storey shear coefficient = Qi max/r;mi.g; n = number of storeys;

5. For given values of mi, lKi, Ci, in a linear system or values of mi, lKi, zKi, Ci, Q+ dyi in a non-linear system, the following values of earthquake response are usually calculated :

di for Mi max at each floor;

Mi for di niax at each floor; S.C. at each floor.

di max and pi;

Mi max;

6. In a linear system, the distribution of iKi for a given distribution of di max can be calculated.

7. A systematic analysis of twenty-storey buildings of steel and reinforced- concrete construction has been carried out and some of the results obtained have been published?

EARTHQUAKE RESPONSE OF A FIFTEEN-STOREY BUILDING

In this paper, the earthquake response of a fifteen-storey reinforced-concrete frame building with masonry shear walls will be presented. The values of mass and rigidities of the idealized model have been determined

from the construction of an actual building with some assumptions and modifications. These values are shown in Table 1. The computed natural periods of the buiIding are lT = 1.46 sec, zT = 0.650 sec,

ST = 0.417 sec, etc., and the damping coefficient Ci is determined by the assumption that the fraction of critical damping of the first mode is 5 per cent. The relation between Qyi and .lKi is determined by the following assumptions:

Qyi u ,Ki% and dYi=* = 0.5 cm. It is assumed that the ratios of lKi to zKi are about 2 at the upper storeys and gradually increase up to about 5 at the lower storeys. Accelerations of earthquakes used as input data are shown in Table 2. Various structural responses of the system in linear and non-linear states have

been calculated for the inputs shown in Table 2. Some of them are presented below. In Table 3, computed values of di max and pi in a non-linear system for various

inputs are presented. In Table 4, values of Mi for di max, Mi max, di for Mi max at each storey and

di max to pulse (a) are presented for both linear and non-linear systems. The distribution of lKi of a modified system which has a uniform value of

di max at each storey to pulse (a) has been computed and shown in Table 5 together with storey shear coefficients. For comparison the corresponding values of the original system are also shown in the table.

1. Earthquake Response of Tall Buildings, Parts I and II, by T. Hisada, K. Nakagawa and M. Izumi, July and August 1964. (BRI Reports No. 18 and 20.)

64

T. Hisada, M. Izumi M. Hirosawa

Structural response of a tall building to earthquake shocks

CONCLUDING REMARKS

The computed values shown in Tables 3 to 5 may serve for the earthquake resistant design of this type of structure against earthquakes of pulse type. The values may also be referred to as criteria for evaluating the damage to structures and other elements.

TABLE 1. Dynamical properties of model

Storey mi ,Ki ,Ki Ci dyi Qyi (i) (lod t.sa/cm) (t/cm) ít/cm) (t.s/cm) (crn) 0)

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

364 478 457 454 466 468 471 473 476 482 488 498 521 323 477

75 167 21 1 280 350 477 558 762 796 989

1080 1210 1 560 1810 1130

38 76 87 106 122 156 169 218 215 250 260 278 342 379 270

1.74 3.88 4.91 6.51 8.14 11.10 13.00 17.70 18.50 23.00 25.10 28.10 36.30 42.10 26.30

0.893 0.73 1 0.689 0.642 0.607 0.562 0.541 0.500 0.495 0.468 0.458 0.445 0.41 8 0.403 0.453

67 122 145 180 21 3 268 302 381 394 463 495 539 652 729 512

TABLE 2. Input accelerations

Notation Type of shock u rnax 7 (sec)

One half-cycle sine pulse One-half-cycle sin0 pulse One-half-cycle sin0 pulse One-half-cycle sine pulse Rectangular pulse Rectangular pulse Rectangular pulse Rectangular pulse El Centro earthquake, 18 May 1940 NA. Comp.

200 gal 0.33 g 200 gal 0.33 g 200 gal 0.33 g 200 gal 0.33 g 0.33 g

0.2 0.2 0.3 0.3 0.2 0.2 0.3 0.3 -

1. 7 = period of pulse.

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.Fi I I

r'

66

T. Hisada, M. Izumi M. Hirosawa

Structural response of a tall building to earthquake shock

67


TABLE 5. Linear response to one-half-cycle sine pulse (200 gal, T = 0.2 sec). Comparison between original system and modified system (uniform di (*T = 1.46 sec for both systems).

Original Modified i

S.C. ,Ki di max (t/cm) (cm) S.C. ,Ki di max

ít/cm) ícm)

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

75 167 21 1 280 350 477 558 762 796 989

1 080 1210 1560 1 840 1130

1.420 1.190 1.230 1 .O80 0.951 0.755 0.695 0.546 0.559 0.479 0.464 0.437 0.357 0.317 0.530

0.30 0.242 0.205 0.177 O. 153 O. 137 O. 125 0.117 0.110 0.105 0.100 0.097 0.093 0.092 0.089

158 323 451 551 63 1 695 746 786 819 845 867 885 902 91 1 917

0.562 0.562 0.562 0.562 0.562 0.562 0.562 0.562 0.562 0.562 0.562 0.562 0.562 0.562 0.562

0.25 0.221 0.200 0.181 0.164 0.149 0.136 0.125 0.115 0.105 0.0981 0.091 5 0.0855 0.0819 0.0765

N. N. Ambraseys

T. Hisada

R. W. Clough

T. Hisada

R. W. Clough T. Hisada

Discussion

How does the calculated response compare with the actual behaviour of the building?

The results of this analysis are not directly applicable to the building (Rabotnicki Dom) in Skopje, but may be used as criteria to evaluate the effect of the earthquake on the structure and masonry walls of the building.

Did you make a non-linear analysis of the response of the building to the El Centro earthquake?

The non-linear response of the building to the El Centro earthquake has been calculated. The inter-storey deflection of each . storey is not so much different from the linear one except for the upper storeys.

How long a segment of the El Centro record was used? A segment of 8 sec from the beginning of the shock was used for this analysis.

68

D. N. Roustanovitch Institut de physique de la terre Moscou

. Essai d‘étude séismique des zones épicentrales sur l’exemple du tremblement de terre d‘Ashkhabad du 6 octobre 1948

Le 6 octobre 1948, la capitale du Turkménistan, Ashkhabad, et ses environs, ont été détruits par un tremblement de terre. Le séisme coûta plusieurs milliers de vies humaines et causa des dommages matériels énormes à la république. L’intensité du séisme a atteint le degré IX et, en queiques endroits, par suite des

mauvaises conditions du sol, le degré X. L’étude du tremblement de terre effectuée d’après les données des stations séismologiques éloignées de l’Union soviétique et de l’étranger, a démontré que les contraintes de pression, dérivées d’une analyse des ondes séismiques, ont agi en une direction presque perpendiculaire aux structures principales du Kopet Dag. L‘étude de la zone épicentrale du tremblement de terre d’Ashkhabad a démontré que les répliques ont eu lieu principalement dans deux zones situées immédiatement à l’est et à l’ouest d‘Ashkhabad. Le plus grand nombre de foyers étaient situés à une profondeur variant entre 10 et 12 kilo- mètres. Cela a permis de supposer qu’à de plus grandes profondeurs l’équilibre n’a pas été troublé et que les mouvements principaux n’ont pas dépassé la surface du socle cristallin. D e nouvelles mesures géodésiques ont démontré que dans la région du tremble-

ment de terre, les points de triangulation se sont déplacés en direction horizontale vers le nord-est, de 0,7 à 1,78 mètre. Selon les données du nivellement, une élévation maximale de + 0,329 mètre a été remarquée au nord-ouest de la ville, et un abaissement maximal de - 0,219 mètre au sud-est de celle-ci. Les données séismologiques et géodésiques ont montré que le tremblement de

terre d’Ashkhabad s’est produit dans la zone de tension maximale du Kopet Dag et dans la dépression du Pretkopetdag. Par suite de la pression fronto-horizontale de la couverture du Kopet Dag dans la zone d’abaissement, il s’est produit un déplacement horizontal des sédiments mésokanéozoïques “mous”, sur la surface du socle cristallin compact. La principale faille et le déplacement maximal se sont produits dans la région centrale sous Ashkhabad et au nord de la ville. Ici les tensions secondaires ont été tout à fait éloignées. Dans cette région, des déplacements horizontaux importants se sont produits, tandis qu’à la surface du sol l’intensité a atteint le degré IX. Le déplacement des “blocs” est survenu en

69


direction du sud vers le nord dans deux zones, qui restent verticale par rapport à la direction de propagation du Kopet Dag. Ainsi l’aile ouest du “bloc” s’est élevée et celle de l’est s’est abaissée. Dans l’évolution des processus techniques, dans le mécanisme des grands

tremblements de terre, il n’y a pas seulement glissement sur la surface du socle suivant le cas cité, mais aussi d’autres déformations.

Discussion

M. Kasumovic Was it necessary to establish an international network of accelerograph stations, or did the existing national seismological networks provide all necessary data?

D. N. Roustanovitch Data obtained from remote stations can help us to understand the mechanism of an earthquake, but they cannot enable us to determine the dimensions of the focus or its depth. Detailed information concerning the focal mechanism, the fracture zone and the activation of faults discovered by geological observation can only be established from data obtained through long- term instrumental observations, with the assistance of regional seismic stations.

N. N. Ambraseys What was the maximum intensity and the magnitude of the Ashkhabad earthquake of 1948?

D. N. Roustanovitch The magnitude of the Ashkhabad earthquake, which occurred in 1948, was M = 7.0. The intensity of the earthquake was IX, and as much as X in isolated places, due to bad soil conditions.

70

S. V. Medvedev Institute of Earth Physics, Moscow

Seismic zoning with reference to the incidence of earthquakes

The seismic zoning of an area is carried out on the basis of: (u) the study of earthquakes occurring in the region concerned; (b) the discovery of the laws governing the occurrence of earthquakes of different intensity; (e) the analysis of the geological conditions under which earthquakes occur; and (d) the investigation of special features accompanying the occurrence of earthquakes in all parts of the globe. Investigation of the geological conditions accompanying earthquakes has shown

that seismic foci are usually found in zones where tectonic movements are currently taking place in opposing directions. Relative displacements of geological structures in the contact area produce considerable shearing stresses, resulting in cracks. Earthquakes and their intensity are indicators of current tectonic activity in the earth’s crust. The frequency with which earthquakes occur depends upon the intensity of

the earthquake itself. From data provided by seismic stations, diagrams have been prepared that show the incidence of earthquakes of various degrees of intensity in different seismic regions. The lower the intensity of earthquakes the more numerous they are. Diagrams showing the incidence of earthquakes are based on the number of

earthquakes of various degrees of intensity, standardized as to both time and space. This is necessary in order to enable the seismic activity of various regions to be compared. Severe earthquakes rarely occur more than once in the same zone, and it is

difficult to evaluate their incidence directly. A constant relationship between the number of slight and severe earthquakes, as expressed in the slope of the magnitude- frequency curve, makes it possible to determine quantitatively the incidence of severe earthquakes during the short periods of operation of seismic stations that record minor earthquakes. A seismic zoning map shows the zones of different seismic danger in a parti-

cular area. It is the basis for planning the construction of buildings in seismic regions, and it shows what effect earthquakes will have on buildings.

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Instrumental observations in the epicentral region of a severe earthquake are undertaken with the object of determining the zones in which earthquakes occur, their depth of focus and the energy of minor shocks. These observations are made in the area of maximum seismic activity, by means of a set of five highly sensitive seismological stations. Such stations are equipped with instruments manufactured in the Soviet Union. The relation between the energy released at the focus (Eo), the maximum intensity

at the surface (lo) and the depth of focus (h) is expressed by the following empirical formula :

Io = u, log Eo + a, log h + u3. The coefficients al, u, and u, are determined on the basis of experimental data. They vary slightly in different seismic regions. In one region, for example, a, = 0.9, u, = 3 and u, = 8.7. The relation between maximum iptensity at the surface (I,,), the magnitude of the

earthquake (M) and the focal depth (h) is expressed by a similar formula: Io = bl A4 + b, log h + b3.

The constant coefficients b,, b, and b, are determined empirically. For example, for a focus 20 k m below the surface:

lo = VI1 when A4 = 5.7; Io = VI11 when M = 6.3; Io = 1X when M = 7.0.

The preparation of the scale of earthquake intensity revealed quantitative data relating to earthquakes of various intensities (I). For example, where the intensity I = VII, earth movements have the following values: accelerations of 0.05 to 0.1 g within the range of periods between 0.1 and 0.5 sec; velocities of 4 to 8 cm/sec within the range of periods between 0.5 and 1.5 sec; a density of seismic energy between 27.106 and 134.105 ergs per cm2. The use of energy as an index of the intensity of an earthquake is very useful

in designing buildings that are subject to plastic deformation. The data concerning the occurrence of earthquakes on the earth’s surface apply

to the same soil conditions throughout a particular zone. The intensity of the earthquake must be corrected and reduced to average soil conditions. Average soils include clays and loamy soils in a hard state, and also sands and pebble beds where the ground-water horizon is 6-8 metres deep or more. Rocky and semi-rocky unweathered soils are the least affected by earthquakes.

In such soils, the intensity of an earthquake is reduced by one degree. In the earthquake that occurred in Skopje on 26 July 1963, buildings standing on rocky soils suffered little. The worst soils, which increase the intensity of an earthquake by one degree, are water-saturated gravel soils, sandy soils and loamy soils. In parts of the city of Skopje near the river Vardar, the ground-water horizon is high. This is why destruction in the 1963 earthquake was so great in those parts of the city. One of the tasks of earthquake engineers is to prepare detailed seismic zoning

maps which take account of local engineering and geological characteristics,

72

S. V. Medvedev Seismic zoning with reference to the incidence of earthquakes

differences in the spectrum of seismic vibrations, and-most important of all-the probability of the occurrence of earthquakes of various intensities. The 1963 Skopje earthquake was recorded by seismological stations in the

Soviet Union from the pre-Carpathian zone in the west to Kamchatka in the east. From the data supplied by these seismic stations it was established that the magnitude of the earthquake was 6 and the co-ordinates 42.00 N. latitude and 21.50 E. longitude. From macroseismic investigations it was possible to establish that the epicentre

of the earthquake was in the centre of the city of Skopje, and had co-ordinates 42.00 N. latitude and 21.40 E. longitude. In the centre, the intensity on the MSK scale was IX. The areas circumscribed by isolines of various intensities are: For intensity IX: SI = 9 sq. km; For intensity VIII: S, = 70 sq. km; For intensity VII: S, = 350 sq. km; For intensity VI: S, = 2,000 sq. km.

The depth of the focus (h km) is determined by using the formula h = 0.22 S, + S,

Here, S, is the area circumscribed by the second isoseist from the epicentre, and S, that circumscribed by the third isoseist. Substituting numerical values, we find that the focus of the 1963 earthquake was 5 km below the surface. Analysis of damage to buildings in the 1963 earthquake in Skopje leads to

the following conclusions: (a) soil conditions had a marked effect on the intensity of the shock; (b) the damage done to buildings varied considerably, which suggests . that the spectrum of seismic vibrations was relatively narrow; (c) buildings with a well-designed reinforced-concrete frame suffered relatively little damage ; (d) the nature of the damage to buildings points to the fact that the most intense shocks were of short duration.

Discussion

A. Zátopek

S. V. Medvedev

A. Zátopek

S. V. Medvedev

M. Kasumovitch

S. V. Medvedev

Could Professor Medvedev give more detailed information on how to apply the figures given to a general case?

Information concerning ground motion was obtained by inter- preting the records obtained in the U.S.S.R., and data published in other countries were also used.

The figures given should then reflect average conditions including geological and tectonic factors. Were these taken into account in deriving these values and, if so, how?

It is of the greatest importance that the interpretation should include seismological, geological and tectonic data.

Est-ce qu’on fait de la micro-régionalisation dans des zones séismiques où il n’y a pas eu de tremblement de terre récent?

Seismic regionalization is carried out on the basis of seismological,

73


M. Arsovski

earthquake engineering and tectonic information. It can be undertaken even if there is no information about disastrous earthquakes in recent years.

Le professeur Ambraseys a indiqué dans son rapport qu’il se produit dans la vallée de Skopje un grand tremblement de terre une fois tous les trente-cinq ans en moyenne, tandis que le professeur Medvedev indique que cela se passe tous les cinq cents ou mille ans. Je pense que le premier considère toute la région, tandis que le deuxième se réfère à une seule zone.

