38.- The Development of the Canadian Building Code for Masonry by

J. F . CUTLER

Canadian Stmctural Clay Associatian, Willowdale, Ontar;o

W. G . PLBWES

National ReJearch COWJcil, Ottawa, Onraria

and

P. T . MIKLUCH1N

P. T. Mikluchin & Assoclates, Toronto , Onlor;o

ABSTRACT

During lhe pas! decade a successful vigorous effort has been made in Canada towards lhe implemento/ali of modem technical del'elopmellts in lhe ,r/rue/ural design of masol/ry. These e.fforrs resulted in lhe codifica­liol1 Df theoretical and experimental results in lhe Jorm Df a modem Building Code. [n developing suilable code regl/latiou.'!, criticai eva/uation of s/rue/ural researe" information reporled by orher cOlllllries !tas been taken ;'110 considera/f 011. The paper explains holV research information /tas been pUI to use in Cal/ada and under­fines lhe COl1odian approach to severa! questiol1s lI'hich may be considered in some lVay specia! to tlle overal/ deve/­opment Df ellgilleered /oad-bearblg mosollry.

1. INTRODUCTION

L' Elaboratíon du Code Canadíen de Construction pour la Maconnerie

Pendant les dix dernieres années un effort intense el couronné de succes a élé fail au Canada pour I'implanta­tion des techniques modernes dons la construction des maçonneries en briques. Ces efforts ont eu pour résultat la créalion d'uII nOUl'eau code de constructioJl moderne et d 'une codification des résultals théoriques el expérimentaux. Ces réglementa­tions onf été établies apres un examen crilique des informations relatives aux recherches concernanl la con­slruction eflectuées dans d'autres pays. Le rapport explique comment celle informatioll a été utilisée au Canada el décrit les Iravaux d ' approche canadiens à I'égard de plusieurs problemes qui peuvent étre considérés d'un intérét parliculier pour le dé­veloppement généralisé des maçon­neries porteuses indusfrialisées.

Die Entwicklung eines Kanadí.chen Baugesetzes für Mauerwerke

Wiihrend des lelzten lahrzehnts sind in Kanada kriiflige Anslrengungell erfolgretch gewesen, moderne tech­nische Entwicklungen bei der Kon­struktion von Mauerwerken nulzbar zu machen. Aus diesem Bestreben sind neue moderlle Richtlinien ais eine Anleitung zum Bauen hervorgegangen. Für die Enlwicklung brauchbarer Gesetzesregeln sind aueh einschliigige Forschungsberichte aus allderen Lãn­dern kritisch ausgewertet und in dte Oberlegungen mil einbezogen worden. Die Abhandlung zeigl, wieForsehungs­ergebnisse in Kanada angewendet wurden ulld unterslreicht die kanad­ischeBehandlung verschtedener Fragen, die auf untersehiedliche Weise bet­rachtet werden kõnnen, spezie/l bezüglich der Gesamtentwicklung tech­nischer, lasttragender Mauerwerke.

During the past decade, a successful vigorous e/fort has been made in Canada towards the implemenlation of modern technical developments in the structural design of masonry. These e/forts resulted in lhe codification of theoretical and experimental results in lhe form of a modern building code. This achievement has been due to a policy of frequent revisions and a general progressive attitude on the part of authorities producing the National Building Code of Canada (NBC). At lhe same time a very important contribution has been made by the Cana­dian Structural Clay Association in the form of educa­tional activities, collating. and assessing these modern lechnical developments.

the amoun! of structural testing of masonry has been limited.

In developing suitable code regulations, criticai evalua­tion of structural research information reported by other countries has been taken into consideration. This paper will not, Iherefore, dwell on ,tructural matters that have been, or will be reported by others. lt wilt, however, explain how research information has been put to use in Canada, and underline lhe Canadian approach lo several questions which may be considered in some way special. to the overall development of engineered load-bearing masonry.

2. EARLIER BUILDING CODES Masonry research in Canada has been confined

mainly to material, and construction problems, such as durability, water penetration, efflorescence and staining. compatibility of mortar and units , and winter construc­tion. The main source for this work, was the Division Df Building Research of the National Research Counci!. but owing to limited numbers, and the availability of very large testing machines in the country as a whole,

233

In 1937. the Toronto Building By-law, the leading Canadian code of Ihat time, required that masonry walls at the bollom storey of a len-storey building be 27 lo 40 in. Ihick or more, depending on the type of building and conditions. This one sentence is suffieient to indicate that during lhe fir,t half or this century, masonry design in Canada consisted of the ,ame Iradi­tional, very conservative rules, that universally prevailed.

