analysis of the weightiness of site effects on reinforced concrete (rc) building seismic behaviour:...

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICSEarthquake Engng Struct. Dyn. 2007; 36:1363–1383Published online 19 March 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/eqe.685

Analysis of the weightiness of site effects on reinforcedconcrete (RC) building seismic behaviour: The Adra town

example (SE Spain)

Manuel Navarro1,∗,†, Francisco Vidal2, Takahisa Enomoto3, Francisco J. Alcala4,Antonio Garcıa-Jerez1, Francisco J. Sanchez1 and Norio Abeki5

1Department of Applied Physics, University of Almeria, Spain2Andalusian Institute of Geophysics, University of Granada, Spain

3Department of Architecture, University of Kanagawa, Yokohama, Japan4Technical University of Catalonia, Barcelona, Spain

5Department of Architecture, University of Kanto Gakuin, Yokohama, Japan

SUMMARY

The damage distribution in Adra town (south-eastern Spain) during the 1993 and 1994 Adra earthquakes(5.0 magnitude), that reached a maximum intensity degree of VII (European Macroseismic Scale (EMSscale)), was concentrated mainly in the south-east zone of the town and the most relevant damage occurredin reinforced concrete (RC) buildings with four or five storeys. In order to evaluate the influence of groundcondition on RC building behaviour, geological, geomorphological and geophysical surveys were carriedout, and a detailed map of ground surface structure was obtained. Short-period microtremor observationswere performed in 160 sites on a 100m× 100m dimension grid and Nakamura’s method was applied inorder to determine a distribution map of soil predominant periods. Shorter predominant periods (0.1–0.3 s)were found in mountainous and neighbouring zones and larger periods (greater than 0.5 s) in thickerHolocene alluvial fans. A relationship T = (0.049 ± 0.001)N , where T is the natural period of swayingmotion and N is the number of storeys, has been empirically obtained by using microtremor measurementsat the top of 38 RC buildings (ranging from 2 to 9 storeys). 1-D simulation of strong motion on differentsoil conditions and for several typical RC buildings were computed, using the acceleration record in Adratown of the 1993 earthquake. It is noteworthy that all the aforementioned results show the influence ofsite effects in the degree and distribution of observed building damage. Copyright q 2007 John Wiley &Sons, Ltd.

Received 15 June 2006; Revised 30 December 2006; Accepted 29 January 2007

KEY WORDS: soil conditions; landform classifications; S-wave shallow structure; microtremors; siteeffects; dynamic behaviour of RC buildings; Adra earthquake; strong motion simulation

∗Correspondence to: Manuel Navarro, Department of Applied Physics, University of Almeria, 04120, Almeria, Spain.†E-mail: [email protected]

Contract/grant sponsor: CICYT; contract/grant numbers: AMB99-0795-C02-02, REN2003-08159-C02

Copyright q 2007 John Wiley & Sons, Ltd.

1364 M. NAVARRO ET AL.

1. INTRODUCTION

The softness of the ground surface and the thickness of surface sediments have been observedas two important local geological factors that affect the level of earthquake shaking. Their localvariations can lead to spatial seismic intensity differences and may have a remarkable influence onthe level of building damage and on significant earthquake damage distribution even in the casesof moderate earthquakes.

Since the 1985 Michoacan earthquake (Mexico), building damage has been studied analysingthe contribution of site response [1], in particular when resonant phenomenon appears. Thisstrong influence of site effects on damage distribution has produced extreme consequences, forexample, in the earthquakes of Leninakan, Armenia (1988), or Loma Prieta, California (1989).More recent destructive earthquakes (e.g. Northridge, California 1994; Kobe, Japan 1995; Izmit,Turkey 1999 or Chi–Chi, Taiwan 1999) have shown how unconsolidated soil and sediment depositswere responsible for important modifications in ground motion amplitude in a range of periodsand how building damage increases when the fundamental vibration period of the building isclose to the predominant period of the soil motion [2]. In the case of the Izmit earthquake on17 August 1999, the non-uniformity in earthquake damage distribution indicates the site effectsassociated with alluvial basins such as motion amplification and low-frequency enhancement,unfavourable to the structures of longer periods [3]. In Adapazari city, these authors report thatfive to six storey buildings located over deep alluvial soil were the most adversely affected by theearthquake.

The relationship between soil amplification and the level of damage has been recently confirmedfor several large earthquakes (Mw>6.5) and analysed with regard to deep soil structures andtall buildings. For example, in the 1948 Fukui (Japan) catastrophic earthquake, the degree anddistribution of damage were strongly correlated with local soil conditions of the Fukui basin,where the ratio of totally collapsed houses ranged from 60 to almost 100% [4]. In the 1967Caracas (Venezuela) earthquake it was noticed that the most affected areas were linked to geologicalcharacteristics like the thickness of the quaternary alluvial [5]. In the 1999 Izmit (Turkey) earthquakethe varying ground characteristics underlying Adaparazi city played a dominant role in the extentand distribution of ground motion intensity and in the consequent building damage [3, 6]. In the1999 Chi–Chi (Taiwan) earthquake, Seo et al. [2] found, using strong motion data, that earthquakemotions would be amplified several times in the period range from 1 to 3 s in the Taipei basin,and such ground motion would be very effective for tall buildings.

A large number of observational studies show that local amplification effect has played a rolein the seismic damage distribution in urban areas for several moderate earthquakes [7–11]. Theagreement is generally quite satisfactory, though only qualitative. Our work tries to estimate how theresulting ground motions might interact with the built environment in the case of small earthquakes,not very thick soil layers and damage in buildings of less than six storeys.

In some large earthquakes (e.g. Northridge or Kobe) it has been proved that the pattern of peaksin ground acceleration and in ground velocity reflect the source proximity and rupture processcausing significant directivity effects. However, site effects are still very important and explain theground motion amplification caused by surface geology and the degree of building destruction andits spatial distribution. Nowadays, it is thought that site effects are more significant in the lowershaking levels associated with small and moderate earthquakes, particularly at higher frequencies[12]. For this reason, the analysis of local site effects has a special relevance in regions of smalland moderate earthquakes, like in south-eastern Spain.