An earthquake may recur somewhere in the republic of Mace donia in a few decades. There may be an earthquake of intensity IX in the same place-for example, in Skopje-once in about 1,000 years.

S. V. Medvedev

74

B

Geological and seismological investigations in the Skopje area Etudes géologiques et séismologiques dans la région de Skopje

A. Zátopek Charles University, Prague

The Skopje earthquake of 26 July 1963 and the seismicity of Macedonia

The main task of the author in the Unesco mission to Skopje, December 1963 to March 1964, was to assemble and analyse data on the earthquake of Skopje of 26 July 1963, and to present a detailed description and map of the seismicity of the region.

THE MAIN EARTHQUAKE OF 26 JULY 1963, 04 H 17.2 M G M T

Macroseismic results

Epicentral intensity: Io = 90 MM (Modified Mercalli scale), determined after maximal real effects of statistical character without any reduction. Macroseismic epicentre: 410 59.6' N., 21° 26.0' E., near the centre of the city

(assumed in the pleistoseistic zone of heaviest destruction). Macroseismic focal depth: h = f 5 km. Macroseismically shaken area: about 200,000 km2 (revised estimated value),

macroseismic field extending in the NNW.-SSE. direction. Macroseismically determined magnitude : M = 6.0 units of magnitude, energy

released E = IOz1 ergs. Zone of damage : destruction and heavy damage in Skopje, damage to buildings

observed along the Vardar river from about 7 k m west to 10 k m south-east of Skopje? Distribution of macroseismic intensities : map of isoseismals constructed at the

Skopje seismological station, and revised by the author. Macroseismically felt after-shocks : maximal intensity 60 MM, 26 July 1963,

04 h 53 m GMT; further after-shocks observed throughout 1963 and the first half of 1964, twenty-one of them with intensity 5 O MM; felt in the city of Skopje, on the slopes of the Skopska &na Gora, and at the foot of the Vodno ; continuation of after-shocks is expected. General characteristic of the main earthquake : a shallow tectonic earthquake,

77

Geological and seismological investigations in the Skopje area

composed of several shocks and having a typical small block structure mechanism (with probable tiltings of several blocks) within the epicentral area.

Microseismic results

Epicentral co-ordinates: 420 00.5‘ N., 21° 27.3’ E. (determined by means of data of the four nearest stations). Origin time: H = 04 h 17 m 11.1s GMT f 0.24 s. Focal depth: h = f 5 k m (adopted macroseismical value). Main phase velocities: Pn 7.85 km/sec; Pg 5.60 km/sec; Sn 4.5 km/sec;

Sg 3.35 km/sec; local anisotropy for Pg: along the main tectonic structures (NNW.-SSE.), about 5.55 km/sec; across the structures (approximately east-west), about 5.48 km/sec. Instrumental magnitude: M = 6.0 (mean value of 11 Eurasiatic and American

stations); approximate focal energy E = 1O2I ergs. Mechanism: first motion at the Skopje station toward WNW., then toward

NNE. Distribution of compressions and rarefactions : preliminary nodal and auxiliary planes position NW.-SE. and SW.-NE. respectively. After-shocks : over 400 after-shocks registered up to 1 January 1964 (Mainka

pendulums of the Skopje stations). Special registration by Hagiwara shortperiod seismometers from 11 September until 11 November 1963. Location’ of twenty-five after-shocks, recorded by Hagiwara seismometers, by distance and azimuth has given two active zones: northern line extremities 420 07’ N, 210 23‘ E and 420 01’ N, 210 32’ E; southern line extremities 420 03’ N, 210 21’ E and 410 58’ N, 210 28’ E. The southern line was found definitely to be responsible for the main shock of 26 July 1963, 04 h 17.2 m GMT.

SEISMICITY OF MACEDONIA AND OF THE SKOPJE REGION

Macedonia

A list of epicentres of Macedonia with epicentral intensities Io > 60 (only such come into consideration with respect to effects on structures) has been compiled for the period 1893 to 1900-1963. In the list are shown the year, date and time (GMT) of occurrence, epicentral co-ordinates, epicentral intensity Io, equivalent radii r1,I-i of circles dividing intensities of I and 1-1 degrees MM, and in a number of cases foca! depths and magnitudes M are introduced. Macroseismic magnitudes were determined according to the Kárník‘s formula M = 0.55 I, + 0.93 log h + 0.14. In seven cases instrumental magnitudes are given. An adjustment leads, in the range of intensities used and for small depths, to

an empirical relation, valid for the territory of Macedonia as a whole, M = 0.58 I, + 0.7. The number of earthquakes having the epicentral intensity Io and having

occurred in Macedonia during the period considered may be empirically expressed

78

A. Zátopek The Skopje earthquake of 26 July 1963 and the seismicity of Macedonia

by an adjusted formula log N = 4.31 - 0.43 Io, or, reduced to a one-year interval, log Ni = 2.47 - 0.43 Io. That means one loo earthquake in about sixty-eight years, 90 in twenty-five years, 80 in ten years, 70 in three and a half years, and 60 in one year, respectively. The geographical distribution depends, naturally, upon the activity of individual regions. Substituting M for Io we get log N = 4.83 - 0.74 M, and log Ni =

2.98 - 0.74 M, respectively. This would mean one earthquake of magnitude 7 in 158 years, one of magnitude 6 in about 29 years, etc. A map of epicentres, Io > 60, for the period 1893 to 1900-1963 has been pre-

pared. In this map the energy density distribution of foci, situated on the Mace- donian territory, is also represented, expressed in Q units (1 Q = 1015 ergs/100km2 year). The region of Valandovo (with earthquakes of M = 6.7 and M = 7.1 on 7 and 8 March 1931, respectively) is the most important autochtonic seismic area of Macedonia. The region of Skopje appears to be the second one. The information is incomplete as regards the effects of earthquakes the foci of which are situated in the neighbouring seismic areas. Focal depths of autochtonic Macedonian earthquakes most probably do not

exceed 20 km. Thus the Macedonian earthquakes are to be classified as superficial and shallow. Energy release and elastic strain rebound as a function of time for Mace-

donian foci show periods of elevated activity in 1910 (southern Macedonia), 1920-1923 (northern and north-western Macedonia), 193 1 (Valandovo, south- eastern Macedonia), and 1963 (Skopje region). External sources of strong earthquake effects observed in Macedonia: the area

of Pishkopeya (eastern part of Albania) in the west; the Prizren region (southern Serbia) in the north-west; the regions of UroSevac, Gnjilani and Orechovac (southern Serbia) in the north; the valley of the Struma river (western Bulgaria, especially the Simitli region) ; some effects observed in southern Macedonia have to be ascribed to foci situated in Greece; strong shocks observed in the south- west of Macedonia come often from south-eastern Albania. Data available permit only a partial estimation of the energy liberated in these regions (loz2 to loz3 ergs in western Bulgaria, 1904; UroSevac (southern Serbia), near loz2 ergs, 10 August The distribution of intensities for I = 6O for 1901-1963 has been studied.

The influence of external sources may be seen when comparing this distribution with that of energy release. Maps of maximal intensities have been constructed on the basis of the distri-

bution of intensities. The maximal isoseismals follow from a superposition of maps of isoseismals plotted for individual shocks. The basic seismic intensity background of 60 MM forms a belt going through the central part of Macedonia roughly in the WSW.-ENE. direction. Outside this zone, all structures must be protected against seismic danger in a measure given by the maximal intensity. Inside the zone, buildings protected against wind effects would be already automatically protected against earthquake, but it is not excluded (see ruins of Stobi) that a destructive earthquake may occur. (A swarm of more than fifty weak

' 1921).

79


earthquakes was located near Gradsko in central Macedonia on 19-20 October 1963.)

Region of Skopje

A map of the seismic activity in the Skopje region has been prepared. Studies of the elastic strain rebound characteristic show the main active periods in 1910, 1921 (followed by moderate activity 1921-1935) and 1963 (preceeded by a relatively very quiet period).

80

N. N. Ambraseys Department of Civil Engineering, Imperial College of Science, University of London

General characteristics of the Skopje ear thquake-a summary

On 26 July 1963, at 05.17 hours local time, northern Macedonia was shaken by a severe earthquake. It killed 1,070 and injured over 3,000 people. The shock destroyed or damaged beyond repair 40 per cent of the houses in Skopje and about 2 per cent of the houses in neighbouring settlements. The earthquake was felt with an intensity equal to, or greater, than III on the

Modified Mercalli Intensity Scale (MM) over an area of about 40,000 square miles. The maximum intensity of the shock, assessed on a statistical basis by the writer, did not exceed IX (MM). The Skopje earthquake was a medium magnitude (M = 6) surface shock of

the kind that in a sparsely populated region might have caused little concern. Unfortunately, it occurred in a densely populated area and it had appalling consequences. It literally destroyed more than 30 per cent of the houses in Skopje without causing their collapse. From the ground, some distance away, or from the air, it was practically impossible to detect any damage to the city. Like the Agadir earthquake of 1960, the unique characteristic of the Skopje

earthquake was the extreme localization of destruction which was concentrated within an area of not more than 5 square kilometres, typical feature of a shallow, medium-magnitude shock that happened to occur very close to a city; most of this area was occupied by Skopje. As a matter of fact, the effects of the earthquake could not be seen outside a radius of 6 kilometres around the city. There is little doubt that the destructive movement of the ground was, for all

practical purposes, of a shock type directed in most cases from east-south-east to west-north-west, and that this was confined within a limited area which included the largest part of Skopje. So far, the results of the field studies show that the predominant ground acceleration, on the average, was very high and of a very brief duration. The destructive part of the ground motion, being very violent but of very short

duration, ceased before there was time to produce total collapse of the majority of the damaged structures which, after the earthquake (whose total duration did not exceed 5 sec), were left shattered on the verge of collapse but still standing.

81


It may be argued perhaps that a building left standing on the verge of collapse does not necessarily mean a shaking of short duration, since it takes different amounts of time to shake down buildings of different types. But in Skopje more than 1,000 buildings of widely different types were shattered and brought to the verge of collapse, and even the weaker ones were left standing. A careful, but not exhaustive, search in the epicentral region did not disclose

any definite case of faulting. It is questionable whether the existence of fissures in alluvial ground in a number of places in the city, as well as some kilometres north and north-east of Skopje, provided evidence of faulting. The foundation strata in Skopje consist of alluvial deposits underlain by marls.

The alluvium is generally gravel with sand and silt, and it is very well compacted. With a very few exceptions, Skopje had no foundation problems, its superficial geology being, for al! practical purposes quite uniform. The ground-water level is about 2 to 10 metres below the surface and it varies seasonally, responding quickly to river fluctuations. The analysis of the available seismological, geotectonic and damage data

suggests that during the earthquake the city with its surroundings underwent a sudden mass movement, perhaps of considerable irreversible displacement, and that the destruction was brought about by a sudden readjustment of a number of tectonic blocks of the mosaic structure of the Vardar zone on a part of which Skopje stands. Apparently, a number of blocks underwent a tilting to the north- north-east and a simultaneous abrupt sliding to the north-west. Neighbouring blocks slid in other directions. I believe that the greater part of the released seismic energy was spent in dis-

placements of the ground in a powerful unidirectional shock of extremely brief duration. While it cannot be said that subsequent oscillatory movements of the ground were of no consequence, it remains true that these, so far as severe damage is concerned, were of secondary importance. Also minor movements of the ground in other directions accompanying the destructive part of the shock are not precluded. Tall reinforced-concrete skeleton structures, modern engineering constructions

such as factories, mills, bridges, dams, underground installations, highway embankments, railways, which had not been designed to resist earthquake forces but had been well designed and constructed for normal operation conditions, suffered little damage. Two concrete dams near Skopje suffered absolutely no damage. A few pipes of the water distribution system in the city and some underground

telephone cables were damaged by the fallen buildings or by heavy debris. Brick wall structures suffered more than any other type and accounted for

the largest number of deaths. Mixed construction suffered considerably and although many of these buildings did not collapse they were left completely shattered, beyond repair. Old adobe construction, particularly that with timber bracing, resisted the shock with some damage but behaved far better than the brick or the hybrid construction. Reinforced-concrete skeleton structures suffered comparatively little damage and only two small structures of this type collapsed.

82

N. N. Ambraseys General characteristics of the Skopje earthquake-a summary

Perhaps the tall skeleton structures, up to fifteen storeys, performed far better because they were flexible but also because, being important engineering under- takings in Macedonia, they were constructed with more care and in some cases wind forces were considered in the design. Finally, the pre-stressed construction was totally destroyed after its supporting columns collapsed. The steel mill, which at the time was under construction, suffered only minor damage. In general, the subsoil conditions at Skopje are adequate and cannot be held

responsible for the damage that the city suffered. Also, the design of modern structures, with the exception of the brick wall bearing houses, was in general adequate, although in some cases a little underdesigned and with considerable improper detailing. What was detrimental, was that these modern methods of design were not followed up by equally advanced methods of construction and materials. The extremely variable quality of the building materials and of the methods of construction were found to be more important than the lack of earthquake-resistant design. Considering that structures were designed for static conditions and that the building materials and the methods of construction were admittedly below average, reinforced skeleton buildings performed rather well. There is an interesting point that emerged from the study of the distribution

of damage in Skopje, though not as yet fully explained. The area of the most severe damage correlates surprisingly well with the part of the city that was flooded in November 1962, just nine months before the earthquake. A number of plausible explanations for the observed correlation have been suggested.

Discussion R. W. Clough

N. N. Ambraseys

T. Hisada

N. N. Ambraseys

A. Zátopek

A correlation was shown between the zones of Skopje which were flooded (six months earlier) and the zones of maximum damage. Were there any other characteristics of the soil in the zones which were found to be uniformly the same, and which might tend to explain the damage similarity?

There is nothing I can do about this excellent correlation. All I can say is that the logical explanation, though perhaps difficult to prove or disprove, could be that the area flooded by the recent ovedlow of the Vardar is an area subject to strong seepage gradients. There is also a sharp change in the depth of the alluvium under this area.

Please give your opinion about the horizontal and vertical accelerations of the ground.

So far, the available evidence indicates rather high ground accelerations. I am afraid I cannot say off-hand their exact value, but there are indications that they were close to 15 per cent g, in some places being lower (8 per cent), in others higher (25 per cent). Please do not take these values as final, as my data are as yet rather scanty.

W e should be very careful when speaking about compressional or transverse (shear) waves in the epicentral area of a strong

83


R. W. Clough

D. Gojgic

T. Mitrov

earthquake, where the assumptions of the theory of elasticity or even of the theory of continua are not valid. It was very good that Dr. Ambraseys denoted these movements as “shocks”.

N. N. Ambraseys I absolutely agree with Professor Zátopek’s remark. Skopje is located so near to the earthquake-generating area that no type of wave was probably completely developed. Both longitudinal and transverse waves emitted from a certain part of the focus might be expected to give rise to ground movements discordant with those due to the arrival of similar waves from deeper or more distant portions of the focal “volume”. What I had in mind was the onsetting waves rather than any distinct type of wave. Dr. Ambraseys has presented a convincing case for the conclusion that the Skopje earthquake was of the single impulse, “shock” type, i.e., consisting of a sudden displacement in one direction. When in Agadir after the 1960 earthquake I came to the same conclusion concerning that ground motion. As Dr. Ambraseys has noted, many examples of evidence from both cases .were nearly identical, and I am convinced that both were similar “shock” type quakes.

Professor Ambraseys emphasizes the connexion between the zone of inundation and the zone of maximum damage. M y opinion is that the inundation zone consists of gravel and sand whose Ky makes possible the drainage in some days or hours. In this way I think that the connexion between these two zones is due to the shallow level of the ground water in this zone and not to the Suence of inundation. Dr. Gojgic remarks that the slide I have just shown you emphasizes the connexion between the area of inundation in Skopje and the area of maximum damage. Please remember that I have not emphasized such a connexion myself; it is the facts shown on this slide that do so. I merely state facts which are well known to all of us. Dr. Gojgic believes that it is not the flooding of this area that is, in fact, responsible for the excessive earthquake damage but the high water table that he has observed. This may very well be the case but the obvious correlations with the flooded area cannot be disregarded.