234 The Developrnent of the Canadian Building Code for Masonry On the basis of great numbers of tests on walls and panels performed in various co untries, mainly the USA , the relationship between brick and morta r strength, and wall strengths under compressive axial loads, has been c1arified. The ranges of slenderness and ec­centricity tested, were rather limited; and systematic efforts were nOl made by code authorities to evolve a more rational approach to the analytical system of designo

The number of tests and range of variables explored, were, however, suffieient to gradually relax traditional wall thicknesses to a modest degree. On the basis of those new, developments structural design of masonry in the National Building Code 1960 contained the following basic three requirements:

I. The minimum thickness of load-bearing and non­load-bearing \Valls \Vere as shown in Figure I. Note that the bottom-storey wall of a ten-storey building was still 20-in. thick despi te the relaxations mentioned above.

2. The maximum distance between lateral supports was 20 times lhe wall thickness.

3. Allowable average compressive stresses on the gross area af a wall \Vere as shown in Table lo

These details of traditional design are given here not beca use they are significant in terms af modern masonry design, the subject of this conference, but because they form the background against which analytical design was first applied in Canada.

With the large thicknesses and Iimits on slenderness imposed by the first t\Vo of the above requirements, it was not surprising that in Canada, load-bearing masonry was not a popular method af construction, against lhe

competition af 5teel and concrete, except for some rela­lively low buildings of three or four storeys. The allow­able stresses shown in Table I were somewhat low as compared to British slandards of that day ; but it should

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FIGURE I- Wall thickness requirements: Nat ional Building Code o f Canada, 1960.

TABU! l - AlLOWABLE STRESSES I N BRI CK MASONRY : NATIONAL BUILDING CODE, 1960

Type Df mortar Type of Type of maso"ry Illlirs I masonry M S N O K

Solid Rubble stone 140 120 100 80 -masonry Ashlar granite 800 720 640 500 -

Ashlar limestone and marble 500 450 400 325 -Ashlar sandstone and caS l-Stone 400 360 320 250 -

Solid tll/its, except concrele block. with an ullimate compressive strength of: Over 10 000 Ibf/in 2 500 450 350 250 100 Over 8000 lo 10 ()()() Ibf/in2 400 350 300 200 100 Over 4500 to 8 000 Ibf/in ' 250 225 200 150 100 Over 2500 to 4 500 Ibf/in' 175 160 140 110 75

1500 to 2 500 Ibf/in2 t25 115 100 75 50

Solid concrete block Over I 800 Ibf/in' 175 160 140 100 -

I 200 to I 800 Ibf/in' 125 115 100 75 -Hollow load-bearing units 85 75 70 - -

Cavity Sofjd IIl1ilS, except concrele block, with ao walls ultimale compressive strength of:

Over 2 500 Ibf/in' 140 130 110 - -1500 to 2 500 Ibf/io' 100 90 80 - -

Solid concrete block Qver 1 800 Ibfjin2 140 130 110 - -

1200 to I 800 Ibf/in' 100 90 80 - -

Hollow load-beariflg units 70 60 55 - -Column 1 Colum 2 3 4 5 6 7

-

TABLE 2- ALLOWABLE STRESSES IN BRICK MASONRY; NATIONAL BUILOING CoOE. 1965

Type of n/orlar Type of maSOl/ry T)'pt of masonry tmits

M S ,

Modu/us Solid rnasonry Solid uniLs exCepl solid concrete unit with an ultimate Compression Shear o! Compression Shear

compressive slrcngth of: (lbflin') (lbfl il/') rupture (lb!!in' ) (/b!!il/') (/bfl in')

Over 10000 lbf{in2 500 20 20 450 15 Over 8 000 to 10 000 Ibf/in' 400 20 20 350 15 Over 4500 to 8000 Ibf/in' 250 20 20 225 15 Over 2500 to 4 500 Ibf/in 175 20 20 160 15