Copyright q 2007 John Wiley & Sons, Ltd. Earthquake Engng Struct. Dyn. 2007; 36:1363–1383DOI: 10.1002/eqe

ANALYSIS OF THE WEIGHTINESS OF SITE EFFECTS 1365

Figure 1. (a) Main tectonic units of the Betic Cordillera (southern Spain) and geographical location ofresearch area and (b) geological sketch of Adra town, with I–I′, II–II′ and III–III′ being the geological

cross-sections shown in Figure 2.

Adra town is situated in the SW of the Almerıa province (Southeast of Spain) (Figure 1(a)), oneof the most hazardous zones of Spain from the point of view of seismic hazard. Historical seismicitydata reveal that Adra was affected by near destructive earthquakes in 1522 and 1804 (IX maximumintensity, European Macroseismic Scale (EMS scale)) and in 1910 (mb = 6.2) [13]. Several smallearthquakes (mb = 5.0) in the south-east of Spain, for example, in 1993, 1994 (with an epicentrenear Adra, Almerıa) and in 1999 (with an epicentre close to Mula, Murcia) reached a VII degreeof intensity (EMS scale) and a detailed macroseismic study revealed areas with different intensitieswithin the most affected towns. Furthermore, buildings with the same typology placed in areasunderlain by similar surface geology, showed significant damage differentiation from place to place(some with moderate damages and others undamaged); the only appreciable difference amongstthem was the height of the buildings. In the case of two Adra earthquakes dated 23 December 1993and 4 January 1994, the most relevant damage in Adra town occurred in reinforced concrete (RC)buildings of four or five storeys placed on recent alluvial deposits. The other RC buildings onlysuffered light damage or remained intact, and similar occurred with brick and masonry structuresplaced outside alluvial deposits. Other nearby towns (Berja, Balanegra and Balerma) suffered asimilar degree of damage. This damage pattern could be related to microscale controlling factors ofground motion. This paper is concerned with such an influence, based on geological, morphologicaland dynamic characteristics of the ground surface and the dynamic behaviour of the buildings inthe affected area of Adra town.



In order to analyse the building/site response relationship we have determined geomorphologicalcharacteristics of Adra’s urban area (using geotechnical data, drillings, shear velocity values ofshallow layers, etc.). We performed microtremor measurements in the ground surface and theNakamura method was applied in order to empirically determine ground predominant periods withthese in situmeasurements. Finally, building behaviour features were obtained through microtremormeasurements on the top of the buildings.

2. GEOLOGICAL AND GEOPHYSICAL SETTING

During the last decade different kinds of empirical and numerical methods have been proposedto estimate site effect dependence on surface geological conditions in urban areas. Sometimes,combined approximations from different methods have been applied to estimate site amplificationcharacteristics. Frequently, a first step is to perform a geological microzonation based on geologi-cal and geotechnical identification of materials, their geographical distribution and morphologicalconditions [12]. A set of tectonic, edaphological, geomorphological, hydrological, geotechnical andgeophysical data is then involved in the determination of physical properties at each site. After that,urban area is divided into different zones with almost homogeneous soil conditions allowing us toreduce the number of sites whose seismic response has to be estimated. Geological microzoningserves as a starting point in the application of experimental procedure in order to determine siteresponse based on the measure of ground motions at different sites. Such an estimation can be doneusing strong or weak earthquake motion [14] or by microtremor measurements analysis [15, 16].

From a geological point of view, Adra town is located inside the Alpujarride Complex, InternalBetic Zone (Figure 1(a)) [17]. The older materials (basement) form part of the Adra Unit (Alpu-jarride Complex), a group of metamorphic nappes constituted by Palaeozoic and Permo-Triassicphyllites, schist and micaschist [17, 18]. These are covered by a Plio-Quaternary set of sedimentsthat run from the East to the West of the town with complex geometries and spatial distributions[19–21], marking the Alpujarride bedrock (Figure 1(b)). The Pliocene materials are representedby deltaic facies formed by poorly rounded pebbles in a sandy–clayey matrix, which appear inthe East of Adra town (Figure 1(b)). The Pleistocene is composed of marine and continental sedi-ments. The marine sediments are formed by two marine terraces composed of gravel and roundedpebbles fairly or very loose in a sandy matrix. The continental sediments are constituted of threegenerations of detrital glacis with red and fine silts, fine sands and gravel in a red silty–clayeymatrix both with scarce internal classification. The Holocene is represented by three principal typesof deposits. The most extensive is a mixed alluvial-marine level of fine sands, silts and gravelbetween 2 and 50m thick. The second deposits are represented by overlaying colluvial deposits oflocal rain-fed watercourses that run through the town from North to South, and the recent sandymarine terrace. The last deposits are represented by recent marine terraces and anthropogenicalfillings [21].

All these preorogenic and postorogenic materials were studied in situ and a stratigraphic correla-tion of six boreholes and 15 stratigraphic columns was carried out [21]. A detailed urban geologicmap in 1:5000 scale (Figure 1(b)) has been obtained using borehole data of recent Adra riveralluvial sediment, SPT measurements, VS values, together with geological and hydrogeologicalfeatures of surface materials [22]. Fifty-five N -values from 30 standard penetration tests (SPT) and49 real density values of sediments (dry estimation) have been analysed to geotechnically char-acterize the geological materials (Table I). The ranges of S wave velocity values were obtainedusing the relationships of Imai [23] and Kokusho [24].



TableI.Geological,lithologicalandgeothechnicaldescriptionof

thelithologicalunits

ofAdratown.