How do you explain the fact that in Karpos there was great damage but not in the inundation zone. In the same way, great damage was observed on the ridge Kale-Zajiev Rid, zone without inundation where the buildings had good ground for foundations.

I do not think that the damage in Karpos was the result of a higher intensity or, if you like, of a more intense ground movement. The observed damage in Karpos was the result of faulty construction which was carried out in a systematic way. The excessive damage in other parts of the town does not surprise me. Variations in intensity due to reasons other than those of flooding are obvious, in many instances augmented by improper

N. N. Ambraseys

N. N. Ambraseys

84

Discussion General characteristics of the Skopje earthquake-a summary

M. Borges I

construction or poor quality of materials. I am well aware of the distribution of the damage in the northern parts of the city. There are so many factors involved that flooding of the foundations alone, I agree, should not be taken as the sole cause of maximum damage. would like to support the general conclusions presented by Dr. Ambraseys, concerning the similitude between Agadir and Skopje earthquakes, by showing some slides concerning the first of these earthquakes. In Agadir, in the areas the more severely affected, most of the masonry buildings collapsed. The reinforced-concrete framed buildings in general behaved well. Several of the collapses that occurred in these buildings can be explained by poor construction, mainly in what concerns connexion details. This shows that masonry buildings without convenient horizontal and vertical bracing are not satisfactory for seismic zones. I would also like to call your attention to the necessity of representing earthquake actions in a general way that allows a safe design of structures. As it is possible to design earthquake-resistant structures in an economic way, the problem of considering detailed information on the variation of the seismic actions from joint to joint is then much less important.

85

M. Arsovski Geozavod, Skopje N. Grujic D. Gojgic

Seismological and geological investigations of the Skopje valley and urban area

A set of problems was imposed by the catastrophic earthquake which struck Skopje on 26 July 1963. Geologists were faced with the task of carrying out complex geological and seismotectonic investigations of the Skopje valley, engineering-geological and soil mechanics studies of the urban area, and, from these investigations, of preparing a microregionalization map for town-planning purposes. Such extensive work could only be realized by the great efforts of our profes-

sional staff and the assistance which was provided to us by highly qualified international experts under the sponsorship of Unesco and other organizations within the United Nations family. Immediately after the earthquake, the collection of data on the deformations

which appeared in the urban area and close surroundings of the town was started. Descriptions of different cracks, boiling and other deformations were compiled. A programme of complex geological and soil physics investigations was established. The first step was the very rapid completion of complex geological and seismological investigations whose ultimate objective was to provide a seismotectonic map of the Skopje valley and its limits, as well as a seismic microregionalization map of the wider urban area. The following method was used in this research: 1. Superficial regional geological survey of Skopje valley and its limits over a

total of 2,200 km2. Gravimetric, seismic and geoelectric methods were used in the study of the Skopje valley structure.

2. Detailed engineering-geological and hydrogeological surveys were carried out over the wider urban area at a scale of 1/10,000 using the mapping of the surface, shallow geoelectric drilling and shallow seismic surveys. Survey drillings were carried out with complex soil mechanics and other laboratory analyses. Also samples were studied using seismic, electric and gamma- gamma carrotage. Piezometers were put into a number of drillings in order to study the régime of underground water.

3. A re-survey of the trigonometric and polygonometric network of the urban area was carried out in order to determine vertical and horizontal displacements.

87


4. A partial study was made of dynamic ground deformation by use of the Kanai method.

5. Within the Skopje valley, temporary seismological stations were installed in order to record the after-shocks and to make ground oscillation studies.

These observations are being continued and a part of the results will be presented here by our colleague Roustanovitch. Regional geological investigations together with gravimetric and seismic investi-

gations have given a new picture of the tectonics of the Skopje valley, which appears to be an inserted depression formed as a result of subsidence in this region in the Neogene-Quaternary. While the basic terrains in Macedonia have been elevated, the depression zones have been sliding down. The Skopje valley area represents an inserted tectonic ridge at the boundary of three main zones: the Vardar zone, the zone of west Macedonia, and the horst-anticline of Pelagonia. The sliding tendency of Skopje valley continues to the present day. Results of studies have shown that in the Quaternary (Pleistocene and Holocene)

disturbances occurred within the Skopje depression. The recent seismotectonic activity in the valley is connected with the longitudinal and transverse fault zones. The new geodetic measurements have shown that the Skopje valley area is

subsiding and horizontal displacements are evident. Such phenomena are recorded for the period of the last twenty years (1938-1963). The soil of the wider urban area is composed of monolite and semi-monolite

Quaternary and Tertiary sediments. The formation and geological history of lithogenetic complexes were the essential factors in determining the present physical and mechanical properties of these sediments. The heterogeneity of sediment composition and its engineering-geological characteristics under changing and non-uniform hydrogeologic conditions have had a significant influence on ground behaviour during the earthquake. It must be pointed out particularly that the area of alluvial terrace has a non-uniform underground water régime with significant oscillations of the water table. On the basis of all these results the preparation of a seismic microregionalization

map for the wider urban area has started, involving about 100 km2. In order to prepare this map, it was necessary to decide first of all the basic degree of intensity, based on the study of historical data on previous earthquakes and on particular studies of the earthquake of 26 July, including the opinions of the international experts who have come to Skopje since then. The basic intensity in the Skopje region was evaluated as 8.5 on the M C S intensity scale. The results of these investigations were applied in the preparation of a seismic

microregionalization map, and the calculation of the seismic intensity correction was carried out according to the formula of S. V. Medvedev. The corrections have reflected the composition of the soil, the depth of the underground water table and the soil mechanics characteristics of the ground. On the basis of all these investigations, zones of 8 and 9 degrees intensity were

determined. These regions were subdivided into relatively more favourable and less favourable zones (zone A and zone B respectively).

88

Discussion Seismological and geological investigations of the Skopje valley and urban area

In order to confirm and supplement the existing results, a team of the Institute of Earth Physics was engaged from the Soviet Union. This meeting will be informed of its preliminary results.

Discussion

N. N. Ambraseys

M. Arsovski N. N. Ambraseys

M . Arsovski

A. Zátopek

M. Arsovski

S. V. Medvedev

M. Arsovski

S. V. Medvedev

M. Arsovski

V. Ribaric

Is there any connexion between the tectonics of Macedonia and the unit energy distribution?

Yes, such a connexion does exist. Is there any correlation between the microregionalization and the damage map?

Yes, the zone of maximum damage coincides with the zone of maximum intensity of the micro-zoning map, despite the fact that this map is based on data quite independent of the earthquake of 26 July 1963.

I would like to ask if you have compared the position of located after-shocks with the systems of active dislocations that you have found by geological means? It would be interesting to know because of the type of the corresponding after-shocks which showed, systematically, relatively very strong P-waves. That could correspond to a certain type of motion along these dislocations.

The most active is the system of dislocations proceeding along the south-western slope of the Skopska Crna Cora and there is another system going into the area of Skopje. The north- eastern border of the massive of Vodno, of course, represents a highly mobile zone, too.

I should be glad to hear further details about the seismic microregionalization map.

Zones 8" and 8 b contain relatively safe areas. Such areas occur in the region between the aerodrome and Ucjem, where the gravel is over 10 m thick, extremely compact and very granular, while the ground-water level is relatively low-over 5 m deep. There is another area of this type north of the Vardar, but here the geological conditions are different. Zones 9" and gb contain areas with heterogeneous soils, a high ground-water level and a permissible load of 1-2 kg/cm2. Will seismic microregionalization maps be prepared for other regions?

W e think that microregionalization work should k continued, but whether it will or not depends on material considerations. In my opinion, it should not be confined to the vicinity of Skopje, but should also cover other valleys, such as the Ohrid and Prilep-Bitola depressions, where there are densely populated areas in seismic regions.

What data were used in plotting the directions of soil movement on the seismotectonic map of the Skopje valley?

89


M. Arsovski

V. Ribaric Do these agree? M. Arsovski

The data obtained by a geodetic survey, which was carried out twice, together with geological data obtained on the spot.

There is a correlation between both types of information referred to.

90

D. N. Roustanovitch, V. A. Tokmakov, Moscou D. Hadzievski Observatoire séismologique, Skopje

Institut de physique de la terre,

Études séisrnologiques instrumentales de la zone épicentrale du séisme de Skopje du 26 juillet 1963

Deux problèmes d'importance scientifique et pratique de premier ordre se posent dans la séismologie moderne : u) la justification géophysique et le perfectionnement des méthodes pour la régionalisation séismique ; b) la découverte de nouvelles méthodes pour la prévision des tremblements de terre importants. Ces deux problèmes exigent des observations instrumentales systématiques dans

la région même OU d'importants tremblements de terre ont lieu, c'est-à-dire dans la zone épicentrale. Pour y réussir, il faut déterminer les épicentres, la profondeur des foyers séismiques et l'énergie des tremblements de terre. L'étude des rapports entre les tremblements de terre et les structures tectoniques

actives permet dans certains cas de résoudre le problème de la prévision de l'endroit et de l'intensité des séismes, nécessaire dans la régionalisation séismique. De telles études séismologiques régionales sont effectuées en Union soviétique

dans les régions séismiques actives, notamment le Caucase, i'Asie centrale et l'Extrême-Orient. Au mois d'août 1964, cinq stations séismologiques ont commencé à enregistrer

les chocs dans la région de Skopje: Matka, Brazda, aux environs du barrage de Lipkovo, bains de Kumanovo et Skopje. Toutes ces stations sont équipées d'instruments soviétiques standards avec enregistrement galvanométrique. Les sismographes sont des types suivants: VEGIK, VSH, GSH et VBP. L'enregistrement est effectué par l'intermédiaire d'un galvanomètre type GK-VII, sur un enregisteur du type RS-2 avec une vitesse de déroulement de la bande photographique variant de 120 à 240 millimètres par minute. Les caractéristiques des instruments sont notées dans le tableau suivant:

Sismographe Galvanomètre Ampiification Intervalle du spectre maximale compris

B 25000 GSH 25 O00

VEGIK GK-VI1 20 O00 1,5 - 10 HZ

VSH GK-VI1 35 O00 1,5 - 10 HZ VBP GL-VI1 100 - 150 1,5 - 10 HZ

91

Études géologiques et séismologiques dans la région de Skopje

La station séismologique auprès de l’université de Skopje fait partie du réseau des nouvelles stations séismologiques; elle est équipée de deux sismographes mécaniques du type MAINKA, dont les pendules ont des masses de 450 kilogrammes, et d’un sismographe pour des tremblements de terre locaux du type CONRAD à enregistrement mécanique et avec pendule de 25 kilogrammes. La période des pendules varie entre 2,5 et 7 secondes. Les appareils enregistrent les composantes horizontales des séismes avec une amplification qui varie entre 25 et 150. Outre ces stations fixes, il existe deux stations séismologiques mobiles qui sont

utilisées pour enregistrer des mouvements sur différents terrains. Les stations séismologiques mobiles sont équipées de sismographes du type VEGIK et de galvanomètres du type GB-III, qui composent ensemble l’oscillographe électro- magnétique du type OSB-VI. La combinaison VEGIK - GB-III donne un tableau suffisamment précis des mouvements du terrain dans l’intervalle entre 1,5 et 2,5 Hz. Le service de temps exact est assuré par des chronomètres du type MH dont la marche est contrôlée par un enregistreur semi-automatique des radio-signaux horaires de Moscou et de Londres. Ces radio-signaux sont enregistrés directement sur les sismographes. Le bon emplacement des stations séismologiques et le service de temps permet-

tent une plus grande précision dans la détermination des épicentres et de la profondeur des foyers. Les premières observations ont révélé une haute activité séismique dans la

région. Au cours des deux premiers mois de travail, les stations ont enregistré 100 tremblements de terre locaux qui proviennent en majorité de la zone du tremblement de terre de Skopje, mais on a aussi enregistré des tremblements de terre des régions de Tetovo et Titov Veles. Les épicentres des séismes du 15 au 29 août 1964, qui se sont fait sentir dans la

région de Skopje avec intensité jusqu’au degré IV se trouvent à 6-7 kilomètres au nord-est de la ville. La profondeur des foyers était de 6 à 7 kilomètres. Beaucoup d’autres séismes de petite intensité ont été observés au même endroit. Les études séismologiques permettent de définir les zones d’activité séismique,

ce qui est actuellement d’une grande importance pratique en rapport avec le développement intensif de la construction dans des régions qui ne sont pas encore conquises et qu’on considère comme inabordables. Cela se rapporte surtout aux constructions hydrotechniques, aux barrages et aux grandes centrales hydro- électriques dans des régions caractérisées par des tremblements de terre destructifs. Ayant en vue ces nouveaux problèmes économiques, il est indispensable de

compléter sur le territoire de la Yougoslavie le réseau actuel des stations séismolo- giques par des appareils plus perfectionnés de grande sensibilité, et de créer de nouvelles stations afin de rendre possible l’étude des tremblements de terre de magnitude M >/ 3,5 sur tout le territoire.

92

Discussion

M. Pavlovic

D. N. Roustanovitch

D. Janic

D. N. Roustanovitch

A. Zátopek

D. N. Roustanovitch

N. N. Ambraseys

D. N. Roustanovitch

Etudes séismologiques instrumentales de la zone épicentrale du séisme de Skopje du 26 juillet 1963

Discussion Les observations ont-elles permis de distinguer une migration des épicentres et, dans l’affirmative, quelle est la direction de cette migration ?

Des conclusions sur la migration des épicentres seront formulées à la fin des recherches, quand les répliques auront été étudiées en plus grand nombre et pendant une durée plus longue.

Le projet de recherches géologiques et géophysiques prévoyait, pour la fin des travaux, un forage d’une profondeur de 1 O00 à 2 O00 mètres. D’après les résultats de vos recherches, estimez- vous que ce forage soit nésessaire?

Les forages paraissent nécessaires surtout pour différentes applications : pour apprécier la profondeur des couches du sous-sol, pour déterminer la pente des structures profondes en corré- lation avec les recherches géophysiques. I1 serait souhaitable d’opérer des forages atteignant 4000 mètres en deux ou trois points au moins.

You have recorded more than 200 weak shocks in the Skopje region. Such a number allows a use of statistical methods. Did you make any statistical study of observed after-shocks?

La région de Skopje est très active du point de vue séismique. Les données recueillies au moyen d’instruments pendant une longue période permettent de savoir dans quelle mesure l’activité séismique s’est modifiée et d’évaluer la probabilité statistique de la répétition des tremblements de terre, sur la base des mesures recudlies à l’aide des instruments.

Are boreholes 2 k m deep needed? If so, would the results be of practical or academic interest?

Des forages profonds atteignant 3 O00 ou 4 O00 mètres auront une grande importance pour la régionalisation séismologique du territoire, pour l’étude de la structure géologique des couches supérieures et pour la solution d’un ensemble de problèmes théoriques.

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D. A. Lilienberg Institut de gbographie, Académie des sciences de l’URSS, Moscou

Qualités géomorphologiques structurales et mouvements tectoniques contemporains dans la vallée de Skopje

1. L’utilisation des méthodes géomorphologiques a été introduite depuis peu dans l’étude des mouvements tectoniques jeunes avec des résultats particulièrement intéressants. En rapport avec cette tendance, une discipline spéciale est apparue dans la géomorphologie : la géomorphologie des structures. On emploie actuellement les méthodes géomorphologiques structurales pour

l’étude des régions séismiques actives et des champs pétroliferes, pour la décou- verte de gisements de minéraux utiles existant dans le sous-sol, pour la construction de centrales hydro-électriques, etc. Des spécialistes de cette nouvelle discipline sont form& avec succès dans les facultés de l’Union soviétique.

2. On inclut dans ces nouvelles méthodes de géomorphologie structurale tous les procédés d‘études des conditions géomorphologiques normales, c’est-à-dire les surfaces de nivellement des terrasses fluviales, lacustres ou marines, ainsi que leur déformation, les transformations du réseau hydrographique, les profils longitu- dinaux et transversaux des vallées, la répartition de faciès et les conditions des dépôts des jeunes formations sédimentaires, le rapport entre le relief actuel et les structures néotectoniques et les différentes configurations déformantes de forme complexe du relief sous l’influence des actions tectoniques et en particulier celles de caractère violent. Actuellement l’étude et la prévision de la séismicité dans des régions montagneuses ne sont pas possibles sans une sérieuse analyse géomor- phologique des structures.