I 500 to 2400 Ibf/in' 125 15 15 115 10

Solid concrete units Over I 800 IbrJin2 175 15 15 160 10

I 200 to 1 800 Ibf/in' 125 10 10 115 10 Load-bearing structural clay tile units

Over I 800 lbf/in2 125 15 15 100 10 Load-bearing hoUow concrete 1000 to 1399lbf/in2 100 6 6 90 5 Load-bearing hollow units 700 to 999 lbf/in2 85 6 6 75 5

-Solid rubble stone 140 15 15 120 10

Grouted masonry Solid units, with an ultimate compressive strength of: Over 10000 Ibffin2 600 30 Over 8000 to lO 000 IbCfin2 500 30 Over 4500 to 8000 Ibfjin2 350 25 Over 2500 to 4 500 Ibf/in' 275 25

I 500 to 2 500 Ibf/in' 225 25

Cavity walls Solid units. except concretc units with an ultimate compressive strcngth of :

12 12 110 10 Over 2 500 Ibf,l in2 140 1 500 to 2 500 lbfjin2 100 12 12 80 10

Solid concrete units Over 1 800 Ibfjin2 120 10 10 90 8

I ZOO to I 800 Ibf/in' 100 10 10 80 8

Hollow Joad-bearing concrcte units over 1 000 Ibf/in2 70 8 8 50 6

Colurnn I 2 3 4 5 6 7

Modll/us o! Compressioll

ruplure (/b!lil/' ) (/b!/in' )

15 350 15 300 15 200 15 140 10 100

10 140 10 100

10 75 5 5 5 10

10 100

30 -30 -25 -25 -25 -

10 -10 -

8 -8 -

6 -

8 9

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Shear (/b!!il/' )

10 10 10 lO -

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236 The Development of the Canadian Building Code for Masonry be pointed ou! Ihat Ihey were taken lo be allowable for any slendemess ratio up to h/t ~ 20 and no special guidance ar admonition was given to take into aceount eccentricities af vertical loads, ar transverse loads. To counterbalance this, however. even those stresses were seldom, if ever, atlained in lhe low-rise Iype of bearing wall buildings generally buil!.

For low-rise buildings, lhe rule-of-Ihumb method of masonry wall design had only one merit, simplicity ; there were, however, tWQ major disadvantages:

I. Even among otherwise sophisticated engineers, Ihere was little understanding of masonry wall behaviour and no guidelines were given, by which the structural behaviour of an unreinforced masonry wall could be satisfaclorily predicled in a given situation.

2. The conservative restrictions 00 thickness and allowable stress did nol allow full exploitation of lhe pOlenlial slrenglh of Canadian bricks which range from 2500 lo over 10000 Ibf/in'; 80% being 8000 Ibf/in' or better.

3. RECENT DEVELOPMENTS

Aboul 10 years ago il beca me evident lo our Codes and Standards Committees, and lhe Canadian Masonry Associations, that a more ralianal flexible, general engineering design melhod, would have to be established if full advanlage could be taken of progresso

This was forcibly broughl home by lhe pioneering sludies, and remarkable achievements by Haller in Switzerland. Numerolls articles in European and British literature also showed thal masonry in Canada was in danger of lagging behind. Successive revisions of Brilish Standard C.P.III demonslrated , Ihal a start could be made towards introducing new design methods in Canada.

Accordingly in lhe 1965 edilion of lhe National Build­ing Code, permission was given for the first lime to design plain masonry on lhe basis of an engineering analysis ralher Ihan by convenlional and slandard engineering rules. The design regulations were very brief, and a deliberale elfort was made to be conserva tive aI this early slage.

AI Ihal lime il was decided nOI lo increase lhe basic axial compressive stresses, as indicated in Table 2.

lt will be recalled Ihal Ihese exisling allowable stresses were laken 10 be applicable for ali slendemess ralios up 10 h/r ~ 20. In construcling a table (see Table 3) of reduction factors for slenderness and eccentricity to lake "/r ~ 20 as a datum and allow slress increases for lesser slendernesses and reduclions for grealer Ihan 20. This ralher unusual approach perhaps deserves an explanalion.

In order nOI lO upsel unduly the conslruclion induslry and lhe design professions in one sudden step, il was also necessary lo retain in lhe code, the old rule-of­Ihumb design wilh ils own lable of slresses. Since de­signers in Canada had little experience in designing masonry for slress il was Ihoughl LO be potenlially confusing lo provide a second lable of basic slresses for analylical designo By taking lhe convenlional allowable stresses aI h/r ~ 20 as datum il seemed Ihal designers could not go 100 far wrong in Iheir early elforts 10 apply lhe new design concepls, especially since they were now called upon 10 lake imo accounl eccentricity which had not been specifically required before.