Characteristic

sof

thematerials

N-value

Thickness

Density

VS,by

VS,by

Geological

(SPT

)(m

eters)

(g/cm

2)

Imai

Kok

usho

Und

erlying

form

ation

Lith

olog

y(average)∗

(average)†

(average)∗

(198

1)(198

7)materials

Rem

arks

Schist

Solid

rock

>50

Morefracturedand/or

meteorized

(>50

)‡—

——

——

areasarevu

lnerable

toland

slides

Phyllites

Solid

rock

——

——

——

Vulnerableto

landslides

Pliocene

Gravels

with

>50

>20

−1.90§

389

402

Hardrock

Vulnerableto

land

slides

cong

lomerates

clayey

matrix

(>50

)(5)

Non

-saturatematerial

MarineterraceI

Medium

and

30–3

51–

51.75

–1.80

297

317

Hardrock

Vulnerableto

land

slides

MarineterraceII

thicksand

s(32)

(2)

(1.86)

Perchedgrou

ndwater

(high

perm

eability)

GlacisI

Clayeysand

s35

–50

1–50

1.80

–1.90

340

344

Marineterraces

Capable

ofland

slide

GlacisII

andgravels

(41)

(12)

(1.86)

andhard

rock

Non

-saturatematerial

Allu

vias

Medium

gravels

25–3

01–

51.80

–1.85

240

240

Hardrock

Vulnerableto

liquefaction

andsand

s(27)

(2.5)

(1.82)

Smallalluvial

aquifer(high

perm

eability)

Allu

vial

fanof

Fine

limes

and

15–2

01–

51.85

–1.90

266

262

Thick

tofin

eItis

norm

ally

coveredby

fillin

gsAdrariver

sand

s18

3.5

(1.88)

sand

sVulnerableto

liquefaction

Medium

gravels

30–4

01–

501.80

–1.90

259

259

Pliocene

and

Quite

vulnerable

toliq

uefaction

andsand

s(34)

(22)

(1.83)

hard

rock

Major

delta

icaquifer(highand

medium

perm

eability)

Recentmarine

Thick

tofin

e15

–20

>15

1.75

–1.85

205

205

Hardrock,

Vulnearable

toliq

uefaction

terrace

sand

s(17)

(7)

(1.81)

GlacisII,

(softmaterials)

TerraceII

and

alluvial

fan

ofAdrariver

Phreatic

aquifer(highand

medium

perm

eability)

Filling

sBlocksand

<10

Several

<1.70

160

160

Several

Quite

vulnerable

toliq

uefaction

gravels

(8)

(4)

(1.65)

Verysoftmaterials

∗ The

averagevalues

have

been

performed

weigh

ting30

measurements.

† The

averagethickn

esson

everygeolog

ical

form

ationhasbeen

carriedou

tweigh

tingthethickn

essof

thestratig

raph

iccolumns

andbo

reho

les

performed.

‡ InsomefracturedareastheN-valueshave

been

lower

than

50.

§Estim

ated

valueaccordingto

theob

tained

valueby

thesematerials

belong

ingto

closeareas.



Figure 2. Geological cross-sections in Adra town; their location appears in Figure 1. The horizontal andvertical scale is given in meters.

The spatial distribution of the main Plio-Quaternary normal fault systems identified in theAdra town is shown in Figure 1(b). The two fault systems, trending N60–90E and N155–180S,respectively, initially affect the basement, and later, the Pliocene and Pleistocene sediments in agrowth faults generation model as a result of recent tectonic activity in sedimentation (Figure 2).Taking into account the different basement topographical bench marks, obtained from boreholedata, and surface measurements of visible fault scarps, it is thought that these faults continue underquaternary alluvial sediments, shaping the bedrock (Figure 2).

According to the above mentioned data, the Adra urban area can be divided into three maingeomorphological zones: the first one is in the higher and older part of the town (located onhills and slopes) delimited by normal faults ENE–WSW direction. The VS values obtained from



surface seismic refraction profiles are 750m/s up to a maximum depth of 10m and 1350m/sfor the underlying unaltered rock. The second one is formed by narrow valleys that generallyfollow another fault system trending N–S. And the third one is the deltaic plain of the Adra river,where three litoseismic domains can be recognized according to litological features and sedimentthickness. The thickness of Plio-Quaternary sediments grow from the mountain border (delimitedby ENE–WSW fault scarp) towards the sea in a SSE direction. The sediments of the domainclose to fault scarp and hill slopes (first domain) are composed of continental glacis, Holocenecolluviums and sandy alluviums of the Adra river. These sediments have a thickness of 5–10m(estimated from borehole data). At sea direction (second domain) alluvial sands of the Adra riverwere only detected with an average sediment thickness between 25 and 35m. In this location, thesubstratum deepens to the South and is fractured [25]; an approximate VS value of 330m/s forthe surface layer overlaying a 560m/s sediment layer was obtained from seismic refraction data[26]. Near the sea (third domain) the Holocene and Pleistocene sediments are composed of finesand and clay overlaying the basement that is located at a depth of 50–60m.

The metamorphic bedrock has an N -value of less than 50 due to the existence of severalsystems of conjugate fractures. The alluvial materials (N -value between 25 and 30 and averagedensity of 1.82 g/cm3) and the not very dense deltaic alluvial water saturated materials bothoverlay the metamorphic substratum. The Pleistocene marine terrace’s N -values are between30 and 35 and its average VS value is around 300m/s. The Pleistocene continental glacis aresignificantly stable and they have an average N -value of 41, an average density of 1.86 g/cm3 andan average VS value of around 340m/s. The thickest Holocene deltaic alluvial materials saturatedin water have N -values between 15 and 20 and an average density of 1.78 g/cm3, making themhighly prone to seismic amplification [27]; their average VS value is around 260m/s. The recentanthropogenic unconsolidated fillings and recent marine terraces are the materials most prone toseismic amplification due to low N -values (<10 and 17, respectively), low density (<1.70 and1.81 g/cm3, respectively), a high rate of water saturation and low velocity values (160 and 200m/s,respectively) [21].