3. La formation du relief de la vallée de Skopje, pendant la période néotectonique, a consisté dans la modification de vieilles structures tectoniques dont le résultat a eté le caractère plissé des effondrements nouvellement apparus, des modifications des strates adossées sur les massifs montagneux voisins et d’une haute séismicité. Les caractéristiques principales du relief sont justement dues à des mouvements tectoniques récents particulièrement actifs au quaternaire.

4. Au néogène, dans la région de Skopje, vers le rebord synclinal du Vardar, se

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sont produits des effondrements transversaux auxquels sont liées les baies de Markova Reka et les baies lacustres de Ljubenci. A ce phénomène, ont correspondu de fortes élévations du massif de Pelagonija et de Sar Planina, produisant une inversion du régime tectonique. Bien que de nombreuses failles aient présenté des caractéristiques de cassure profonde et qu’à la fin du pliocène des épanchements basaltiques se soient produits (M. Arsovski, etc., 1964), on n’a pas observé de grands contrastes dans les couches du pliocène, qui limitent ici les bassins lacustres, ni dans la zone montagneuse voisine.

5. Le quaternaire est caractérisé par des contrastes plus accusés du relief et des mouvements tectoniques. On y remarque un soulèvement progressif des zones élevées des massifs montagneux et une réduction des affaissements dans leur partie intérieure. L’amplitude courante de ces mouvements quaternaires va de 500 à 600 mètres d’élévation pour 120 mètres d’affaissement (cuvette du Bas Lepenec), de 200 à 260 mètres (cuvette de Vardar - Markova Reka) contre 350 mètres d’élévation sur les rebords montagneux. Dans les zones des plus grands contrastes, les gradients de mouvement atteignaient 300 mètres au kilomètre (le long du courant inférieur de Markova Reka). Le bassin lacustre, avec 560 à 580 mètres d’altitude sur ses rives, tel qu’il existait au quaternaire, s’est affaissé et la morphogenèse de la vallée a été soumise à l’influence du fleuve Vardar et de ses affluents. Le fond de la vallée a subi un effet de compartimentage en petites structures tectoniques de différentes amplitudes et caractéristiques des mouvements, et en plusieurs cas avec inversion du style tectonique et du relief. D e tels phénomènes se sont reproduits dans d’autres dépressions de Macédoine

(Pelagonija, la vallée de Kicevo, la crête de Bitola-Dzavate, etc.). Les anciennes zones de faille jouent de nouveau pendant la période quaternaire, et des failles nouvellement formées pendant cette période sont caractérisées par une grande activité. On aperçoit un renouvellement et une augmentation subite des contrastes des mouvements se produisant le long de la faille de Kisela Voda, surtout le long de la vallée de Markova Reka où ils s’unissent aux mouvements inverses de la partie élevée du golfe pliocène de Markova Reka et de la portion d’âge quaternaire la plus basse de la faille Vardar - Markova Reka. A l’intersection avec la crête Osoj-Vodno, les contrastes de mouvement diminuent le long des failles. Cela est visible sur le relief où les couches sédimentaires sont inclin‘ées vers la cuvette Vardar - Markova Reka. La crête Osoj-Vodno est formée sur toute sa longueur de terrasses lacustres du pliocène supérieur et du quaternaire inférieur. Des épi- centres de nombreux tremblements de terre sont situés sur les crêtes (A. Zátopek, 1963). On aperçoit aussi une augmentation des contrastes le long de la grande faille de Katlanovo - &na Gora OU l’on a aussi de nombreux épicentres de tremblements de terre. Dans la partie de l’axe transversal et surélevé du fond de la vallée de Skopje, à partir de la crête de Vodno vers &na Gora, se forment des mouvements longeant la faille Brazda-Skopje dont le résultat est la formation d’élévations isolées (Orizare, Kale, etc.). Cette activité tectonique serait en liaison avec certains anciens tremblements de terre ainsi que des séismes locaux constatés par le réseau des stations temporaires soviétiques (D. N. Roustanovitch).

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D. A. Lilienberg Qualités géomorphologiques structurales et mouvements tectoniques contemporains dans la vallée de Skopje

La zone à mouvements opposés de Kumanovo-Skopje, qui partage la vallée de Skopje en deux, mérite une mention particulière. La partie d’affaissement se trouve au sud-est de la plaine de remblaiement holocène, tandis que les zones élevées se situent au nord-ouest. L’augmentation des oppositions de mouvement s’est déroulée de l’est vers l’ouest. En effet, les élévations pliocènes de l’ouest ont montré pendant le quaternaire une forte tendance à l’élévation. Les hauteurs du côté esCont subi une inversion tectonique en même temps que le relief. Au cours du quaternaire récent, elles ont constitué une région d’affaissement et de comble- ment actif de sédiments alluvionnaires et pluviaux, tandis que dans le quaternaire moyen et ancien elles s’élevaient. Au-delà de cette zone sont disposées les bordures est des zones relativement élevées de Treska-Vardar, situées dans la zone d’affaissement du Vardar. Les épicentres de nombreux tremblements de terre enregistrés par le réseau des stations temporaires soviétiques (D. N. Roustanovitch) se trouvent dans cette zone. Les élévations situées au nord de Skopje sont disposées selon une direction

nord-nord-ouest, et constituent une série de vallées et de crêtes. Elles sont entre- croisées d’une série de failles généralement d’âge pliocène et quaternaire. C’est au milieu de ces failles qu’apparaît une série de cassures du quaternaire dont l’activité ou l’apparition continue jusqu’à l’holocène. Ainsi la faille qui passe au travers du secteur de l’aciérie affecte les sédiments du quaternaire récent. Ces jeunes failles peuvent servir de foyer de tremblement de terre. Le contraste des mouvements se voit aussi le long de certaines failles anciennes

comme, par exemple, celles qui apparaissent le long de la base nord de la crête Osoj-Vodno. D e même que pour les zones élevées, on distingue les zones affaissées quaternaires anciennes et celles nouvellement formées qui sont du même âge. Ainsi la cuvette Vardar - Markova Reka apparaît comme un phénomène hérité de l’affaissement mio-pliocène. En même temps la cuvette du Bas Lepenec s’est installée à l’emplacement de l’ancienne élévation et son fond est composé de minéraux paléozoïques (M. Arsovski, N. Grujic, D. Gojgic, 1964). A présent les affaissements de la cuvette du Bas Lepenec se sont atténués et ne sont pas perceptibles dans la structure des zones supérieures des terrasses holocènes de 1,5 à 2 mètres. Au commencement de l’holocène, les oscillations étaient encore fortes, comme le montrent la surface de l’alluvium et les terrasses de 4 à 5 mètres. Ces deux cuvettes sont séparées par la partie relativement plus élevée de la dépres- sion Treska-Vardar où l’alignement des sédiments quaternaires vane de 4 à 5 mètres de hauteur à l’ouest, à 10 à 20 mètres à l’est. Les cuvettes anciennes sont les plus actives.

I

6. Sur la base des recherches déjà effectuées, est en train de s’achever la carte de la structure géomorphologique des environs de Skopje sur laquelle on distingue des zones de différentes directions, des zones à contrastes et des amplitudes de mouvement. On y indique l’âge de la formation de ces zones, le rapport avec la structure jeune ou ancienne et les daérentes étapes de l’activité tectonique.

7. Les mouvements tectoniques jeunes et la haute séismicité de cette région rendent

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indispensable l’établissement d’un polygone spécial géophysique dans la région de Skopje, pour l’étude des mouvements actuels. A l’heure présente, des polygones complets pour l’étude des mouvements tectoniques actuels sont installés ou en voie d’installation dans beaucoup de pays : URSS (Crimée, Caucase, Turkestan, Kirghizie, Baïkal, etc.), Etats-Unis d’Amérique, Japon, Bulgarie, Roumanie, Tchécoslovaquie, etc. Depuis le tremblement de terre du 26 juillet 1963, La région de Skopje intéresse le monde entier, et représenterait un lieu idéal pour l’établisse- ment d’un tel polygone. I1 est recommandé que les études qui y seraient effectuées puissent faire partie du programme de l’Institut de séismologie de Skopje. Les recherches doivent avoir un caractère complexe. I1 est nécessaire de constituer un réseau géodésique auxiliaire de première classe, dans lequel on pourrait inclure les points les plus stables possédant des bornes géodésiques. Dans tous ces réseaux il est recommandé d’effectuer des recherches sur les conditions morphologiques des bornes installées. Dans l’implantation des nouvelles bornes, il est recommandé de ne pas perdre de vue l’étude des conditions géologiques et morphologiques de la région avec des bornes différentes pour l’étude des mouvements continus et des failles jeunes et anciennes. Dans la structure morphologique de base, y compris l’axe montagneux de Skopje, il faut installer des repères avec socle. Les mesures doivent être faites chaque année, mais dans un cas de séisme important, il est recommandé de les effectuer immédiatement après le mouvement. I1 est souhaitable de procéder à l’agrandissement du réseau polygonométrique de la ville, en y incluant le Grand Skopje ainsi que les collines au nord de la ville. Le long des zones possédant des failles actives (Koumanovo, Skopje, Katlanovo, Kisela Voda, Brada), il faut installer un réseau de repères spéciaux pour l’étude des mouvements verticaux ainsi que pour l’étude des mouvements horizontaux dont l’intérêt scientifique et pratique est considérable. Dans les études géomorpholo- giques à entreprendre, il faut inclure l’observation des changements dans le processus géomorphologique actuel, ainsi que l’étude de la sédimentation, des signes d’augmentation de sédimentation et des signes d’augmentation de mouvement le long des failles. I1 faut réunir les données détaillées concernant la géologie, la tectonique et la géomorphologie, et amorcer immédiatement des études complexes des zones sud-est et nord de la ville. Tous les éléments obtenus par ces études pourront donner des informations

précises sur l’accumulation des tensions dans les couches supérieures de l’écorce terrestre ainsi que les éléments pouvant servir à établir des pronostics de séismicité. Les savants soviétiques peuvent contribuer à l’élaboration détaillée d’un programme pour l’étude des mouvements tectoniques actuels à Skopje et contribuer à la formation de cadres par l’établissement d’une méthodologie scientifique appropriée.

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J. A. Mechtcherikov Institut de physique de la terre, Académie des sciences de l’URSS, Moscou

Mouvements actuels de l’écorce terrestre et leur étude dans la région de Skopje (résumé)

1. L’étude des mouvements récents de l’écorce terrestre est un problème actuel pour la géophysique, d’une grande valeur théorique et pratique. Pour favoriser le développement de ces recherches dans le cadre de l’Union internationale géodésique et géophysique (UGGI), une commission permanente a été créée, avec la participation de savants de 28 pays. A la 13e assemblée générale de l’UGGI, tenue à Berkeley, aux États-Unis, en 1963, un projet international de recherche “Les mouvements actuels de l‘écorce terrestre”, a été adopté par ASSO SO- ciation internationale de géodésie. Le projet prévoit : u) l’élaboration d’une carte mondiale des mouvements actuels

de l’écorce terrestre, à commencer par des cartes d’Europe et d’Amérique du Nord; b) un réseau mondial de polygones spéciaux pour l’observation des mouvements de l’écorce terrestre dans les différentes zones tectoniques et dans différents *

continents. L‘observation dans les polygones doit s’effectuer par diverses méthodes (méthodes géodésiques, géophysiques et géomorphologiques). On a prévu un échange international des matériaux de recherches. A la confé-

rence des géophysiciens des pays socialistes tenue à MOSCOU, au mois de mai 1964, on a adopté des résolutions pour créer un réseau de polygones d’observation des mouvements de l’écorce terrestre, pour élaborer des cartes des mouvements actuels dans différents pays et une carte de toute l’Europe de l’Est. I1 est souhaitable que la Yougoslavie prenne part à cette recherche internationale en raison des travaux entrepris dans, la région de Skopje.

2. L’étude des mouvements actuels de l’écorce terrestre a une grande importance pour l’analyse de la séismicité et pour l’estimation de la sécurité des grandes constructions. Les mouvements rapides (séismiques) et lents (séculaires) de l’écorce terrestre sont de toute évidence étroitement liés. Les mouvements lents qui ne sont pas sentis par l’homme (élévation, dépression, déplacement de l’écorce terrestre) se produisent partout et sans cesse, bien qu’avec une intensité variable dans les différentes régions. Les chocs séismiques sont observés dans les zones des déformations, là où

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Etudes géologiques et séismologiques dans la région de Skopje

s’accumulent les tensions suffisantes dans l’écorce terrestre. Pour bien connaître les conditions dans lesquelles se produisent les forts tremblements de terre, semblables à celui du 26 juillet 1963 à Skopje, il est nécessaire d’étudier les défor- mations de la surface terrestre qui se développent lentement. Des données sur les mouvements actuels de l’écorce terrestre ne peuvent être

obtenues qu’en utilisant des méthodes complexes : a) des méthodes géodésiques (déterminations répétées des altitudes et des distances à l’aide de nivellements, de triangulations et de mesures linéaires) ; b) des méthodes géophysiques (observation des inclinaisons, déterminations répétées extrêmement précises de la force de la pesanteur, etc.) ; c) des méthodes géomorphologiques (étude des formes les plus jeunes du relief et des sédiments). Dans la région de Skopje, les données géodésiques et les études géomorpholo-

giques ont révélé l’existence de mouvements récents, aussi bien verticaux qu’horizontaux, de l’écorce terrestre. Ces mouvements sont très contrastés; même à l’intérieur du polygone de la ville de Skopje, ils ont des directions et des vitesses différentes (déplacements verticaux entre 18 et 190 millimètres pendant la période de 1937 à 1963, déplacements horizontaux entre 40 et 80 millimètres). Un intérêt particulier s’attache à la découverte de quelques failles récentes et l’activité tectonique holocène.

3. I1 est nécessaire de continuer à mener systématiquement à l’avenir les recherches sur les déformations lentes de l’écorce terrestre dans la région de Skopje, dans les buts suivants: a) La construction d’une carte des mouvements actuels de l’écorce terrestre

(élévation, affaissement, déplacements horizontaux) dans la vallée de Skopje, et ensuite dans toute la Macédoine. La préparation de Sette carte exigerait des

. mesures répétées du réseau géodésique d’appui de premier et deuxième ordre (nivellement, triangulation). Pour obtenir les valeurs absolues des mouvements verticaux, les nivellements répétés doivent être liés au niveau de l’Adriatique et à d‘autres mers. A l’avenir il sera nécessaire de répéter les observations géodésiques d’appui après sept à dix ans. Aussi est-il souhaitable d’étendre le réseau au sud-est de la ville. b) La création dans la région de Skopje d’un polygone géophysique pour

l’observation permanente de la “vie” des structures séismiques actives par des méthodes géodésiques, géophysiques et géomorphologiques. Dans ce but il serait nécessaire d’installer des repères spéciaux sur les deux côtés des cassures jeunes (actives), de répéter systématiquement les mesures géodésiques, et de faire des photos aériennes répétées. Des recherches détaillées et systématiques de ce genre pourraient être entre-

prises, par exemple dans la région de Brazda, autour de la faille de Kisela Voda, autour de l’usine sidérurgique, dans la région de Matka et dans la vallée du Treska. D e telles recherches sur les mouvements lents de l’écorce terrestre, par des

‘ méthodes géodésiques, géophysiques et géomorphologiques, pourraient s’inclure dans le programme de travail du futur Institut de séismologie à l’université de Skopje.

C

Earthquake engineering studies in Skopje Études séismotechniques effectuées dans la région de Skopje

J. Despeyroux Ingénieur civil des ponts et chaussées, délégué des commissions techniques du Bureau Sécuritas, Paris

Les enseignements du séisme de Skopje du 26 juillet 1963

L'observation in situ des constructions éprouvées par un tremblement de terre constitue une des principales sources de connaissances dont nous disposons en matière de génie sismique. Contrairement à ce qui concerne la résistance des constructions aux sollicitations normales, domaine dans lequel nous bénéficions d'une expérience quotidienne, notre expérience pratique de leur résistance aux sollicitations d'origine sismique ne peut progresser que de façon discontinue, à l'occasion des séismes destructeurs qui surviennent ici ou là dans le monde. C'est dire l'importance que revêtent, pour l'avancement du génie sismique, les missions d'enquête sur place après séisme. Des semblables études ne doivent évidemment pas se limiter à l'enregistrement

des observations et à la seule accumulation de faits matériels; il convient, si l'on désire en tirer des leçons et des moyens d'action pour l'avenir, de confronter à chaque instant les phénomènes observés aux résultats indiqués par la théorie, de façon à leur trouver une explication rationnelle, les rattacher à un corps de doctrine cohérent et, le cas échéant, dégager les données nouvelles susceptibles d'influer sur la théorie. L'interprétation correcte des phénomènes implique, préalablement à l'analyse du comportement des constructions, une importante contribution à l'enquête macrosismique, notamment en ce qui concerne l'évaluation de l'intensité avec laquelle la secousse a été ressentie; elle nécessite également que tous les faits soient replacés dans leur contexte séismotectonique et géologique. L'étude du séisme de Skopje du 26 juillet 1963 s'est révélée instructive sous bien

des rapports.