TABLE 3- REDUCTION FACTORS FOR SLENDERNESS ANO ECCENT­RlelTY: NATIONAL BUILDING COOE, 1965

Slress factor (Eccelllriciry o/ vertical

Slendemess

loading as a proporlion 01 file rhickness 01 lhe

member) ralio

O 116 1/4 1/3

10 1·30 0·97 0·86 0·73 12 1·23 0·90 0·78 0·64 14 1·\7 0·83 0·70 0·54 16 1'11 0·76 0·63 0'45 18 1·06 0·70 0·54 0·36 20 (-(lO 0·61 0·47 0·27 22 0·94 0 ·56 0·38 0'19 24 0·89 0·48 0·31 0·10 26 0·83 0-42 0·23 O·()() 28 0·77 0·34 0·16 O·()() 30 0·)2 0-27 0·08 O·()()

Linear interpolation between values for lhe stress factors is per­missible .

As can be seen from Table 3, lhe slenderness ralios were allowed lo go as high as h/r ~ 30 since Ihis seemed lO be justified by available data. The maximum slender­ness was, however, limited to one-third the thickness; since lhe committee was of lhe opinion that any greater eccenlricilies should nol be allowed aI Ihal lime; unlil furlher sludies had been made, and praclical experience gained.

Referring back to lhe stress reductions for slenderness and eccentricity, lhe actual rales of decrease were selected after studies of test results reported from Switzerland, Britain and other countries.

4. MATERIALS ANO INSPECTION

From lhe outset the Code Committee was very conscious of lhe need for high-qualily malerials, construclion praclice, and inspeclion of high-rise buildings if engin­eered masonry was lo bo encouraged. This is especially important because freezing conditions are continuous for almOSI 6 months in mosl parts of Canada.

The Nalional Building Code and lhe Standards of lhe Canadian Slandard Association had for many years spelled out in delail lhe materiaIs and conslruction requiremenls for qualily masonry. For example, clay brick specifications contain requirements for sampling and slrenglh testing, freeze Ihaw durability, dimensional tolerances, warpage tolerances, suction rate, etc. Similarly Ihere are slandard specificalions for ali the materiaIs used in morlars, as well as for their preparation and mixing, workability, and water retention properties.

1t was realized , however, lhat in many of the low and less imporlant buildings common lo bearing-wall CÓI)­slruction of lhe pasto Ihere was often a lack or inspeclion and qualily control on sile. This had resulled in a cerlain disregard for good construction practices among conlraClors and workers in lhe field.

For high-rise engineered masonry structures, inade­qualely conlrolled morlars, parlial filling of joints, and unprotected construction in freezing weather, could nOl of course be lolerated. Since these were already pro­hibited by our codes, lhe key lo adequale conSlruclion lay in improved insl'ections. The 1965 code, Iherefore, required thal where masonry is designed by analytical ralher Ihan rule-of-Ihumb procedures, there shall be

J. F. Cutler, W. G. Plewes and P. T. Mikluchin 237 at alI times 'special engineering ar architeclural super­vision:

The codes cf some countries deal with this situation by permitting lower stresses where supervision is lacking. The Canadian authorities, however, hold to the view that beeause of the thinness of the walls and praetieabJe heights possible with engineered masonry, it should not in any circumstances be permitted except under inspection.

5. REINFORCED MASONRY Most of the remarks 50 far have referred to plain masonry. Reinforced masonry af course has a Iong history but was nOI much in favaur in Canada for normal contruction, owing to lhe general lack af interest in bcaring·wall masonry eonstrlletion there. With the expected advent af high-rise masonry construction however, it was evident lhat reinforcement af masonry would be desirable in some situations. Detailed require­ments for reinforced masonry were introduced ioto lhe National Building Code. These essentially followed ASA A41.2 Building Code Requirements for Reinforeed Masonry, a doclIment of long standing whieh need nol be reviewed herc.

6. ACCEPTANCE

This concludes lhe brief summary af the introductioll of the analytieal or engineering approaeh to the design Df masonry in Canada.

Initially, it was n01 received with any great enthusiasm , except by a few masonry oriented and experienced designers. This was not due to any particular resistance among designers , but beca use engineers and architects were almost totally unllsed to thinking of masonry as ao engineering material. capable af more economic and imaginative construction.