3. PREDOMINANT PERIOD OF SOILS FROM MICROTREMORS OBSERVATIONS

The reference site methodology has been a traditional way to estimate site effects due to sur-face geology and is generally expressed as the spectral ratio between horizontal seismic motionrecordings at the surface of soft layers and at suitable outcropping bedrock [28, 29]. The applica-tion restrictions of this technique are the existence of a suitable rock outcrop near the soft sitesand being able to achieve earthquake records in each area (a great inconvenience in regions ofmoderate or low seismic activity). Among the non-reference site dependence techniques devisedto overcome this restriction, is the horizontal to vertical microtremors spectral ratio technique(H/V), first proposed by Nogoshi and Igarashi [30] and later described and brought back to theengineering community by Nakamura [31]. It is now one of the most frequent and successfullyemployed technique in the last decade.

The Nogoshi–Igarashi–Nakamura technique (more commonly known as the Nakamura tech-nique) is based on the assumption that microtremors are composed of several waves, but the maincomponents are Rayleigh waves propagating in the soft surface layer overlaying a stiff substra-tum [32]. The H/V technique is based on various assumptions: firstly, the vertical component ofmicrotremors is not amplified by soft surface layers. Secondly, the effect of the Rayleigh waves



on microtremors is the same for the vertical and horizontal components [33]. And thirdly, thehorizontal to vertical motion ratio at the base layer is approximately 1, for the frequency range(0.2–20Hz). Consequently, the H/V spectral ratio of the microtremors recorded at the surfaceturns into an adequate estimation of site response. This conjecture is supported by empirical, theo-retical and numerical results [15, 32, 34–36]. These studies suggest the ability of the H/V spectralratio technique in the estimation of the resonance frequencies (mainly when a sufficient velocitycontrast between layers exists) but does not seem to be able to provide a reliable estimation of theamplification of surface ground motion [32].

The microtremor measurements in the urban area of Adra town have been carried out usingtwo pairs of high-sensitivity seismometers, which have a natural period of 1 s, to record the hor-izontal and vertical components of background noise at each site. Microtremors were recordedat 160 sites with a 100 × 100m dimension grid. In general, the recommended length of therecords depends on the stability of the signal and maximum expected period [37]. From geologicaland geophysical data, the period domain related to the predominant period of subsurface groundshaking characteristics expected in the study area is from 0.1 to 0.5 s. Therefore, it has beenrecorded that a time window of 180 s sampled at a rate of 100Hz was allowed at each observationpoint.

Figure 3. Examples of spectral ratio in sites of Adra town with different soil conditions: (a) hard soil(phylites); (b) medium hard soil (schists); (c) soft soil (alluvial fan); and (d) soft soil (thicker alluvial

fan). (See Figure 5 to locate these sites in the study area.)



Figure 4. Continuous microtremor measurements for stationarity analysis: (a) tunnel site (hard rock)and (b) beach site (soft soil).

Since microtremor spectra could be affected by nearby sources, special care has been taken tocarry out the measurements as far as possible from nearby disturbances caused by heavy machin-ery facilities, household appliances, traffic or by pedestrian passing near the instrument duringmicrotremor measurements, as such kinds of noise are transient and do not show the stationarycharacteristics of ground vibrations [38]. Lombaert and Degrande [39] have experimentally provedthe influence of nearby traffic in soil-induced vibrations, and even soil characteristics are recog-nized as an important influence. They found that overestimation of the soil response at higherfrequencies gradually decreases as the source of transient traffic increases. Furthermore, specialcare has been taken with the presence of spurious peaks in the H/V ratio where spectral amplitudeof ground microtremor was very small, mainly in the points of hard soil placed outside of theurban area analysed. Consequently, a first inspection of the recordings and a Fourier analysis ofthe signals were carried out in each point, and contaminated portions of the record were removedfrom the analysis; in the worst cases, the measurements were repeated at the same point in morefavourable conditions.

At each point, seven time domain records (free from transient disturbances) were Fouriertransformed and Fourier spectra were digitally filtered applying a Parzen smoothing window



Figure 5. Predominant period distribution map from microtremor data analysis and location of damagedRC buildings with four or five storeys (dark block) in Adra town.

with a bandwidth equal to 0.3Hz. The spectral horizontal amplitude was computed for eachfrequency as the geometric average of the spectral amplitudes obtained for the two horizon-tal components. The calculation of the H/V spectral ratio was made after smoothing the spec-tra in order to obtain the predominant period of the soil response at each site(Figure 3).

Continuous microtremor measurements were carried out over 24 h at two sites with differ-ent geological conditions with the purpose of checking the stationarity of the H/V spectralratios. The first site was located inside a tunnel at the centre part of the town, a hard rockoutcrop composed of schists and phyllites of the Adra unit. The second one was located ona soft soil site, at the south-eastern part of the town, near the beach, on hollocene alluvialfan deposit from the Adra river. The shape of the H/V spectral ratio obtained was very flatat the tunnel site and had very clear predominant peaks at the soft soil site. The fundamen-tal frequency is identical over time at each site, showing the stability of H/V spectral ratio(Figure 4).

A map of the predominant periods of the studied urban area was prospected (Figure 5),in order to establish the seismic microzonation in terms of fundamental periods of surfacegeology. The shorter predominant periods (from 0.1 to 0.3 s) correspond with the rock sites(northern part of the town) and with Pleistocene and Holocene thinner deposits (at the mid-dle of the town); the largest periods (greater than 0.5 s) have been detected in the southeasternpart of the town, formed by alluvial fan deposits from the Adra river. The resonance periodvalues increase with the increasing depth to the sediment/basement interface. In some places



(see Figure 5), Alcala-Garcıa et al. [21] have obtained long periods near to hard zones, this beingdue to local artificial deposits caused by farming or the filling of gullies and slopes.

The comparison of the resonance periods, revealed by peaks in the H/V spectral ratio ofmicrotremors, with a general classification of sites based on general geological data, shows anapproximated relation between them, but the general geological map neither explains isolated zoneswith periods greater than 0.3 s nor reflects the changes in predominant periods due to thicknessincreasing in the soft soil zones. Nevertheless, comparing the predominant period map with ageological microzoning, based on a more detailed landform classification using a Quaternary map,geomorphological features, borehole geotechnical data and S-wave velocities, a better correlationcan be found.