CONTEXTE SÉISMOTECTONIQUE ET GÉOLOGIQUE

Tectonique de la Macédoine

La Macédoine se présente comme une zone de montagnes coupées de profonds fossés d'effondrement. Les massifs anciens du Rhodope et du Pelagon sont en

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Études séismotechniques effectuées dans la région de Skopje

voie de fracturation et leur instabilité est aggravée par la proximité de la charnière articulant les chaînes des Karpates et des Balkans. La dépression Morava-Vardar, en particulier, est constituée par une longue chaîne de fossés tectoniques encadrés de hautes montagnes. Parmi ces derniers, figure le bassin de Skopje. La géologie du bassin de Skopje est dominée par la présence de l’accident

constitué par le sillon du Vardar. I1 s’agit d’une étroite bande de terrains méso- zoïques comprimée entre les massifs schisteux anciens du Rhodope et du Pelagon. Cette bande se développe de la mer Egée aux environs de Belgrade. L‘activité sismique de cet accident est confirmée par les séismes de Rudnik

(Serbie), en 1927 (intensité IX), et de Valandovo, sur le Vardar (à 120 kiIomètres au sud-est de Skopje), en 1931.

Géologie de Skopje

En dehors du grand accident dont il vient d’être question, il convient de citer les accidents locaux constitués par les failles découpant dans le Vodno des graben orientés ouest-sud-ouest - est-nord-est. Dans la plaine, ces failles disparaissent sous des épaisseurs considérables de dépôts ultérieurs. Ces failles n’ont pas rejoué lors du séisme du 26 juillet 1963. Les dépôts néogènes (argiles, grès et marnes) affleurent au pied du Vodno, sur

la rive droite, et surtout sur la rive gauche où ils ont donné l’éperon de la Citadelle. Dans la vallée elle-même, ils ont été enlevés par le Vardar et remplacés par des alluvions quaternaires récentes. D’une façon générale, il existe une certaine opposition entre les rives droite et

gauche : d’un côté, les alluvions quaternaires hétérogènes se présentant sous forme de lentilles confuses en épaisseur pratiquement indéfinie, de l’autre, des affleure- ments ordonnés de dépôts néogènes pouvant éventuellement servir d’appui à des formations récentes. Les niveaux des nappes phréatiques sont également différents : alors que la nappe ne remonte pas à moins de 6 mètres de profondeur sous les parties bâties de la rive gauche, on la rencontre souvent à 2 mètres sur la rive droite.

ASPECTS MACROSISMIQUES DU SEISME .

Comme le séisme d’Agadir, le séisme he Skopje du 26 juillet 1963 a été un séisme de puissance relativement faible puisque sa magnitude, suivant diverses évaluations, est comprise entre 5,8 et 6,O. Il n’a dû d’être aussi sévèrement destructeur qu’à la proximité de l’agglomération par rapport à l’épicentre (10 kilomètres environ). La destruction plus ou moins complète des habitations traditionnelles archaïsan-

tes à base de matériaux locaux assez pauvres n’apporte aucune indication inté- ressante sur la distribution des intensités à l’intérieur de l’agglomération. Par contre, la considération des dommages occasionnés aux autres catégories de constructions permet de considérer, compte tenu des facultés de résistance que l’on peut attribuer à chacune d’elles, que la secousse a été ressentie avec une

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J. Despeyroux Les enseignements du séisme de Skopje du 26 juillet 1963

intensité IX (MM) au nord d’une ligne définie en gros par la route de Tetovo, la périphérie du quartier de Bunjacovac, la voie ferrée de NiS à Ohrid. Au sud de cette ligne l’intensité n’est plus que de VIII. On retrouve cependant une enclave d’intensité IX sur les pentes du Vodno

(Faculté de médecine, hôpital) et inversement dans la zone IX une enclave d’intensité VI11 correspondant à la colline néogène dominant la rive gauche du Vardar jusqu’au col la séparant de l’éperon de la Citadelle. La notion d’intensité est au demeurant assez imprécise. I1 convient de signaler

que des modifications ont été proposées à l’échelle de Mercalli modifiée en vue de rendre moins subjectives les appréciations. I1 serait intéressant de reprendre l’étude, si toutefois la chose est encore possible, avec la nouvelle échelle.

COMPORTEMENT DES CONSTRUCTIONS DE DIVERSES NATURES A SKOPJE

Les constructions rencontrées dans la ville de Skopje peuvent en général se ranger dans l’une des cinq catégories suivantes : a) constructions traditionnelles archaïsan- tes; b) constructions en maçonnerie classique plus ou moins anciennes; c) constructions récentes mixtes maçonnerie - béton armé; d) immeubles à ossature en béton armé; e) constructions spéciales.

Constructions traditionnelles archaïsantes. Leur comportement n’apporte aucun élément d’information nouveau ou intéressant. Nous nous limiterons à l’étude du comportement des autres catégories.

Constructions en maçonnerie classique. Le séisme de Skopje a largement. confirmé ce que l’on savait déjà et ce que laisse prévoir la théorie au sujet du comportement des constructions de ce type: il s’agit en général de constructions massives, rigides, donc de période propre courte, et, comme telles, susceptibles de réagir de façon très sèche à l’ébranlement sismique. Le défaut de résistance à la traction les sensibilise encore à cette action.

Constructions mixtes maconnerie - béton armé. 11 s’agit de constructions dans lesquelles les éléments porteurs sont constitués par des murs en maçonnerie (le plus souvent de briques) recevant des planchers en béton armé. Ces constructions présentent évidemment par rapport aux constructions en

maçonnerie classique l’avantage de la présence des planchers, qui constituent autant de diaphragmes horizontaux susceptibles de solidariser à certains niveaux les organes porteurs ou de contreventement.

Il subsiste cependant dans ces ouvrages une grave faiblesse : l’absence d’élé- ments verticaux susceptibles d’équilibrer des efforts de traction. Certains constructeurs, se basant sur une analyse défectueuse des efforts agissant

dans les panneaux de maçonnerie, nient l’utilité de ces éléments dans la construction antisismique. Le comportement des immeubles de Karpos met un point final à la controverse : les chaînages verticaux sont aussi indispensables que les chaînages horizontaux.

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I1 convient de remarquer également que le séisme de Skopje a illustré en plusieurs occasions le fait qu’en l’absence de chaînages verticaux, les murs de refend, c’est-à-dire des organes dont la raison d’être est de stabiliser les bâtiments, peuvent au contraire se transformer en dangereux instruments de ruine en projetant dans la rue les façades qu’ils ont normalement pour mission d’épauler.

Constructions à ossature en béton armé. On peut noter que, d‘une façon générale, les effondrements complets de structures en béton armé ont été très rares et tiennent à des circonstances spéciales. Les défaillances constatées sont le plus souvent locales et n’ont pas, en général, entraîné de mort d’homme. On note même de nombreux bâtiments, surtout parmi les immeubles élevés, qui ont remar- quablement supporté la secousse. Une grande partie des défaillances observées correspond à des manquements

plus ou moins graves aux règles de l’art. I1 est permis de penser qu’une stricte observation des règles de l’art eut considérablement réduit le nombre, ou diminué la gravité, des sinistres constatés. Le comportement favorable des immeubles élevés est en bon accord avec la

théorie qui, pour les séismes à épicentre rapproché, attribue aux structures de longue période propre, des réactions plus modérées que dans le cas des structures rigides. La présence de systèmes de contreventement efficaces correctement étudiés à l’égard des sollicitations dues à l’action du vent est également à l’origine des résultats satisfaisants constatés.

Constructions spéciales. Nous rangeons dans cette catégorie les ouvrages très élancés, tels que cheminées d’usine, minarets, et les structures spéciales rappelant les ouvrages d’art (voiles minces, etc.). L‘examen des structures élancées est particulièrement intéressant en ce sens

qu’il met en évidence l’intervention de sollicitations mettant en jeu les modes supérieurs de vibration, comme le fait prévoir la théorie.

INCIDENCE DES CONDITIONS D E FONDATION

On sait l’importance que jouent les conditions de fondation dans la résistance d’une structure aux secousses telluriques. En règle générale les constructions sur sol meuble se trouvent placées en situation plus défavorable que les constructions sur sol ferme. Cela tient d’une part au fait que, toutes choses étant égales par ailleurs, les amplitudes du mouvement du sol sont plus grandes dans le cas des sols meubles que dans le cas des sols rocheux, d’autre part au fait que, dans le cas des sols meubles, les fondations sont exposées à subir des déplacements diffé- rentiels qui introduisent dans la structure des efforts parasites au moment même où elle a besoin de mobiliser toutes ses facultés de résistance. Pour illustrer ce point de vue, on peut faire les remarques suivantes: a) il existe

une différence, pas toujours très sensible, étant donné la diversité des constructions, mais réelle, entre les quartiers situés au-dessus du quaternaire en profondeur

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I. Despeyroux Les enseignements du séisme de Skopje du 26 juillet 1963

pratiquement indéfinie, et ceux situés sur les alluvions plus fermes ou sur les formations du néogène ; b) certains effondrements se groupent en zones préféren-- tielles; le cas est particulièrement net à Karpos, où des bâtiments effondrés s’ali- gnent curieusement sur une même bande de terrain, au milieu de bâtiments simi- laires beaucoup moins sévèrement atteints. Ces différences de comportement s’expliquent par le fait que ces bâtiments sont situés sur d’anciens thalwegs dans le lit majeur du Vardar, comblés par des alluvions peu résistantes; c) les constructions dotées de fondations profondes, comme les mosquées, et surtout leurs minarets, ont eu en général un comportement plus satisfaisant qu’on eût pu l’attendre: d) les effondrements et les graves dommages observés dans la Maison de l’armée et la Banque nationale sont dus en grande partie au fait que leurs fondations ont été affaiblies par des pompages inconsidérés pratiqués dans la nappe phréatique les entourant.

REMARQUES CONCERNANT LES CARACTERISTIQUES DE LA SECOUSSE ET COMPARAISON AVEC LES PHÉNOMÈNES OBSERVES A U COURS D’AUTRES SEISMES RECENTS

Les séismes d’Agadir et de Skopje

Nous avons eu déjà l’occasion de signaler que le séisme de Skopje du 26 juillet 1963 présentait de grandes similitudes avec celui qui détruisit Agadir le 29 juillet 1960. I1 s’agit dans les deux cas de séismes de faible magnitude mais cependant sévère- ment destructeurs du fait de la faible distance existant entre l’épicentre et la localité éprouvée. Dans les deux cas, on est frappé par le fait que les constructions basses en

maçonnerie, et d’une façon générale les constructions rigides de courte période propre, ont subi des dommages considérables, alors que les constructions souples et élevées ont, comparativement, beaucoup moins souffert. Cette différence de comportement traduit la présence dans l’ébranlement moteur d’une forte proportion de composantes que nous dirons de “haute fréquence” (3 à 6 hertz), et d’une moindre proportion de “basses fréquences” (moins de 1 hertz).

Les séismes de Mexico et d’Alaska

Si par contre on se réfère à l’état des destructions dans la ville de Mexico lors du séisme du 28 juillet 1957, on constate que la situation est inversée par rapport à Agadir ou Skopje: ce sont les constructions élevées qui ont le plus souffert, alors que les dégâts causés aux constructions basses apparaissent comme très modérés par rapport à ceux subis par les immeubles hauts. On se trouve en présence d’un mouvement du sol dans lequel, au contraire de ce qui a été observé à Agadir et à Skopje, les composantes basse fréquence l’emportent sur les composantes haute fréquence. L‘étude du séisme d’Alaska du 27 mars 1964 fait ressortir une situation comparable. Au cours de ce tremblement de terre, à Anchorage,

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Études séismotechniques effectuées dans la région de Skopje

de larges crevasses se sont ouvertes dans le sol. Les constructions basses ont été sévèrement éprouvées toutes les fois qu’elles se sont trouvées sur le trajet ou à proximité d’une de ces crevasses. Par contre, elles sont restées pratiquement indemnes dès que, se trouvant à quelque distance des zones éboulées, elles n’ont été soumises qu’au mouvement communiqué par le sol de fondation. Inversement, les immeubles élevés, encore qu’ils n’aient donné lieu qu’à de très rares effondrements, ont été très sévèrement éprouvés, dans une proportion qui contraste de façon saisissante avec la bonne tenue des constructions basses. On se trouve donc en présence d’un cas OU les composantes haute fréquence

étaient pratiquement inexistantes, et où, par contre, les composantes basse fré- quence se sont trouvées suffisamment bien représentées pour mettre en oscillation les bâtiments de longue période propre.

Discussion

Si l’on compare les séismes d’Agadir et de Skopje d’une part, ceux de Mexico et d’Anchorage d’autre part, on peut résumer la question en remarquant, pour les deux premiers: qu’il s’agit de séismes de faible magnitude (5,75 et 5,9 respectivement) dans lesquels l’ébranlement sismique n’avait à parcourir, pour atteindre la localité, qu’une distance courte (3 kilomètres et 10 kilomètres respectivement) à travers des terrains fermes, le foyer se trouvant dans les deux cas à une faible profondeur (3 kilomètres de la surface environ); pour les deux derniers: qu’il s’agit au contraire de séismes de grande magnitude (73 et 8,3 respectivement) dans lesquels l’ébranlement n’a atteint les localités dévastées qu’après un long parcours (environ 300 kilomètres dans le cas de Mexico et 100 kilomètres dans le cas d’Anchorage). I1 s’agissait de foyers assez profonds. Les localités dévastées étaient situées sur de très importantes formations de sols meubles. On peut imaginer que la composition spectrale de l’ébranlement originel est

différente suivant qu’il s’agit d’un séisme de petite ou de grande magnitude, ou d’un séisme de foyer profond ou superficiel. Mais cela n’est qu’une hypothèse sans fondement. Il est certainement plus juste de considérer d’une part que les séismes de grande

magnitude sont des séismes d’une durée relativement longue, en rapport avec la quantité d’énergie libérée: en ce cas, les composantes de basse fréquence disposent d’un plus grand nombre de cycles pour amener le sol et les constructions plus près de leurs conditions de résonance; d’autre part qu’au cours de la propagation, les sols rencontrés, et spécialement les sols meubles, jouent le rôle de filtres sélectifs étouffant les composantes haute fréquence, et laissant passer pratiquement sans amortissement les basses fréquences. Enfin il n’est pas exclu que, sur de longues distances, l’intervention des ondes

de surface (de grande période) ne vienne contribuer à détériorer la situation au détriment des immeubles très hauts.

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J. Despeyroux Les enseignements du séisme de Skopje du 26 juillet 1963

CONSEQUENCES POUR LA DEFINITION DES INTENSITES

Les exemples qui précèdent montrent en quoi la définition actuelle des intensités sismiques est critiquable: c’est que l’on est conduit à attribuer à la secousse des intensités différentes suivant les propriétés dynamiques de la classe de constructions à laquelle on s’intéresse. Dans le cas d’Agadir ou de Skopje, on est conduit à parler d’intensités très élevées, si on se base sur les destructions dans les constructions basses. Inversement, dans le cas d’Anchorage, on est amené à des estimations dérisoires suivant ces mêmes critères. On serait donc exposé à des erreurs considérables dans le cas de localités où ne

serait représenté qu’un seul des types de constructions ci-dessus. Cela met en évidence l’impossibilité qu’il y a, à priori, de caractériser par un seul nombre - l’intensité - un phénomène aussi complexe que l’ébranlement sismique. Les modifications de l’échelle d’intensité récemment proposées apportent une

amélioration appréciable en ce sens qu’elles tiennent compte, beaucoup mieux qu’on ne le faisait par le passé, de la qualité des constructions et de leur aptitude à supporter une secousse tellurique. II conviendrait à notre avis, pour être complet, de diviser, à priori, les constructions en un certain nombre de classes (par exemple trois classes correspondant respectivement à des fréquences propres de l’ordre de 10 à 3 hertz, 3 à 1 hertz, moins de 1 hertz) et, pour les intensités destructives, de donner les évaluations correspondant à chacune des trois classes. Ces indications reflèteraient d’une certaine manière la forme du spectre de réponses.