Sinee the requirements in the eode gave only the bare principies Df materiais and design it was necessary to embark 011 a programrne af education among design engineers. A major step was the publieation of a Clay Masonry Manual sponsored by the Briek & Tile Institute of Ontario, and eompiled by two of the present authors­Cutler and Mikluehin. This explained how to approaeh the design of high·rise thin·wall masonry, illustrated with design examples. Of eonsiderable help a lso in this regard were the publications of the Clay Produets Teehnieal Bureau in Great Britain and the Struetural Clay Produels Institute in the USA. These were widely circulated to engineers, architects and building officials, to inform them of aetivities elsewhere and provide furlher examptes.

As a result af lhi5 activity, interest in masonry bearing-

TABLE 4-ALLOWABLE STRESSES I N BRICK MASQNRY: NATIONAL BUlLDING CODE 1970-SuPPLEMENT No. 4

Compressiye Assumed compressiye strellgth 01 brick srrengrh 01 masonr)', r m, (lbJlin2)

IInits .

(lbJlin2) Type M mortar Type S mortar Type N mortar

14000+ 4600 3900 3200 12 000 4000 3400 2800 10 000 3400 2900 2400 8 000 2800 2400 2000 6 000 2200 1900 1600 4 000 1600 1400 1200 2 000 1000 900 800

wall eonstrlletion inereased, at first slowly, and then very rapidly, 50 that aI present there is scareely any major city in Canada where there is not a high-rise masonry structure under construction and numerous other projeets on the drawing boards.

7. RECENT CODE ACTIVITIES, 1970 EDITION OF NATIONAL BUILDING CODE

ft should be brietly mentioned that the masonry require­ments of the National Building Code are soon to be revised .

About lhe same time that lbe 1965 euilion of NBC appeared, aClive interest in engineered masonry arose in the USA; and with the resourees at their disposal the Structural C1ay Produets Assoeiation and other USA laboratories have made intensive sludies and many tests to bring their own codes up to date. This work has been followed c\osely in Canada and has been partieipated in to some degree at the eommittee stage. Consequently the National Building Code of Canada is being revised this year following c\osely the reeommendations of the 'BlIilding Code Requirements for Engineered Brick Masonry ' published by the Strlletural C1ay Produets Institute. The SCPI recommendations will provide the means for a welcome and needed expansion of the brief ru les given in the present (1965) NBC.

The new SCPI Code will not be deseribed here in detail since this is the perogative of the USA representa· lives . However, in adapting it to Canadian requirements a special approach was taken 00 several points.

In lhe Canadian Struetural Design Manual, Supple· ment No. 4, NBC, 1970, the b.sie eompressive strength of the masonry, f'm' may be established by tests of prisms or alternatively by tests of units and morta r with

TABLE 5 - ALLOWABLE STRESSES IN PLAIN BRIC K MASONRY ; NATIONAL B UILDlNG CeDE I 970-SuPPLEMENT No. 4

DescriptiolJ Designaria" Alfoll'oble slresses (lbJlin2)

Compressive, axial Walls Im 0'251'm Columns Im 0·201'm

Compressive, flexural Walls Im O·325/'m Columns Im 0'261'm

Tensile. flexural Normal to bed joints

Mor S mortar f, 36 N mortar f, 28

Parallel to bed joims Mor S morlar f, 72 N mortar (, 56

Shear vi'm. but not to Mor S morta r 'm

exceed 50 N mortar 'm , I /' m. but not to

exceed 3S

Bearing on masonry On full area Ih 0·25 I' On one-third area or less Ih 0'3751'm

Modulus of elasticity Em lOOO/'m. butnot to exceed 3 ()()() 000

Modulus o f rigidity E, 400 /' m. but not to exceed 1 200 000

238 The Development of the Canadian Building Code for Masonry the correspondiog value of f'm selected by interpolating from the values given in Table 4 and the allowable stresses must not exceed the values given in Table 5.

By either approach at least five prelimioary tests are required and f 'm is to be taken as the average test result reduced by a factor (I - I! 1'/ 100) \Vhere v is the test coefficieot of variation. This is similar to the SCPI statement, and io an approximate way allows for the oatural statistical variability of materiaIs and testing.