The map of ground predominant periods provides a quick guide to study the influence of thesurface ground motion on the dynamic behaviour of buildings and damage distribution occurredin RC building structures of Adra town during the 1993 and 1994 earthquakes.

4. DYNAMIC BEHAVIOUR OF ADRA RC BUILDINGS

The natural period and the damping factor of buildings are key parameters to determine thedynamic behaviour of building structures to dynamic loading and can be used to estimate thepotential damage of a building during future earthquakes [40]. Ambient vibrations, generatedby natural and human activities, are random amplitude solicitations transmitted to the struc-tures by the soil and foundations and are strong enough to excite the building vibrationmodes [41].

The analysis of microtremor measurements is a quick, easy and inexpensive way to assess thedynamic behaviour of building structures and has been applied in several parts of the worldto estimate the natural period of buildings. The application of this analysis is based on thefact that microtremor spreads on the building structure and is amplified at periods close tothe natural period of the building. Kobayashi et al. [1] determined, using microtremor mea-surements, the natural period of vibration (T ) and damping factor (h) of undamaged build-ings in Mexico city, after the 1985 earthquake, obtaining a relationship T = 0.105N for RCbuildings with 5–30 storeys (N ). They found that buildings with a natural period from 0.5to 2.0 s and damping factor h = 0.05 suffered the most severe damage. Midorikawa [42] ap-plied microtremor measurements to constitute a data bank of vibration period and dampingfactor of existing buildings in Santiago de Chile and Vina del Mar (Chile). More recently,Enomoto et al. [5] obtained T = 0.06N using this technique on RC buildings in Caracas city(Venezuela).

Similar research studies on T –N relationship for RC buildings for several cities of Spain havebeen carried out. Kobayashi et al. [43] empirically determined T = 0.051N for Granada; Enomotoet al. [44] obtained T = 0.048N for Almeria, and Espinoza [45] T = 0.046N −0.048 for buildingsin Barcelona. Recently, Navarro et al. [40] collected full-scale data on 89 RC buildings in Granadacity and found the relationship T = (0.049 ± 0.001)N for swaying motion, very close to thoseobtained by Kobayashi et al. [43].

In this study, the dynamic behaviour of RC building structures in Adra town has been obtainedin an empirical way by using microtremor measurements. The main goal is to determinate therelationship between the oscillation period of the swaying motion (in its longitudinal and transversalcomponents) and the number of storeys the building has.



Figure 6. Typical reinforced concrete building structure constructed in Adra town after 1970s.

The structures under analysis were typical RC building constructed in Adra after 1970s. Thecriterion to select the investigated buildings was the regularity both in elevation and plant (generallyalmost square or rectangular), consisting in RC frames with unidirectional floors composed of RCjoists and ceramic or concrete arches (Figure 6). The columns are about 3.1m high, spanning 4–5m,and beams connecting them forming a quite regular grid. The columns are about 0.35×0.35m2 atthe first floor and reducing their dimensions to the upper floors. The room space is divided by thecurrent tradition partitions of hollow clay brick walls, 7 cm thick in the interior plant and 25 cmin exterior, without earthquakes-resistant design (Figure 7). The foundation is formed by clampedfootings of RC.

The microtremor measurements were performed at the top of 39 RC buildings (from 2 to9 storeys) using a three-component seismometer with a natural period of 1 s and flat responsebetween 1 and 10 s. The sensors were oriented one to the longitudinal direction, one to thetransverse direction and the third to the vertical direction, respectively. The signals were am-plified depending on site characteristics, and after being integrated, the signal proportionalto displacement was directly recorded on a lap-top personal computer. The recording time ineach observational point was 3min, sampled at a rate of 100 samples per second, avoidingany cultural noise generated into the structure during the measurement process, which can



Figure 7. Plant view with infill brick walls and structural elements.

saturate the record in a frequency band (usually uncoupled with the fundamental period of thebuilding).

The fast Fourier transformation (FFT) was applied to every record in order to calculate thespectral characteristics of displacement response in the longitudinal and transversal components.Fourier spectrum was computed by smoothening with a Parzen’s window of 0.3Hz in width.The Fourier amplitude spectrum shows a pronounced peak, centred at the fundamental period.



Figure 8. Examples of amplitude spectra of longitudinal motions for RC buildingswith different number of storeys.

Table II. Average natural period and standard deviation for each RC building’s storey number.

Number Number of Longitudinal period Transverse periodof storeys buildings TL (s) TT (s)

2 3 0.16 ± 0.02 0.18 ± 0.043 14 0.17 ± 0.04 0.18 ± 0.044 5 0.22 ± 0.02 0.21 ± 0.035 5 0.19 ± 0.04 0.22 ± 0.066 3 0.26 ± 0.04 0.29 ± 0.017 2 0.30 ± 0.09 0.28 ± 0.128 5 0.42 ± 0.04 0.41 ± 0.049 1 0.42 ± 0.03 0.41 ± 0.01

Figure 9. Relationship between the natural period T and the number of storeys N by the swaying motion.R, coefficient of correlation; SD, standard deviation.



This peak is more pronounced for the case of longer periods (higher buildings) as shown inFigure 8. Table II contains details about the average natural period of building structures and itsstandard deviation estimated in this study. The empirical relationship obtained for Adra town isT = (0.049 ± 0.001)N (Figure 9), with a correlation coefficient of 0.96 and an average standarddeviation of 0.04.

5. RESPONSE SIMULATION OF RC BUILDING USING SDOF MODEL

In order to simulate the dynamic behaviour of buildings in Adra during the earthquakes of 23December 1993 and 4 January 1994, we have assumed a simplified model of the surface shearvelocity structure of the soil and the incident seismic waves in the basement show the samecharacteristics. It was achieved considering the amplitude spectrum due to the subsurface conditionsof the soil and the characteristics of building response (conditioned by natural period and dampingratio).