Discussion O. Werner

J. Despeyroux

O. Werner

J. Despeyroux

F. CaEovii:

Possède-t-on une expérience en maçonnerie renforcée seulement avec les éléments verticaux?

Des bâtiments à rez-de-chaussée, ou à rez-de-chaussée plus un étage, ainsi traités, ont été éprouvés à Bougainville (Algérie) par deux séismes d’intensité VI11 survenus en 1958. Leur comportement a été satisfaisant, les désordres étant limités à de légères fissurations. Inversement, pour quelques autres maisons qui n’avaient pas été convenablement traitées, les dégâts ont été beaucoup plus graves.

Ces armatures verticales sont-elles reliées par des liens aux murs adjacents?

Les chaînages horizontaux et verticaux forment un réseau à trois dimensions.

W e cannot make such a negative statement, regarding masonry buildings only, because many of them collapsed. W e must bear in mind that in most cases no static analysis with respect to the horizontal load had been made. On the other hand, earthquakes with high frequencies of ground motion-such as happened in Skopje-are the most dangerous for stiff masonry buildings. Vertical reinforced columns are one, but not necessarily the best way to prevent the failure of damaged buildings

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Etudes séisrnotechniques effectuées dans la région de Skopje

(the brick walls will crack before the reinforcement comes in action). W e can achieve the same result with reinforced bars placed horizontally or with pre-stressing. Further research is needed on this problem.

I1 n’est pas question d’interdire l’utilisation des maçonneries porteuses, mais de pallier les graves défauts que présente ce matériau, en particulier son défaut de résistance sensible à la traction, et surtout le caractère fragile de sa rupture. Les chaînages verticaux liés aux chaînages horizontaux constituent une solution qui n’évite peut-être pas la fissuration des panneaux de maçonnerie, mais maintient des possibilités de résistance après fissuration. La mise en place d’armatures dans les joints pourrait constituer une solution, mais son efficacité peut être considérablement diminuée, ou même annihilée par la sen- sibilité de ces panneaux aux effets d’un excentrement des charges verticales et leur aptitude au flambement: l’ouverture d’une fissure horizontale, ou même la mise en traction d’une face du panneau, peut détruire l’adhérence de ces armatures et les rendre inutiles.

L’expérience du tremblement de terre du 10 novembre 1940 à Bucarest confirme les points de vue de M. Despeyroux. Un bâtiment ayant rez-de-chaussée et six étages, construit en 1928 avec la maçonnerie portante et avec des chaînages verticaux en béton armé, espacés de 6 à 8 mètres, s’est comporté très bien pendant le tremblement de terre de 1940. Même pendant la guerre, quand il a rqu des secousscs et des vibrations à la suite des bombardements des édifices avoisinants, il n’a pas eu à en souffrir. D e même, l’édilìce de la nouvelle Banque nationale qui a été bombardé pendant la guerre s’est très bien comporté, du fait que la maçonnerie était exécutée en même temps que le béton armé. Vu les dimensions respectables des maçonneries imposées par des considérations d’architecture, celles-ci se comportaient comme des maçonneries portantes. Cela a été mis en évidence au cours du bombardement de 1944.

J. Despeyroux

A. Beles

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K. HoloEev Skopje Dj. Solovjev Sarajevo


The earthquake which struck Skopje on 26 July 1963 caused great destruction because it occurred in a town which at that time numbered about 200,000 inhabitants. The consequences expressed in human as well as in material losses were therefore very severe. During this century strong earthquakes have occurred in the Vardar river

valley (catastrophic earthquake in Valandovo), and in the vicinity of Belgrade and Ljubljana. It has to be noted that almost nothing was done to study this problem . The catastrophe in Skopje has changed this situation and the Association of

Yugoslav Laboratories was requested by the Federal Government to collect extensive technical documentation on the damage to structures, in order to arrive at conclusions regarding the influence of earthquakes on construction. The Materials Testing Institutes of Skopje, Belgrade, Zagreb, Sarajevo and

Ljubljana collected the technical information under the leadership of the Skopje Institute. The collection of technical documentation is now complete and includes the thirty-five volumes exhibited at this seminar. These contain documentation on 266 different structures which were surveyed and studied. The more detailed survey of thirty-two structures involves structural analysis and comparison with the real behaviour of the construction. W e hope that this work will be finished by the end of this year and that the experience obtained will serve in the preparation of the new regulations for earthquake-resistant construction. W e have to mention, regarding earthquake effects on individual structures,

that the material from the Skopje earthquake is so extensive that we can give in this short and limited report only a few of the most important and most characteristic details. The influence of the earthquake is considered in relation to constructions,

public works and buildings. No severe damage occurred to public works. The water supply and sewage

systems suffered little damage and they continued to work normally except where affected by the collapse of buildings. Reinforced-concrete bridges and railway

1 1 1

Earthquake engineering studies in Skopje

culverts did not suffer any damage. The steel railway bridge over the Vardar river suffered horizontal displacement only in its upper part. The railway line was undamaged. Roads did not suffer damage except ZelezniEka Street, opposite the railway station, where small deformations and waving was remarked. A geodetic survey has shown slight displacements of the ground in certain places. The conclusion can thus be drawn that no remarkable displacements of the ground occurred. Buildings in Skopje present a great variety of floors, construction systems,

materials and ages. The majority of them suffered significant damage, and some were partially or completely destroyed. According to the data gathered by the Statistical Institute of the Socialist

Republic of Macedonia, the damage to housing may be expressed in terms of the percentage of flats, living space and population affected by the earthquake, as follows :

. Flats Living space Population Degree of damage damaged affected affected

(7%) (%) PL)

Collapsed 8.5 , 7.05 8.5 Heavily damaged (most buildings have to be demolished) 33.6 29.9 36.4

Moderately damaged 36.3 39.9 30.6 Slightly damaged 19.0 19.8 20.3 Undamaged 2.6 3.4 4.2

Different types of construction behaved differently in particular zones of the town. However, general characteristics of the earthquake effects may be summarized. All buildings can be divided into the following types, according to their purpose,

type of construction, building material used and their dimensions : L. Buildings in light material:

(u) adobe; (b) adobe or brick buildings with wooden beams.

(u) old buildings with wooden slabs; (b) old buildings with reinforced-concrete slabs ; (c) new buildings with transverse bearing walls; (d) new buildings with longitudinal bearing walls; (e) buildings with bearing walls in both directions.

3. Combined system buildings, that is to say a combination of bearing walls made of brick with reinforced-concrete columns.

4. Reinforced-concrete frame buildings with brick filling. Here a distinction has to be drawn between housing skyscrapers and usual skeleton buildings for public or housing purposes. Some of the skyscrapers have a rigid core made

2. Massive buildings :

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K. HololEev Dj. Solovjev

The & m c e of the earthquake of 26 July 1963 on constructions in Skopje

of massive concrete around stair. walls, while others have no particular rigidity points.

5. Industrial and exhibition halls. Concrete, pre-cast concrete and steel structures are included in this category.

6. Structures for special purposes, such as chimneys, mosques, churches, theatres and similar buildings.

Because the shock effects were quite different for different types of structure, it is necessary to give a short description of characteristic. damage suffered by each type.

BUILDINGS IN LIGHT MATERIAL

Most old buildings of one storey are built of adobe. Some old buildings of one or two storeys have wooden beams with either adobe or brick filling. These buildings are mostly located in the old part of the town on the left bank of the Vardar river. The adobe constructions are built in a most primitive way without the use of

straw or any similar material in making the adobe, Earth mortar is used as a junction material without straw. It is understandable that such structures did not possess even a minimal resistance to earthquake, and they nearly all collapsed. Concerning buildings with wooden beams, it can be remarked that some of

them are not classical structures with all necessary rigidity points; rather the rigidity is provided by the filling walls. In these cases the damage was significant especially where the wooden skeleton was badly placed. On the other hand, buildings with complete wooden frames usually stood up to the shock. In the cases where the skeleton was correct, partial collapse of filling, which is mainly of weak material, often occurred.

MASSIVE BUILDINGS

The majority of housing and public buildings in Skopje date from the period between the two World Wars or after the Second World War. They are made of bricks with more or less use of reinforced concrete and they possess the characteristics of massive buildings. Because of the variety of types and forms of massive buildings, the damage to them will be considered under five sub-headings. Old massive buildings, of one or two storeys, made of bricks with lime mortar

without concrete or reinforced concrete. These buildings have wooden beam ‘slabs supported by the walls and as a rule they have neither concrete collar-beams nor iron junctions. Buildings of this type were constructed mainly between the two wars and exceptionally after the Second World War. Their resistance to earthquake shocks is not great and in spite of relatively

light slabs and small height they suffered heavy damage. Their main weakness is

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insufficient tying between walls because of the lack of collar-beams, so that the damage was produced partially by the break between walls and by the subsequent collapse of the slabs. Old massive buildings, of one or two storeys, made of brick with lime mortar

and with reinforced-concrete slabs. For buildings of this type concrete collar-beams were used for wall junctions, with or without some slight reinforcement. These buildings were also constructed between the two wars and after the Second World War. The system of construction of these buildings cannot be included in any of the usual groups without difficulty. It is characteristic of them that their walls are always 38 cm thick regardless of the load they bear, while the internal sections are used freely as bearing or non-bearing walls with different thicknesses. The behaviour of these buildings varied from structure to structure, and really

depended on the position of the bearing walls with respect to the direction of earthquake wave propagation, the quality of building material and local hydrogeological conditions. In principle these buildings behaved better than the previous group and only exceptionally have they collapsed. The damage is characterized mainly by cracks in the walls, damage to inter-window columns and destruction of the corners of the structure.

New massive buildings with transverse bearing walls. These buildings were designed and constructed after the Second World War and they represent the most widespread type of housing in Skopje. Whole communities as well as whole new buildings in the town itself were built in this way. These buildings are mainly of three or four storeys plus attic. From the construction point of view, these buildings are characterized by

transverse bearing walls of brick in lime mortar. The thickness of the walls in the older buildings is 38 cm, while that in new buildings, for instance in Karpog II community, is only 25 cm. The external transverse walls are also 25 cm thick, while the middle wall is usually only 12 cm thick. Partition walls within flats are only 7 or 12 cm thick. The slabs are mostly of semi-prefabricated reinforced concrete, reinforced brick

or concrete supporters with concrete block filling, Monolithic concrete slabs are rarer. The buildings usually have collar-beams, although the amount of reinforcement in most cases is minimal. Stairs are made of reinforced concrete, with consoles or connecting plates. Longer buildings are divided by dilatational joints. The degree and type of damage to these structures depends mainly on the orien-

tation of the transverse bearing walls with respect to the direction of wave propagation. If the bearing walls were oriented nearly parallel to the direction of propagation of the earthquake wave, the structure suffered greater or less damage, depending on other factors, but as a rule did not collapse. Typical damage and deformation of these structures was as follows: oblique crossed cracks in bearing walls, sometimes so wide that the longitudinal walls left their vertical position. Especially severe damage occurred to transverse walls where they were not connected to the façade walls. The wall mass collapsed outwards in triangular form. The same phenomenon was observed at the façade corners.

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Particular damage occurred to walls which were weakened by the cutting of electrical conduits or chimneys. On the other hand, identical or similar structures in which the bearing walls

were oriented across the earthquake wave propagation vectors suffered much heavier damage or collapsed. It can even be said that most collapsed constructions were of this type and with this orientation of the bearing walls. This is well illus- trated in the Karpos' community, where there were identical buildings oriented in both directions. All collapses with tragic consequences occurred to buildings in which the seismic wave struck directly on the bearing walls. The thin middle wall, 12 cm thick, provided practically no rigid support for the bearing walls and collapse of the buildings followed the failure of the bearing walls. It is characteristic even of longer structures of this type, divided into several sections, that the eastern end section collapsed while the other sections remained standing, sometimes even without serious damage. Even when buildings oriented in this direction did not collapse, they suffered

very severe damage, mostly in the form of oblique or crossed-oblique cracks in longitudinal walls, displacement of inter-window columns and bending of transverse walls, so that ground-floor windows took the form of rhombs. With regard to the distribution of damage with height in buildings of up to

four storeys, the worst damage usually occurred at the ground floor. In higher buildings, besides heavy damage to the ground floor, the first storey was also heavily damaged, sometimes more seriously than the ground floor itself. It was characteristic that, even in the most heavily damaged buildings, the slabs

remained ' practically intact regardless of whether they were monolithic, semi- prefabricated based on reinforced-concrete supports, reinforced brick, or concrete slabs with block filling. This leads us to the conclusion that the damage was produced exclusively by the collapse of walls, either because of their dimensions or quality weaknesses, or because of their lack of rigidity. The cellars in these buildings remained always completely without damage.

This was valid for all buildings in Skopje, even for those which collapsed. Cellar walls were as a rule made of massive concrete. The roof constructions, if correctly made, did not suffer any damage. If they

were non-rigid or badly made, the whole roof collapsed. Gable walls enclosing roofs were destroyed in many places. New massive buildings with longitudinal bearing walls. This type of construction

is less common than the buildings described previously. They represent less housing and much more public buildings. It is characteristic of this type of building that the slabs represent longitudinal

walls made of bricks mainly in lime mortar, usually 38 cm thick but sometimes 25 cm. Longitudinal rigid walls do not always exist and it is usually the case, especially in schools, that the section walls are made of half-brick, sometimes without connexion to the bearing walls. The slabs are of the same type as in buildings previously described. In these buildings the inter-window columns, which are usually made of brick,

are often of insufficient dimensions.

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As in buildings with transverse bearing walls, the degree and the type of damage depends to a great extent on the shock direction. But while the buildings with transverse bearing walls collapsed mainly when they were attacked directly on bearing walls, the buildings of this latter type resisted the shock better, because of the fact that they were bigger. And, on the contrary, the buildings attacked along bearing walls collapsed because of very weak inter-window columns, whole slabs falling one on top of the other. In general this was the most frequent type of collapse in structures of this kind. The buildings which resisted collapse suffered minor damage similar to that

observed in buildings with transverse bearing walls. In buildings of up to three storeys, the heaviest damage occurred to the ground floor, a little less to the first floor, and much less to higher floors. The most frequent type of cracks are diagonal crosses, while significant damage to building corners was also remarked. Also, in buildings of this type, the destruction of secondary walls was much rarer than in the case of buildings with transverse bearing walls.

Massive buildings with bearing walls in both directions. The buildings of this type with bearing walls in both directions were constructed mainly between the two wars. They include the bigger housing buildings and the more important administration buildings. A very small number of these buildings date from after the Second World War. The number of these buildings is much smaller than that of the buildings described above. They were constructed according to the regulations which were in force between

the two wars and therefore the walls are thicker than those which are built today for the same height of building. For four-storey buildings, the thickness of walls at the ground floor is 51 cm, decreasing gradually with height. In this way is achieved not only a strengthening of the section but also a lowering of the centre of gravity of the structure. The foundations of these buildings are complicated, often irregular, while the

slabs are very often supported on one or other wall depending on the architectonic conditions. The slabs are usually of monolithic reinforced concrete. Because of varying foundation conditions, the damage varied from one building

to another and even from one part to another of the same building. It is remarkable that damage occurred to all storeys and was not limited to the ground floor. No building of this type collapsed, but the degree of damage depended on many local factors. Some buildings remained very slightly damaged. W e cannot give more general characteristics of damage to tKese buildings

because of their irregular foundations and complete lack of uniformity.

The following conclusions regarding the consequences of seismic shock apply to all types of masonry buildings : 1. Taking into account structural calculation analyses and the results of a masonry

building survey, it can be stated that the height of such buildings should be limited because they represent unfavourable structures in earthquake areas, being infiexible, massive and heavy.