Ao examioatioo of Table 2 (NBC, 1965) and Tables 4 and 5 (NBC, I 970- Supplemeot No. 4) quickly indicates the increased stresses allowed in the new design manual.

AIso introduced for the first time are Tables 6 and 7. These establish the compressive strength of masonry using clay tile and hollow block. Although the allowable stresses indicated in Table 6 are based on the mandatory 1965 allowances, it is to be hoped that the tile-testing programme now under way at Texas University \ViII eventually produce a table of strength allowances

TABLE 6 CoMPRESSIVE STRENGTH Df MASONRY USING CLAY TILE AND HOL­LOW BLOCK: NATIONAL BUILDlNG CoDE 1970-SuPPLEMENT No. 4

Compressive srrengrh of ullit~· · (lbf/in')

6000+ 4000 2500 2000 1500

Assumed compressive s/reI/crI! of cOl/aele b/ock masonry or stl"llct/lral

file masQnry (/'111) (lbJJin2)

Types M afld S mortar I Type N mortar

2400 2000 1550 1350 11 50

1250 1250 1050 950 800

• Based 00 gross cross-sectional area for masonry of solid units and filled hollow unils; nel cross-secrional area for masonry of hollow units.

similar to that for brick. The NBC Masonry Committee thought Ihat there should also be requiremeots for regular testing during construction of a building, in much the same way as concrete is ordinarily tested. Therefore NBC, 1970, Supplement No. 4, will require a prism test, or test of units and morta r (whichever method is being used) for every 5000 ftZ of wall and at Ieast five such tests for a building. Although experience is lacking on the value of the coefficient of variation Iikely to be ob­tained in field tests, it was initially assumed to be much the same as for concrete; that is, about 15 % on the average. Accordingly, it is required that no field test result should fali below 80 % of the value of f' m deter­mined from the preliminary teSlS. This should occur only about one in 200 occasions unless the strength or variability of the masonry is actually lower respectively thao determioed from the preliminary tests.

8. DESIGN FOR EARTHQUAKE

Aoother subject requiring serious consideration by Canadian Building Code authorities was the malter of earthquakes. For many years the only area in Canada where the earthquake possibility was regularly coosidered in design was on the West Coast of British Columbia. In receot years it has become recogn ized that large areas of the country, especially in the SI. Lawrence Valley vicinity, Quebec and Ontario, are in active earthquake zones of various intensities. An earthquake lOne map has been constructed for the couotry; the zooes are rated as follows:

Zone No. 3 2 I O

Seismicfactor R 4 2 I O

TABLE 7- ALLOWABLE STRESSES IN PLAIN CoNCRETE BLOCK MASONRY ANO SntUCTURAL CLAY TILE MASONRY; NATIONAL B UILDING CODE 1970--SuPPLEMENT No. 4

I A/lowable slresses (lbJ/in2)

Description Designario" Solid or Ho/low filled-cell units lmits

Compressive, axial Walls f. 0'20!'m 0'225!'oo Colwnns f. 0'18!'m 0 ·20 f'm

Compressive, flexural Walls /O, 0'30/'m 0'301'111 Columns /O, 0'24f'm 0'24 I'm

Teosile, fiexural Normal to bed joints Mor S mortar /o 36 23 N mortar . f, 28 16

Paraltet to bed joints Mor S morta r /o 72 46 N mortar fi 56 32

Shear Mor S mortar v. 34 34 N mortar Vn 23 23

Beariog on masonry 00 full area fh 0·25 I'm 0·25 1'", 00 one-third area ar Jess fh 0 '375/'m 0'375/'00

Modulus of eJasticity En looo/'m but nol to 1000/'m but not to cxceed 3 000 000 exceed 3 000 000

Modulus of rigidity E. 400/'m but not to 400/'m but oot to exceed 1 200 000 exceed 1 200 000

---

J. F. Cutler, W. G . Plewes and P. T. Mikluchin 239 The seismic factar, when inserted in the appropriate

code formulae, gives the relative severity af the forces for which design is to be made. For example, the design forces in zone 3 will be faur times as large as those in zone I.

As a consequence of this belated recognition of the earthquake problem, building code requirements will require earthquake design in a number af major cities where it was not required before. In arder not to upset the whole building tradition at once and in recognition of the fact that much research and study is still needed, lhe NBC, 1970, will arbitrarily exempt, at least for the present, certain buildings from earthquake designo

For masonry, thase exempted, are buildings in zones O, I and 2, three storeys or less in height, and 6000 ft' or less in area. There is a proviso, however, that these are not institutional or high-hazard buildings. As a conse­quence of this decision small buildings in lOnes 0, I and 2 may still be designed by the traditional rule-of-thumb method, if the designers or builders do not wish to undertake an analysis.