Taking into account the geomorphological characteristics of the studied area mentioned aboveand the shear velocity values obtained from seismic refraction data in Adra town [26], we haveconsidered three cases of soil structure, all of them composed by single layers 10, 30 and 60mthick, respectively, with a shear velocity of 560m/s overlaying a basement with a shear velocity of1340m/s (Figure 10). The simulated strong surface motion, due to superficial soil conditions, hasbeen calculated using one-dimensional SH wave propagation from basement to surface, consideringa finite number of homogeneous plane-layered elastic medium overlying an elastic half-space,assuming a 5% damping factor. The strong motion observation station in Adra Town is located onhard rock, which is the basement of each soil model. The three components acceleration record

Figure 10. Assumed subsurface soil structure model of Case 1, Case 2, Case 3. The strong motionobservation station in Adra Town is located on hard rock. It is the basement of each soil model andobserved strong motion was input at the basement level and calculated the seismic response wave at the

surface by using one-dimensional SH wave propagation from basement to surface.



Figure 11. Assumed single degree of freedom system as computational simple model for the RC building,from four to eight storeys RC buildings. The natural period of each model was evaluated from the resultobtained by ambient noise measurement, such as the relationship between the natural period and number

of storeys of RC buildings. And the damping factor is assumed to be 5% in each SDOF model.

on the basement in Adra town corresponds to the 1993 earthquake, 5.0 magnitude (Richter scale)and epicentre near Adra town. It has been used as input in the sediment layer in all cases.

The dynamic behaviour of RC buildings has been simulated for three typical buildings with fourstoreys (low building), six storeys (middle building) and eight storeys (high building), consideringa natural period of 0.20, 0.29 and 0.39 s, respectively (Figure 11). It has been assumed an averagedamping factor of 5% in all cases according to results obtained for buildings of the same typologyand size in the research zone [40, 43, 44].

The response at the top of the buildings has been simulated assuming building structure as adamped single degree of freedom (SDOF) system, considering calculated seismic response at thesurface for each type of soil structure analysed. An example of the time histories of the buildingresponse computed for the three typical building structures mentioned above when the sedimentthickness is 30m (Case 2) and their corresponding amplitude spectra are shown in Figure 12.

The maximum absolute acceleration values have been calculated at the top of three typicalbuildings in its N–S (Figure 13(a)) and E–W (Figure 13(b)) components, from the simulatedstrong motion for each type of soil used in this study.

6. DISCUSSION AND CONCLUSIONS

Based on the landform classification, ground and RC building microtremor measurements, geo-graphic distribution of damages caused by the 1993, 1994 Adra earthquakes and strong motionsimulation on alluvial ground surface and at the top of building structures, it is possible to explainwhy damage was mainly located in a particular zone of Adra town and to infer some noteworthyconclusions.

Using geological, hydrological, geotechnical and geophysical data, the Adra urban area hasbeen divided into three main geomorphological zones. The first one corresponds to the zones



Figure 12. An example of the methodology followed to simulate ground and building responsesduring the 1993 earthquake in Adra: (a) E–W component of strong motion record in Adra town; (b)simulated strong motion on sediment soil site with a thickness of 30m (Case 2); (c)–(e) simulatedstrong motions at the top of buildings with four, six and eight storeys, respectively; and (f) amplitude

spectra of the previous strong motions.



Figure 13. Comparison between maximum acceleration values simulated at the top of three differentheight buildings considering simulated strong ground motion for three sedimentary structures:

(a) N–S component and (b) E–W component.

on hills and slopes (the higher and older part of the town) with a thin layer (less than 10m) ofVS = 750m/s over an unaltered rock with VS = 1350m/s. A second one is formed by narrowvalleys following a fault system trending N–S. The third one is the deltaic plain of Adra river,composed of three litoseismic domains with Plio-Quaternary sediments with a thickness of around5–10m, 25–35m and 50–60m, respectively (estimated from borehole data). The thickness ofPlio-Quaternary sediments grow from the mountain border towards the sea in a SSE direction.

The strong contrast of seismic velocity between sediment and bedrock in Adra’s urban area isadequate to apply the Nakamura method and to successfully identify the fundamental resonantperiod of soil. The predominant period distribution of soil is clearly related to the subsurface soilconditions. The shortest predominant period values, between 0.1 and 0.3 s, are found in mountainlandform zones and Pleistocene and in Holocene thinner deposits (north and middle parts of thetown). In these parts, in small isolated zone, long periods have also been detected due to theartificial filling of gullies and slopes. Larger periods (greater than 0.5 s) are found in thickerHolocene alluvial fans (south-eastern part of the town).

The natural period of the RC building structures has been determined from microtremor mea-surements analysis. The relationship T = (0.049± 0.001)N , has been empirically obtained, whereT is the natural period of swaying motion and N is the number of storeys. This result shows agood concordance with the ones obtained by other authors in several cities of south-eastern Spain:Granada city [40, 43] and Almeria city [44] and slightly defers from the one obtained by Espinoza[45] in Barcelona city (north-eastern Spain).

It is interesting that the varying ground characteristics underlaying Adra town played a dom-inant role in the degree and distribution of ground motion intensity [46] and in the consequentbuilding damage in the town. Most of the damage incurred during the 1993 and 1994 earthquakescorresponds to RC buildings with four and five storeys, placed on sediments around 30m thickwith a predominant period of nearly 0.2 s.

The simulation of strong ground motion computed using an acceleration record at the basementof the 1993 earthquake and three different surface structures with a thickness of 10, 30 and 60m,respectively, show peak acceleration on sediments which are 30m thick (corresponding to soilstructure underlaying damaged RC buildings) and is five times larger (Figure 12(b)) than peak



acceleration recording at the outcropping bedrock (Figure 12(a)). This result clearly indicates thatthis alluvial soil structure significantly amplifies the ground motion in a range of periods near tothe natural period of the buildings (Figure 12(f)).