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2. The weakest parts of masonry buildings are the inter-window columns and corners of the buildings.

3. Collar-beams which do not go through all storeys and which are badly reinforced do not provide good rigidity in a masonry building.

4. Non-rigid walls, either bearing walls or partitions, represent weak points in the structure.

5. Console stairs are always dangerous and in some buildings are responsible for severe damage.

6. Massive reinforced-concrete stairs very often caused heavy damage to walls because the difference in material between stairs and walls results in different frequencies of vibration.

7. Unsymmetrically shaped buildings of several blocks should be provided with dilatation joints between blocks because the differential movement of the blocks produces gaps at these points.

The results of structural analysis which has been made of some buildings of this type are in close agreement with the observed damage to the structures. In one case where two buildings of the same type and same height stood next to each other, the damage to one was much greater than to the other. One building had a roof of brick and the other of “Eternit”. Structural analysis showed that the seismic forces were much stronger on the first than on the second building and, in fact, the first suffered by far the greater damage. This example also showed how important is the position of the centre of gravity of a structure. Structural calculation has shown that in masonry buildings damage is produced

because on displacement the maximum load on the wall material is exceeded. For this type of building, the thickness of walls and the quality of brick and mortar must therefore be carefully specified. In all masonry buildings for which calculation has been made, the most dan-

gerous is the first mode of vibration; the load due to the second and third modes may be neglected.

COMBINED TYPE BUILDINGS

Buildings of this type are very common in Skopje, especially in the centre of the town. They are relatively high buildings in which the ground floor is used for commerce, while the upper storeys are used for housing and business purposes. The foundations are sometimes irregular because of special architectonic conditions. A characteristic of this type of building is the combination of reinforced- concrete columns with brick bearing walls. The distribution of columns is not strictly regular, both columns and bearing walls being placed according to the whim of the designer. In blank walls, there are very often no columns and the whole load falls on the brick. The slabs are the same as in the type of structures already described. Damage to this type of structure was generally very severe although collapse

was not observed. In most cases, destruction or heavy damage occurred to the

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brick bearing walls which had to provide rigidity to the structure. Heavy damage was caused to reinforced-concrete columns because of the lack of rigidity to resist horizontal forces for which they had not been designed. In general, this type of structure appears to be very unsuitable for earthquake

areas. The structural analysis of these buildings is still in progress, so we limit our-

selves to statements which can be made on the basis of results already available:

.

1.

2.

3.

4.

In

The majority of these buildings have columns only at ground-floor level, where heavy damage was concentrated, the concrete columns breaking with nicking and bending of the reinforcement. These columns are often cross- sectioned with very bad junctions with the slabs. From the point of view of safety, the relation of the centre of mass of the structure to its geometric centre is of great importance. Their separation results in torsion and twist of the building, causing severe damage. In order to achieve rigidity of the structures, brick walls were usually put in the stairways. Because of differences between their elastic properties and those of reinforced-concrete columns, such walls cannot fulfil this function. Therefore, in order to get the needed safety in earthquake areas they have to be put symmetrically in relation to the centre of mass and with approximately the same elastic properties as the concrete. The construction of buildings, in which columns partially replace walls, has to be carried out very carefully and the columns have to be well anchored into the wall mass. The lack of close connexion results in separation and destruction of the columns. conclusion, this type of structure has shown itself unsuitable for earthquake

areas, because of the variety of materials employed.

SKELETON CONSTRUCTION BUILDINGS

There were many reinforced-concrete skeleton structures in Skopje before the earthquake. We have to distinguish between two types of such building : (a) pure reinforced-

concrete skeleton buildings ; (b) reinforced-concrete skeleton buildings with rigid cores or rigid walls. All of these buildings, ten to fifteen storeys high, resisted the earthquake quite

well: none of them collapsed and none suffered heavy damage. Damage to these buildings was limited to: (a) appearance of fissures in the

columns and skeleton ties; (b) separation and partial destruction of façade or section walls; (c) fissures in filling walls. While the massive buildings and combined buildings were not designed to

withstand wind forces, this factor was always taken into account in the dimensioning of the skeleton buildings. Although wind forces are much weaker than earthquake forces, this was sufficient to protect the buildings from heavier damage and collapse.

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K. Hololkv Dj. Solovjev


Seismic forces were reduced because of the elasticity and the flexibility of these structures, so that heavy damage did not occur. The study of these buildings is in the final stage but definite conclusions are

not yet drawn. W e can, nevertheless, make some statements based on a detailed analysis of some of the structures.

Core structures. In most buildings, rigidity is provided by the massive concrete walls. If the earthquake wave was in the plane of a rigid wall, the wall resisted the forces and suffered no severe damage. If the wave was transversal to the plane of a rigid wall of massive concrete,

the wall suffered severe damage, with horizontal fissures. These fissures were usually worst on lower floors under the slabs and above the floor; in middle floors they decreased or completely disappeared, but they reappeared on upper floors. Such a distribution of fissures coincides well with the results of structural

analysis if the common work of the columns and the massive walls connected to the slabs is considered. The approximate methods which are usually used for rigid wall calculations give results which are in significant disagreement with reality, and more exact methods have to be used for the seismic calculation. The observed fissures in rigid concrete walls and the results of detailed struc- ,

tural analysis both show that these walls have to be reinforced. It should be mentioned that in many cases rigid walls were made of very bad

concrete, which further contributed to the damage. Buildings without core. Since these skeleton buildings were usually designed to

resist wind forces (i.e., horizontal forces received on the columns), these buildings resisted the earthquake well, with only slight damage to the columns and ties. The quality of material was usually good and therefore the reinforced-concrete

skeleton was able to resist a bigger load than that for which it was designed. In skeleton buildings, the second or even the third mode of vibration is often observed, so that the damage to tall buildings often occurred at the lower and upper storeys, while the middle storeys suffered less damage.

INDUSTRIAL AND OTHER STRUCTURES

In this short report, attention has been concentrated mainly on housing and similar buildings because they suffered the greatest damage. Industrial structures were less damaged, mainly for three reasons :

1. They are located on the outskirts of Skopje where the earthquake shock was rather weaker, as can be seen from the earthquake intensity map.

2. Industrial structures are usually lighter and more flexible than housing structures.

3. Usually these structures are designed to resist certain horizontal forces. All these factors contributed to giving these structures greater resistance to dynamic shocks and consequently the damage was not so severe.

119


In this short preliminary report, some aspects which are still under study could not be included, especially those related to combined and skeleton structures. The influence of soil ductility on the structure bearing capacity is particularly

related to skeleton systems, and the analysis of this problem is still in progress.

120

F. CaToviE Zavod za Raziskavo Materiala in Konstrukcij, Ljubljana

Preliminary results on testing of brick walls

The earthquake which destroyed or damaged many brick buildings in Skopje in 1963 reminded us that we must take into account much greater horizontal forces in the' statical analysis of such buildings than we had done before. On the other hand we need much more precise data on the behaviour and strength of brick walls subjected to combined' vertical and horizontal forces. The first tests on brick walls subjected to combined loads were carried out at

Zavod za Raziskavo Materiala in Konstrukcij (Institute for Material and Structure Research) in Ljubljana almost immediately after the earthquake in Skopje. Later a more extensive programme was formulated under the sponsorship of Direkcija za zgrade I.V.S.R. Makedonija, Skopje. This programme and further research is now being carried out. Here we present some of the results of tests already completed. Comparing the modes of failure of brick walls subjected to vertical load or

combined loads, we can see that they are quite different. A brick wall, subjected to the vertical load only, begins to crack at a certain load, the cracks being vertical and more or less uniformly distributed on the surface. With increasing load, the cracks become wider and longer. Near failure the brick wall is divided by vertical cracks into narrow columns, which lose stability and the brick wall fails. There are some other modes of failure, but they are rarely observed. The brick wall subjected to combined vertical and horizontal loads fails after

a diagonal crack going from one corner to another is formed. The modes of failure show that we cannot deduce the deformability and strength

characteristics of brick walls under complex loads from the characteristics observed under vertical loading only. Tests carried out on brick walls of 100 x 25 x 150 cm showed that as a basic

characteristic of brick walls made of material of various qualities, subjected to horizontal forces, we can take the average shear stress Tk at vertical load N = O to be

horizontal load -

H I; gross area

7 h 3 - -

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With greater vertical load the average shear resistance increases. The increase is given by:

/

Results of tests and theoretical analyses have shown that for normal buildings, made of brick and lime mortar or lime cement mortar, the shear stresses are always decisive, not the normal compressive ones. Since the shear resistance increases with vertical load, it is recommended that

the vertical compressive stress (N/F) should be approximately the same in all bearing walls. On every brick wall we tested, the deformation characteristics were also meas-

ured. On them depends the share of load that falls to a given brick wail at a given floor. Shear deformations are predominant. But if the length of a brick wall element

is less than 1 m (walls between the windows) we have also to take into account deformations due to bending moments. It follows that only the walls aligned in the direction of the seismic forces can resist them; and also that in designing our buildings we must strive to ensure that half the bearing walls are placed in one direction and the other half in a perpendicular direction. An analysis made by V. Turnsek shows that the resistance to seismic forces

can vary as much as 1 : 6, depending on how the bearing walls are placed and how uniform is the vertical load. Tests were also made with the object of finding how to repair and improve the

shear resistance of damaged or existing brick walls. Two ways were proved to be efficient: the impressing of thin cement mortar into walls, and horizontal pre- stressing. By injecting a lime mortar brick wall we could raise the average shearing

resistance by 70 to 100 per cent; and by injecting and pre-stressing, 100 to 200 per cent depending on the amount of pre-stressing. Further research in this field is needed if we are to build economical and also

seismic resistant buildings with our most common building materials.

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S. TerCelj Zavod za Raziskavo Materiala in Konstrukcij, Ljubljana

Preliminary results on testing of reinforced-concrete columns

Under the sponsorship of Direkcija za zgrade I.V.S.R. Makedonija, Skopje, a research programme on testing reinforced-concrete columns was established last year with the objective of producing experimentally damage similar to that suffered by columns of reinforced-concrete buildings in Skopje, and to determine the forces necessary to produce this damage. A further objective of this programme was to find the most efficient means of repairing damaged columns. Twelve reinforced-concrete columns were made with square cross-section and

reinforcement p = p' 0.5 per cent. Their height was 2.60 m. Columns were tested at different combinations of vertical and horizontal loads. Horizontal forces were applied at the top and bottom of columns. The ultimate strength of the columns, thus measured, was plotted in a diagram

together with the analytically calculated values. The ordinates were values of Ml W (M being bending moment and W the section modulus of columns) and the abscissa were values of N/F (N being axial load and F the cross-section of a column).

It was found that the calculated values corresponded very well to the experimental values of ultimate loads. Experimentally determined vectors, defined by formula

were on an average 5 per cent greater, while the standard deviation of experimental results was 9 per cent. In such a way, the generalized results could be applied in order to find the

ultimate forces corresponding to the damaged columns. It must be noted that in cases where all the columns of a given storey were

damaged or failed, we can determine only the lower limit of the forces which acted on the building. But when there were only a few columns damaged, we can estimate the forces quite accurately. Further analysis showed that the safety of a reinforced-concrete building sub-

jected to seismic forces cannot be expressed conveniently in terms of stresses.

123


All twelve columns were repaired after failure. We used three methods of repairing them : pre-compacting, injecting and casting damaged portions of columns anew. It was found that only the first and last ways were efficient.

Some experiments were also made in order to increase the bearing capacity of existing columns. Circumferential pre-stressing gave very good results.

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A. Velkov

Analysis of damage to fourteen-storey buildings in Skopje

In the village of Karpoi there are three identical reinforced-concrete buildings, fourteen storeys high, with an attic, the rest of the buildings being made of brick. These three buildings are 44.60 m high, and their rectangular foundation measures 14.74 x 42.50 m. Each building is made of a number of reinforced-concrete frames, with sup-

porting cross-bars placed laterally. Inside the building there is also a supporting core (the staircase), made of reinforced concrete, strengthened with horizontal and vertical struts of reinforced concrete. The bearing walls in both directions are made of lightweight bricks and are 25 cm thick. The flooring is of single massive iron slabs. Two of the buildings have founda-

tions of massive, strongly reinforced concrete slabs, strengthened with tapering struts around the columns, and the third building stands on strip foundations in both directions. The foundations go down 2.50 to 3 m below the surface of the earth, and ground-water level at the time of construction was also 2.50 to 3 m below the surface. Monolithic on-the-spot construction methods were used for all the buildings.

The first was completed in 1961, and at the time of the earthquake the third was in the final stages of construction. Average quality concrete steel and 220 and 300 grade concrete were used. Analysis of the damage to these buildings showed that all three suffered more

or less the same degree of damage, the principal forms of which were: 1. Cracks in all directions in the concrete nucleus, especially marked near open-

ings such as doors and in places that were weakened because they had to support various kinds of interior equipment.

2. The only cracks in the supporting frames were in the cross-bars in all floors. The columns were not damaged, except in one building, which is discussed below. The cracks in the cross-bars were near the columns and were mostly vertical and rarely slanting. It should be noted that the flooring slabs were damaged in the same places as the cross-bars, which suggests that cross-bars and slabs are similarly affected.

125


3. Transverse cracks were found in the supporting walls principally along the line of the framework and, less frequently, smaller diagonal cracks. This shows that in the transverse direction the walls were, generally speaking, at the limit of elasticity or in the early stages of plasticity. Lengthwise, the front walls parted from the supporting structures, leaving horizontal cracks, but no diagonal marks. This shows that the greatest impact was exerted across the building. The shock produced horizontal cracks in the longitudinal walls, and thus made the longitudinal walls useless as far as the resistancehiof the structure was concerned.

The over-all picture of the damage caused is much the same in all three buildings. The first building was the only one in which the first floor corner pillars collapsed, and nearby pillars were severely damaged. This caused considerable distortion of the whole building in the direction of the impact. Because the damaged buildings-and especially the first-had to be rebuilt, the whole construction, including the framework, nucleus and bearing walls, was analysed, so that a plan for the reconstruction of all these buildings could be prepared. Investigations based on practical calculations suggested that flexible transverse movements of buildings resulting from transverse impacts usually produce seismic stress in the framework. Only 15 per cent of the stress is exerted on the central nucleus, and it rapidly declines vertically, so that stress on the sixth and seventh fioors is insignificant. The bearing walls are affected at all levels, and receive about 10 per cent of the seismic stress. Obviously, the part of the structure that is subjected to the greatest seismic stress while the building is in elastic vibration is the framework, although the moment of inertia of the pillars is several hundreds of times less than that of the nucleus. Other characteristics appear in the lengthwise direction of the structure; here,

the nucleus bears most of the stress, and the framework is so resilient that it is hardly affected at all, compared with the rest of the structure. Further analysis of the building consists in the determination of the limit of

the seismic forces at which the different structural elements cease to work in the elastic state, and enter the plastic state, or at which plastic joints appear in the construction. Analysis showed that at a stress as low as 1.5 to 2 per cent g the outer cross-bars undergo plastic distortion, or at least no longer operate within the limits of elasticity. If the stress rises to 3 per cent g, plastic movement begins in the bearing walls, being characterized by diagonal cracks. In making the analysis, the dynamic characteristics of the building in its original

condition were taken into account. Obviously, as plastic distortion appears, the. structure of the building changes, and this results in a change in the dynamic characteristics of the building. Longitudinally, the nucleus, which is the basic supporting element (made of massive concrete) rapidly changed its structure ; suffering plastic deformation, it broke down into separate diaphragms of vanous shapes and sizes. From the foregoing it may be deduced that in the longitudinal direction of

the seismic impact the building had good characteristics, which ensured that the different elements moved in different ways, and this resulted in the dispersion and

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A. Velkov Analysis of damage to fourteen-storey buildings in Skopje

absorption of the energy throughout the whole height of the building. This is confirmed by the nature of the damage suffered by the building itself. Further- more, the degree of involvement of the pillars is much greater than that of the other parts of the building, a fact which, at all events, ensures the stability of the building. Finally, it should be pointed out that the major distortion in the first building caused by the collapse of the pillar in the south corner was due to local factors. In my opinion, the marked increase of force in this pillar was due to the presence of a large cantilever, measuring 1.90 m, on all floors, and also to the fact that a sub-standard grade of concrete was used. Nor can we exclude the possibility that vertical shocks affected the corner of the building, as a result of specific seismic stress. All these factors led to increased stress on the pillar and caused it to collapse, resulting in severe damage to the building itself. I propose that further work on plans for the protection of this building should

expand the above conclusions, which will serve as a basis for solving the problem of strengthening and protecting buildings of this kind.