For buildings of any height, masonry may be built without reinforcement in zones O and 1. In zone 2, minimum reinforcement is required, regardless af the stresses unless special permission is granted by the building official to omit it. In lOne 3 ali masonry buildings will require reinforcement.

9. DESIGN AGAINST PROGRESSIVE COLLAPSE

Finally, there has been consideration of the problem of progressive collapse following widespread discussion of this subject in British Iiterature. For understandable reasons there has been no firm decision with regard to detailed recommendations. For the present the code merely gives a warning statement:

'Buildings and Structural systems shall provide such structural integrity, that hazards associated with progressive collapse, due to load failure caused by severe overloads, or abnormal loads not specifically covered in this Section, are reduced to a leveI commen­surate with good engineering practice.' An accompanying explanatory document discusses

the question in a more detailed way in terms of the necessity for good connections, aIternate Ioad paths, and so on; but the ultimate responsibility still remains with the designers to consider the possibility of local collapse spreading throughout the structure. Ronan Poiot is not the only case of progressive collapse known. Roofs have collapsed in the same manner in Canada, parti­cularly in large span areas; in most cases these are due to snow loads, but at least one was connected with a gas explosion.

Canadian engineers are continuing to ponder this question; and again tribute must be paid to numerous reports and publications from Great Britain as contri­buting to our thinkiog. Special mention should be made of the informative publications produced by the British brick masonry industry, which are used as exampIes of rational thinking on structural integrity in discussion with designers.

Other additions or changed sections to NBC, 1970, Supplement No. 4, and worthy of further study by designers are:

(a) Grouled Reinforced Masonry

Grouted reinforced masonry is that form of reinforced masonry construction made of two or more wythes af brick or salid concrete units in which interior cavities between wythes are filled by pouring grout therein as the work progresses and in which the reinforcement is placed between the wythes or other specially provided space.

(b) Conlribulion of Flanges in Slruclllral Desigl/ of T­and L-shaped Wal/s

(i) Where shear \Valls intersect a wall or walls to form symmetricaI T or I sections, the effective ftange width shall not exceed one-sixth of the total \Vali height above the levei being analysed and its overhanging width on either side of the shear wall shall not exceed six times the thickness of the intersected wall.

(ii) Where shear walls intersect a wall or walls to form L or C sections, the effective overhanging flange width shall not exceed one-sixteenth of the total wall height above the levei being analysed nor six times the thickness of the intersected wall.

(iii) Limits on effective fiange width may be waived when approved after a review of a written justification.

(iv) The vertical shear stress at the intersection shaIl not exceed the allowable stress in Articles 4.4.3 .11 to 4.4.3 .14. inclusive of the intersection if laid up in true masonry bond as set forth in Artiele 4.4.5.18 .(I)(a) or the allowable shear values given in Arriele 4.4.3.17 if metal bolts or anchors are provided as set forth in Sentences 4.4.5.18 (I) (a), (b) and (c).

(c) Diaphragm Aclion

(i) When fioors or roofs are designed as diaphragms to transmit horizontal forces to walls, the anchorage of the diaphragm to the wall shall be designed to resist the horizontal force.

(i i) Shear on steel anchors shall not exceed the allow­able shear values given in Article 4.4.3.17.

(d) Prefabricated Masonry

lncluded due to lhe present joint CSCA/Department of lndustry PAIT Industrial Building Research programme now under way in Canada .

(e) Construct;on To/erances

(i) Except as permitted in (2) ali masonry shall be built true and plumb.

(ii) In lhe tines and surfaces of columns, walls and arrises: in 10 ft-t in.; in any storey or 20 ft maximum­i· in.; in 40 ft or more-t in.

(iii) For externaI corners, expansion joints and other conspicuous lines: in any storey cr 20 ft maximum­t in.; in 40 ft or more--! in.

(f) CSA A224 Masollry Code of Praclice

It is to be hoped that the development of the 1970 NBC Masonry Section , will be an example to other countries where, by the exchange of data and ideas, a continuous development and improvement in masonry codes can be encompassed.