The simulated strong motion of RC buildings shows different characteristics in amplitude andperiod, depending on the considered soil conditions and the number of storeys the building has.Low buildings (four storeys) present, for soil conditions belonging to Case 2 (Vs = 560m/s;thickness 30m), a maximum acceleration four times larger than for soil condition of Cases 1 and3, respectively (Figure 13). On the other hand, in the soil condition of Case 2, the four storeybuildings have a maximum acceleration between two and four times higher than for six and eightstorey buildings, respectively (Figure 13). This result is especially meaningful if we consider thatthe most important damages that occurred in Adra during the 1993 and 1994 earthquakes, wereproduced in RC buildings of four storeys (natural period of building 0.20 s) located in soils withpredominant periods around 0.20 s.

Finally, it is important to note that all the aforementioned results show that the analysis ofmicrotremor measurements is an effective technique to estimate the dynamic behaviour of soiland building structures, and to find possible resonant phenomena during future small to moderateearthquakes.

ACKNOWLEDGEMENTS

The authors wish to express their sincere gratitude to all those who helped them during the survey, especiallyto local government of Adra town and geotechnical companies: Estudio y Control de Materiales S.L.;Geologıa Hormigon y Suelos Almerıa S.A. and ICC Control de Calidad S.L., who provided us withnumerous borehole data. This research was supported by the CICYT coordinated projects AMB99-0795-C02-02 and REN2003-08159-C02.

REFERENCES

1. Kobayashi H, Seo K, Midorikawa S. Estimated strong ground motions in Mexico city. The Mexico Earthquake,Factors Involved and Lessons Learned (ASCE) 1986; 55–69.

2. Seo K et al. Ground motion characteristics in the Taipei Basin during the 1999 Chi–Chi earthquake. Proceedingsof the International Workshop on Annual Commemoration of Chi–Chi Earthquake, vol. 2, Taipei, Taiwan, 2000;260–269.

3. Bakir BS, Sucuoglu H, Yilmaz T. An overview of local site effects and the associated building damage inAdaparzari during the 17 August 1999 Izmit earthquake. Bulletin of the Seismological Society of America 2002;92(1):509–526.

4. Seo K. A joint microtremor measurement in the Fukui basin to discuss the effects of surface geology on seismicmotion during the 1948 Fukui, Japan, earthquake. Proceedings of the XIth European Conference on EarthquakeEngineering, Paris, Balkema: Rotterdam, 6–11 September 1998.

5. Enomoto T, Schmitz M, Abeki N, Masaki K, Navarro M, Rocavado V, Sanchez A. Seismic risk assessmentusing soil dynamics in Caracas, Venezuela. 12 WCEE, CD-ROM 2000.

6. Cranswick E, Ozel O, Meremont M, Erdik M, Safak E, Mueller C, Overturr D, Frankel A. Earthquake damage,site response and building response in Avcilar, west of Istanbul, Turkey. International Journal for HousingScience and its Applications 2000; 24:85–96.

7. Mucciarelli M, Monachesi G. A quik survey of local amplifications and their correlation with damage distributionobserved during the Umbro-Marchesan (Italy) earthquake of September 26, 1977. Journal of EarthquakeEngineering 1998; 2:325–337.

8. Mucciarelli M, Monachesi G. The Bovec (Slovecia) earthquake, April 1998: preliminary quantitative associationamong damage, ground motion amplification and building frequencies. Journal of Earthquake Engineering 1999;3:317–327.



9. Gallipoli MR, Mucciarelli M, Gallicchio S, Tropeano M, Lizza C. Horizontal to vertical spectral ratio (HVSR)measurements in the area damaged by the 2002 Molise, Italy, Earthquake. Earthquake Spectra 2004; 20(S1):S81–S93.

10. Natale M, Nunziata C. Spectrak amplification effects at Sellano, Central Italy, for the 1997–98 Umbria seismicsequence. Natural Hazards 2004; 33:365–378.

11. Panou AA, Theodulidis N, Hatzidimitriou P, Stylianidis K, Papazachos CB. Ambient noise horizontal-to-verticalspectral ratio in site effects estimation and correlation with seismic damage distribution in urban environment: thecase of the city of Thessaloniki (Northern Greece). Soil Dynamics and Earthquake Engineering 2005; 25:261–274.

12. Navarro M, Enomoto T, Sanchez FJ, Matsuda I, Iwatate T, Posadas A, Luzon F, Vidal F, Seo K. Surface soileffects study using short-period microtremor observations in Almeria City, Southern Spain. Pure and AppliedGeophysics 2001; 158:2481–2497.

13. Vidal F. Sismotectonic of Betic and Alboran Sea region. Ph.D. Thesis, University of Granada (in Spanish withEnglish abstract), 1986.

14. Triantafyllidis P, Hatzidimitriou P, Theodoulidis N, Suhadolc P, Papazachos C, Raptakis D, Lontzetidis K. Siteeffects in the city of Thessaloniki (Greece) estimated from acceleration data and 1D local soil profiles. Bulletinof the Seismological Society of America 1999; 89:521–537.

15. Lermo JF, Francisco S, Chavez-Garcıa FJ. Site effect evaluation using spectral ratios with only one station.Bulletin of the Seismological Society of America 1993; 83:1574–1579.

16. Semblat JF, Duval AM, Dangla P. Numerical analysis of seismic wave amplification in Nice (France) andcomparisons with experiments. Soil Dynamics and Earthquake Engineering 2000; 19(5):347–362.

17. Sanz de Galdeano C. The Betic-Rif Internal Zone. University of Granada: Granada, Spain, 1997; 316 (in Spanish).18. Aldaya F, Baena J, Ewert K. Geologic map of Spain 1:50.000, no 1.057 (Adra). Geological Survey of Spain

1983, Madrid (in Spanish).19. Fourniguet J. Neotectonique et Quaternaire marin sur le littoral de la Sierra Nevada (Andalousie, Espagne). Ph.D.