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S. A. Bubnov Tehnicni Direktor, Gradbeni Center Slovenije, Ljubljana

Problems of earthquake-resistant design and engineering

DEVELOPMENT OF EARTHQUAKE ENGINEERING IN THE W O R L D

In various parts of the world, earthquakes still cause numerous human casualties and enormous material damage. The number of casualties and the extent of damage is by no means decreasing as time goes by; on the contrary, there even exists the danger that the extent of damage will grow, owing to the constantly increasing concentration of population in cities. Should the necessary safety measures not be taken in time, the seismic areas will beyond doubt face a serious hazard. The exact time, intensity and location of an earthquake cannot be determined

in advance. Of course, there are certain regions known to be subject to more or less frequent earthquakes. In regions where earthquakes occur more often, they receive a great deal more attention than elsewhere; in fact, all means are being tried in order to protect the population from the ravages of earthquakes. In regions, however, where earthquakes are rather rare, i.e., where destructive earthquakes take place at intervals of one or more human generations, as for instance at Skopje, constructors tend to forget or disregard the threat of earthquakes. They are often heard to express the view that the additional investment for anti-seismic reinforcement of structures is uneconomic and irrational, on the grounds that the seismic risk is insignificant. If we consider the problem of anti-seismic safety in a somewhat broader per-

spective, we must note that, according to statistics compiled by Unesco, earthquakes caused an annual average of 14,000 casualties during the second quarter of this century; material damage, moreover, exceeded several tens of millions of dollars annually. Therefore, it is not surprising that numerous constructors have started in the last few years to study intensively the problems of anti-seismic safety, in an endeavour to find the most effective means of protection against the effects of earthquakes. Along with the extraordinary dynamic development of science in the last ten years, an awareness of the need for anti-seismic construction of buildings has now developed. Important landmarks in the development of

129


this particular field of science are the world congresses on earthquake engineering, at which leading scientists from all over the world have made known their latest findings. The proceedings of the ñrst World Congress on Earthquake Engineering at

Berkeley, California, in 1956, have been compiled in one volume. The publications of the second world congress, held in Tokyo in 1960, comprise three volumes with more than 2,000 pages. For the third world congress, which will take place in New Zealand at the beginning of 1965, the volume of technical papers is such that the organizers will be unable to publish all articles, but only a limited selection. The entire science is based on theoretical study, experimental research and the

examination of the effects Df disastrous earthquakes in all parts of the world; thus, it is now possible to approach this natural phenomenon with more under- standing, and more effective measures can be taken in order to protect mankind from its ravages. On account of the extreme irregularity of this phenomenon, as well as the extremely complicated reaction of structures to earthquake forces, it is quite clear that it will never be possible to foresee with absolute precision the effects of earthquakes on structures. W e can, nevertheless, feel confident that we are slowly coming to master this problem in such a way that human life and property may be safeguarded. The recent earthquakes at Anchorage and Niigata have proved, however, that

earthquake engineering has still not reached a satisfactory level. Accordingly, the attention paid by Unesco to this problem can very well be

understood. In 1961, the foundations of the International Institute of Seismology and Earthquake Engineering were laid. In 1961-1963, Unesco sponsored seismological survey missions to all parts of the world, with the object of reviewing the situation of seismology and anti-seismic engineering in countries in the main seismic zones. One of these missions visited the Middle East and Mediterranean region, spending some time in Yugoslavia in May 1962. It prepared an elaborate and very significant report. At its twelfth session, the General Conference of Unesco had adopted a resolu-

tion calling for the organization of an intergovernmental meeting on seismology and earthquake engineering. A preparatory meeting of experts took place in Paris from 26 to 28 March 1963, and the intergovernmental meeting itself was held at Unesco Headquarters from 21 to 30 April 1964. At this meeting, delegates of forty countries were present, as well as represen-

tatives of various international associations, Very important conclusions were reached, in particular with regard to the question of seismology and anti-seismic engineering on the largest scale possible.

EARTHQUAKE-RESISTANT DESIGN

In order to assure the anti-seismic safety of structures, the design should first of all be worked out in conformity with the principles of anti-seismic construction,

130

S. A. Bubnov Problems of earthquakeresistant design and engineering

taking into consideration the building materials available. Although this task may appear to be simple, a detailed analysis shows that one has to take into account numerous problems either of general or specifically local character, all being very difficult to solve in a quick and efficient way. Let us first take a look at the problem of design itself; here, an efficient staff

of qualified designers, familiar with the principles of anti-seismic design and local building conditions, is utterly indispensable. Anti-seismic engineering is a comparatively recent science and it should be

understood that it is only in the last ten years that it has developed to a considerable extent. Therefore, it can easily be understood that it has been almost impossible to develop the education of specialist designers and engineers at the same time. It is desirable that the teaching of anti-seismic design be introduced into the curricula of technical colleges and faculties in those countries where earthquakes are likely to occur; the extent of this tuition should, of course, be adapted to suit the syllabus and the general level of knowledge in each particular school. Furthermore, symposia, seminars and technical consultations should be organized in order to give up-to-date information on anti-seismic construction to all those concerned with building. Administrative and governmental organs in countries of the seismic zones

should prepare relevant technical regulations regarding structures to be built in earthquake danger areas. The existing regulations of technically advanced countries may be consulted, though local technical building circumstances should by no means be neglected. Invaluable hints in this direction are contained in the report of the Unesco working group on the principles of earthquake-resistant design and construction (doc. Unesco/NS/SElSM/ó). This report defines the fundamental principles of anti-seismic engineering,

serving also as a cor\cise summary of the views of various experts on the problems of earthquake engineering; there are, naturally, some points where opinions differ slightly from one another, yet there exists a definite possibility of agreement on some general common principles. It is intended that this study should be carried out in future by a permanent

working group nominated by Unesco and consisting of the most distinguished experts of world repute. Notwithstanding the fact that different opinions regarding the effects of earth-

quakes on structures exist (particularly regarding the power spectrum of the effect, soil factors, response of complex structures to seismic forces) as can clearly be seen from the regulations governing anti-seismic construction in different countries, the general tendency is towards the exact determination and gradation of horizontal forces, exercised either at the centre of mass or on the whole structure. In this respect, the regulations adopted by some countries during the last few years are much more stringent than they used to be some ten years ago. An anti-seismic engineering code is incomplete without a seismic micro-zoning

map of the earthquake region; such a map is necessary in order to include in a satisfactory way all local soil factors in calculating the seismic forces. At the intergovernmental meeting, the delegation of the U.S.S.R. put forward

131


precise proposals concerning the compilation of seismic micro-zoning maps. These proposals have proved very useful in the preparation of micro-zoning maps of Skopje and Ljubljana. In Yugoslavia we had, until 1948, regulations which contained provisions for

very small horizontal forces as a result of earthquakes. In areas of maximum probable intensity IX, these forces amounted tó only 2 to 3 per cent of the vertical load. The disastrous earthquake at Skopje demonstrated clearly enough that these regulations were indeed inadequate. Consequently, Yugoslavia has recently been paying special attention to the

elaboration of more up-to-date regulations concerning anti-seismic structures. W e have drawn amply on foreign experience, taking into account at the same time our own situation, particularly as regards the extent of seismic areas, the qualified technical staff available, local ways of building and local building material. The regulations in Slovenia, based on up-to-date findings of earthquake engineering, were drawn up shortly before the Skopje earthquake. At present these regulations are being completed and improved with the co-operation of Yugoslav and foreign experts (T. Hisada, S. V. Poliakov) and will soon be extended to the entire territory of Yugoslavia, including Macedonia. These regulations introduce modern dynamic methods of dimensioning building structures, and are based on a specific spectrum of earthquake forces on structures. This represents a compromise between the various spectra used in different countries in their respective regulations. The latest analysis of spectra observed in recent earthquakes, especially in that of Niigata, confirms the opinion that in certain cases this spectrum is almost a horizontal line. In the technical literature, the problem of expressive shock earthquakes with extremely high, yet very short-term, acceleration is still quite unexplored. The results of research on the Skopje earthquake by N. N. Ambraseys and

A. Zátopek justify the supposition that the Skopje earthquake was certainly of this type. All these circumstances clearly seem to demonstrate that earthquake engineering must develop further, while regulations should be adapted to reflect this development. Our new Yugoslav regulations allow designers to profit from the recent achievements of anti-seismic'engineering. As regards the design of earthquake-resistant structures, there exist several

general principles based on engineering technical grounds which may, provided they are duly taken into consideration, ensure the construction of anti-seismic buildings. Observation of these principles may ensure that a structure is anti- seismic merely because they impose a correct and rational choice of building concept and construction, with proper attention to construction details and to the use of correct materials. Practice, however, has made it clear that it is wholly wrong to combat the effects of earthquakes simply by modifying structures designed for non-seismic areas. Such methods have led to several very expensive and highly uneconomic solutions. Therefore, it should be stressed anew that structures in seismically active zones should be designed from the beginning in conformity with the principles of earthquake-resistant engineering. In other words, seismic areas should develop a specific anti-seismic architecture.

132

S. A. Bubnov Problems of earthquake-resistant design and engineering

This procedure will also make it possible to reduce to a minimum the cost of reinforcing structures to resist earthquake forces. Of course, design staff should be fully familiar with these principles of anti-seismic design. The application of the principles of earthquake-resistant design means, there-

fore, a considerable saving of communal financial resources. Accordingly, this problem should also be considered as one of general interest to the whole community.

THE CONSTRUCTION

The realization of the design itself in accordance with the principles of earthquake- resistant engineering is not sufficient if there is no guarantee that the design corresponds to the requirements in so far as the geometrical form, dimensions and above all the kind and quality of the material are concerned. Poor workmanship and material can easily entirely spoil the effectiveness of

even the best design. Here we encounter not only the question of quality of material, but also the

problem of selection of appropriate material, bearing in mind economic factors. It is well known that engineers are always more inclined to employ local material, because of the high cost of transport. The analysis of the various effects of earthquakes has proved that certain tradi-

tional building materials, such as brick and stone, are extremely inappropriate from the standpoint of earthquake safety. In particular, the traditional full brick of normal shape, which is used very widely in massive construction, has turned out to be entirely unsuitable for earthquake-resistant construction. A wall made of bricks consists of an enormous number of joints, each one of

which conceals in itself the danger of insufficient adhesion, apart from reducing the bearing capacity of the structure. It is evident that the number of joints should be decreased, as much as possible,

if we wish to minimize the risk of damage. W e can infer from this that we should use larger brick elements-provided that we intend to keep brick as a building material-because only thus can the number of dangerous joints be considerably diminished. The entire elimination of this material and the substitution of new materials, more appropriate to the necessities of earthquake-resistant construction, would place certain regions at a considerable disadvantage. In view of the fact that we still live in a period when less appropriate materials

for earthquake-resistant construction are at our disposal, we should take care to ensure that material of first-rate quality is used and that the construction methods assure the best possible effectiveness of this material in case of earthquake. The same applies to material now regarded as more suitable for earthquake-

resistant construction. W e should be always aware of the fact that defects in the quality of materials or in the workmanship, though they may remain concealed under normal conditions, can immediately appear and may even have fatal consequences when the structure is exposed to seismic forces.

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It is therefore essential that, in areas where disastrous earthquakes are to be expected, the engineering industry and construction methods should develop in such a way that earthquake-resistant constructions can be built in the most rational and economic way. At the same time, a very strict control must be carried out over works of engineer-

ing industry and over building sites themselves. Without this effective control of quality of material and of workmanship, even the most modern regdations and the best earthquake-resistant design are useless. Finally, I would like to emphasize that the problem of anti-seismic design and

construction is a very dynamic one. The development of seismology and earthquake engineering should be followed carefully and the achievements of earthquake engineering should immediately find practical application. Only in this way can human life and property be protected against the ravages of earthquakes, within the limits imposed by economic factors.

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General discussion of session C Discussion générale de la session C

M. RadojkoviE

M. Breznik

As much information as possible must be collected concerning the cost of earthquake-resistant construction. The increase in costs due to strengthening a particular building and using different supporting materials must be calculated. N o mention of the earthquake-resistant qualities of steel construction has been made in the above papers. I therefore consider that attention should be drawn to the behaviour of steel buildings in a Skopje steel foundry, which had one, two and three degrees of freedom. The natural periods of vibration of different buildings varies between 1.0 and 2.40 sec. Some of them had heavy loads on the top floor (the third). Despite this, these buildings suffered no significant damage at all when the earthquake occurred, although they were not designed to stand up to earthquakes. It was clear that simply taking into account the action of the wind and the horizontal reactions of cranes was enough to ensure that these buildings were earthquake resistant. The most recent calculations concerning these buildings, according to the new standards of earthquake- resistant construction, showed that there was no need to strengthen such constructions in a transverse direction. Only in a longitudinal direction was it necessary to strengthen them slightly. This showed that steel buildings are very suitable for earthquake-resistant construction, from the point of view of both planning and calculation and the possibility of subsequently strengthening such buildings.

I should like to draw attention to a number of points relating to earthquake-resistant construction which have not yet been satisfactorily elucidated, mostly in connexion with pile foundations. It is known that the effect of earthquakes diminishes with depth. This was established in Japan, in the Hitachi min0, and in the mine at Trbovle, at the time of the disastrous earthquake in Ljubljana in 1895. It is not known, however, to what extent th0 effect of earthquakes diminishes at depths of between 20 and

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Earthquake engineering studies in Skopje Etudes séismotechniques effectuées dans la région de Skopje

30 m, when pile foundations are normally used. It is also not known whether pile foundations should be designed so that they can sway as the ground sways, or whether slanting piles should also be provided, which would take the weight of the horizontal walls. With regard to damage to buildings in Skopje, it should be taken into account that since the war Yugoslavia has paid no attention at all to earthquakes. Our building standards have been designed to overcome the action of wind rather than earthquake stress. W e have therefore built increasingly thin bearing walls, till finally we have been putting up four- or five- storey buildings with bearing walls 25 cm thick. In Ljubljana, eleven- or twelve-storey dwellings have been built with brick bearing walls. W e all realize that such buildings are not capable of standing up to an earthquake of any force. As early as 1959, in Ljubljana, we drew attention to the need for altering our building standards, and as a result Slovenian standards for earthquake-resistant construction were worked out-for the first time in Yugoslavia. These standards were adhered to in the erec- tion of buildings in Skopje, and the new Yugoslav standards for earthquake-resistant construction were based on them.

The strength of brick wall bearing structures is affected by secondary effects such as the bonding between bricks and mortar, strength of the mortar and of the bricks themselves, eccentricities, etc. Various combinations of these effects result in considerably varying strengths of the structures.

N. N. Ambraseys

I. Alpan

D. Janic

J. Petrovski

J. Caëovië

In view of the effect of foundation conditions on the response of structures to earthquake forces, it is most important to study the dynamic properties of soils, i.e., the effect of dynamic forces (shock or vibration) on their strength and deformation characteristics.

In cities which are being rebuilt after an earthquake on their original site, a local survey of sites that have been filled in must be made. If these are not shown on the map of the area concerned, buildings may later be erected on them, and this will considerably reduce their capacity to withstand an earthquake, should another one occur.

The opinion has been expressed that the most economical buildings are those with flexible pillars and rigid floors, made of reinforced concrete or pre-stressed concrete.

I fully agree with the views expressed in the paper by Professors HololEev and Solovjev. In m y opinion, staircases are an extremely rigid part of a building, whether of concrete or of brick, and a way must be found to reduce the bad effect of a staircase on the rest of the structure. The permissible degree of damage to buildings should be determined, from the point of view of

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General discussion of session C Discussion générale de la session C

economy, and the possibility of a recurrence of earthquakes should be taken into account.

M. Kasumovii: All that has been said points to the fact that it will probably be difficult to eliminate all damage to buildings during an earthquake. Builders, however, should make it their object to ensure that there is no loss of human life in earthquakes. The buildings they erect should be able to withstand earthquakes as successfully as those in Niigata, where, although buildings were destroyed, there was no loss of human life.

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of the international earthquake engineering -...

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