Thesis, University of Orleans, 1975 (in French with English abstract).20. Goy JL, Zazo C. Synthesis of the quaternary in the Almerıa litoral, neotectonic activity and its morphologic

features (eastern Betics, Spain). Tectonophysics 1986; 130:259–270.21. Alcala-Garcıa FJ, Espinosa J, Navarro M, Sanchez FJ. Proposal of regional geologic division of Adra town

(province of Almerıa). Application to the seismic zonation. Revista Sociedad Geologica Espana 2002; 15:55–66(in Spanish with English abstract).

22. Pulido Bosch A, Morales G, Benavente J. Hydrogeology of delta of Adra river. Estudios Geologicos 1988;44:429–443 (in Spanish with English abstract).

23. Imai T. P- and S-wave velocities of the ground in Japan. Proceedings of the 9th International Conference ofSoil Mechanics and Foundation Engineering, Tokio, vol. 2. 1981; 257–260.

24. Kokusho T. In-situ dynamic soil properties and their evaluation. Proceedings of the 8th Asian Regional Conferenceof Soil Mechanics and Foundation Engineering, Kyoto, vol. 2. 1987; 215–240.

25. Martınez Dıaz JJ. Sismotectonic analysis of seismical series of Adra: mechanisms of composed earthquakes asresponse to the interaction between active faults in the South-east of Betic Cordillera. Revista Sociedad GeologicaEspana 2000; 13:31–44 (in Spanish with English abstract).

26. Seo K. Joint study on seismic microzonation in Granada Basin, Spain. Report of Research Project, Grant. In-Aidfor International Scientific Research, 1995–1997, Project No 07044136, Tokyo Institute of Technology, 1999.

27. Seed HB, Tokimatsu K, Harder LF, Chung RM. Influence de SPT procedure in soil liquefaction resıstanseevaluacions. Journal of Geotechnical Engineering 1985; 3(12):1425–1435.

28. King JL, Tucker BE. Observed variations of earthquake motion across a sediment-filled valley. Bulletin of theSeismological Society of America 1984; 74(1):137–151.

29. Gutierrez C, Singh K. A site effect study in Acapulco, Guerrero, Mexico: comparison of results from strongmotion and microtremor data. Bulletin of the Seismological Society of America 1992; 82:642–659.

30. Nogoshi M, Igarashi T. On the propagation characteristics of microtremors. Journal of Seismological Society ofJapan 1970; 23:264–280 (in Japanese with English abstract).

31. Nakamura Y. A method for dynamic characteristics estimation of subsurface using microtremor on the groundsurface. Quarterly Report of Railway Technical Research Institute 1989; 30:25–33.

32. Bard P. Microtremor measurements: a tool for site effect estimation? The Effects of Surface Geology SeismicMotion 1999; 3:1251–1279.

33. Bour M, Fouissac D, Dominique P, Martin C. On the use of ambient noise recordings in seismic microzonation.Soil Dynamics and Earthquake Engineering 1998; 17:465–474.



34. Field EH, Jacob KH. The theoretical response of sedimentary layers to ambient noise. Geophysical ResearchLetters 1993; 20(24):1127–1143.

35. Lachet C, Bard PY. Numerical and theoretical investigations on the possibilities and limitations of the Nakamura’stechnique. Journal of Physics of the Earth 1994; 42:377–397.

36. Rodrıguez V, Midorikawa S. Comparison of spectral ratio techniques for estimation of site effects usingmicrotremor data and earthquake motions recorded at the surface and in boreholes. Earthquake Engineering andStructural Dynamics 2003; 32:1691–1714.

37. SESAME European Research Project. Guidelines for the implementation of the H/V spectral ratio techniqueon ambient vibrations: measurements, processing and interpretation. European Commission—Research GeneralDirectorate 2004, 2004.

38. Yamanaka H, Dravinski M, Kagami H. Continuous measurements of microtremors on sediments and basementin Los Angeles, California. Bulletin of the Seismological Society of America 1993; 83:1595–1609.

39. Lombaert G, Degrande G. Experimental validation of a numerical prediction model for free field traffic inducedvibrations by in situ experiments. Soil Dynamics and Earthquake Engineering 2001; 21(6):485–497.

40. Navarro M, Sanchez FJ, Fetiche M, Vidal F, Enomoto T, Iwatate T, Matsuda I, Maeda T. Statical estimationfor dynamic characteristics of existing buildings in Granada, Spain, using microtremors. Structural Dynamics,Eurodyn 2002, vol. 1, Munich, Germany, Balkema: Rotterdam, 2002; 807–812.

41. Dunand F, Bard PY, Chatelain JL, Gueguen Ph, Vassail T, Farsi MN. Damping and frequency from randomdecmethod applied to in situ measurements of ambient vibrations. Evidence for effective soil structure interaction.12th European Conference on Earthquake Engineering, London, U.K., 2002; 869.

42. Midorikawa S. Ambient vibration tests of buildings in Santiago and Vina del Mar. A Report of the Chile-JapanJoint Study Project on Seismic Desing of Structures, The Japan International Co-operation Agency, 1990.

43. Kobayashi H, Vidal F, Feriche D, Samanco T, Alguacil G. Evaluation of dynamic behaviour of building structureswith microtremors for seismic microzonation mapping. The 11th World Conference on Earthquake Engineering,Acapulco, 1996.

44. Enomoto T, Navarro M, Sanchez FJ, Vidal F, Seo K, Luzon F, Garcıa JM, Martın J, Romacho MD. Dynamicalresponse of buildings in Almeria, by means of environmental noise analysis. I Asamblea Hispano-Portuguesa,Aguadulce, Spain (in Spanish with English abstract), 1999.

45. Espinoza F. Determination of dynamical characteristics of structures. Ph.D. Thesis, Technical University ofCatalonia, Barcelona, 1999 (in Spanish with English abstract).

46. Vidal F, Romacho MD, Feriche M, Navarro M, Abeki N. Seismic microzonation in Adra and Berja Towns,Almeria (Spain). Proceedings of the 11th World Conference on Earthquake Engineering, Acapulco, 1996.


analysis of the weightiness of site effects on reinforced concrete (rc) building seismic behaviour:...

Documents