Earthquake Load Provisions of the National Building Code of Canada

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Earthquake Load Provisions of the National Building Code of Canada

-:,r: Ser TIIl N21t2 no. 2l-l e . 2 BtDG

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NATIONAL RESEARCH COUNCIL CANADA

DIVISION OF BUILDING RESEARCH

EARTHQUAKE _{LOAD PROVISIONS OF THE NATIONAL BUILDING CODE} OF CANADA by H . S . W a r d

ANJ

ALYZgD

T e c h n i c a l P a p e r N o . ?Il of the

Division of Building Research

OTTAWA F e b r u a r y 1 ! 6 6

PREFACE

The best judgernent that can be brought to bear on the possibility _{of earthquakes in canada, based on the records} and knowledge available, leads to the conclusion that there are risks that should not be ignored in certain designated areas of the country. consequently, _{the National Building code of} canada recognizes earthquake loads as one of the hazards to structural _{safety and has certain provisions intended to ensure} that structures will have a substantial resistance to the earth-quake loads to which they may be subjected.

Various aspects of the earthquake risk and of the resistance of structures are under continuing study so that the provisions _{of the Code rnust also be kept under review. studies} on the response of structures are a continuing interest of the Building Physics section of which the author of the paper now p r e s e n t e d i s a m e r n b e r . T h e d i s c u s s i o n o f t h e N B C p r o w i s i o n s and the cornparison of them with the provisions of other codes lead to a better appreciation of thern.

Ottawa

F e b r u a r y 1 p 5 6

N . B . H u t c h e o n A s s i s t a n t D i r e c t o r

EARTHQUAKE LOAD PROVISIONS OF THE NATIONAL BUILDING CODE oF CANADA "'

by H . S . W a r d

Many of the building codes of countries subjected to earth_ quakes include provisions for the deterrnination of earthquake loads. The earthquake resistant design of a building is rneant to account for the response of the structure to a cornplex and chaotic ground rnotion, the nature of which is not known in advance. As a result, the

earthquake load provisions of building codes can only specify design and constructional _{requirernents intended to account for the effect of} an arbitrary earthquake.

rt is the object of this paper to discuss the earthquake load provisions _{of the National Building code of canada and to compare} thern with those of sorne other buiiding codes. The provisions incorporate _{sorne of the theoreticat ana practical knowledge of} earthquake engineering that has been accurnulated over the last 30 o r 4 0 y e a r s .

EARTHQUAKE _{DESIGN FACTORS}

A s u r v e y o f n i n e t e e n earthquake resistant reguratiorr" (t) s h o w s t h a t s o r n e o r all consider thirteen rnajor design factors. These are shown in Table r, together with the nurnber of building codes that t a k e a c c o u n t o f e a c h f a c t o r . _{E a c h o f t h e s e f a c t o r s will be discussed} briefly, _{and the rnanner in which some of the buitding codes take account} of the factors w.ill be cornpared.

BUILDING _{CODE METHODS OF CALCULATING AND DISTRIBUTING} BASE SHEAR

Nineteen building codes provide for the calculation an" d i s t r i b u t i o n o f b a s e shear caused by earthquakes. rn all cases the b a s e s h e a r f o r c e , v B , is carculated frorn an expression of the forrn,

V e = ( x W _{( l )}

where K is a function of some of the factors given in Table I and W is the Presented at the syrnposiurn on Earthquake Engineering, vancouver B . C . , 8 - I l S e p t e r n b e r 1 9 5 5 .

z

-total weight of the building. Two different but prevalent design c o n c e p t s l e a d t o a n e x p r e s s i o n o f t h e f o r r n s h o w n i n e q u a t i o n ( 1 ) for the base shear, but the rnethod of distributing the force through the height of the building is different.

The sirnplest design procedure is to assurne that the s t r u c t u r e r n o v e s a s a r i g i d b o d y . T h e b a s e s h e a r f o r c e a t a n y instant is then a function of the total rrrass of the building and the foundation acceleration, ii1, at that instant (Fig. lA). This approach reduces earthquake design to the problern of deterrnining a suitable rnaxirnum ground acceleration that defines a regionts seisrnic activity. The lateral force acting at any floor IeveI is obtained by rnultiplying the weight of that floor by the ratio of the ground acceleration to the a c c e l e r a t i o n d u e t o g r a v i t y ( F i g . l B ) . T h e r i g i d b o d y a p p r o a c h i s u s e d b y t 0 o f t h e I 9 b u i l d i n g c o d e s , i n c l u d i n g t h e J a p a n e s e , w h i c h provides for a value of K between 0. I and 0. Z, depending on the region, the type of structure, and the foundation soil.

Beginning with Biott" (Z) *o"k in the early lgh}ts, an

increasing arnount of work has been devoted to the concept of a flexible s t r u c t u r e . _{I n t h i s c a s e t h e s t r u c t u r e w i l l d e f l e c t w i t h r e s p e c t t o t h e} position of the foundation (Fig. ZAl. lf the structure is assurned to be a lurnped rnass systern, the problern is reduced to the solution of a s y s t e r n o f s e c o n d - o r d e r d i f f e r e n t i a l e q u a t i o n s . T h e s e e q u a t i o n s a r e forrnulated in terrns of the distribution of fi)ass and stiffness in a structure, together with its darnping characteristics and the dynarnic characteristics of the ground rnotion.

The detailed aspect of. this part of. the problern has been

dealt with in a preced.ing paper (3) and we can now consider the situation shown in Fig. ZB. A three-storey building is shown at an instant

during a vibration in the fundamental rnode specified by an angular velocity 6. The lateral inertia force acting at any floor is given by a terrn of the forrnE, 2*, but Fig. 28 show" tt"t this can be written

7 E

as W uJ oh if the rnode of vibration is considered to be a linear function g o f h e i g h t . I n t h i s c a s e t h e b a s e s h e a r , V o , i s g i v e n b y

v g =

Z N

(? '

,1,

*ini

w h e r e N i s t h e n u r n b e r o f s t o r e y s . T h e l a t e r a l f o r c e , F i , a c t i n g a t

( z \

the ith

\or can now be written in the forrn,

n. (942) =

r g Ir/. h. 1 1 N

x w . h .

i = l I 1

The value of the base shear to be used in equation (3) depends on the solution of the second order differential equations. rf the

s t r u c t u r e i s c o n s i d e r e d t o b e a s i n g l e - d e g r e e - o f - f r e e d o r n s y s t e m , then it is possible to calculate the rnaxirnurn base shear acting on the

structure by evaluating the rnaxirnurn velocity of vibration, So., that occurs during the ground rnotion. This introduces the concept of velocity spectrurn curves and Fig. 3 shows srnoothed curves that have been obtained by Housner (41 using actual strong-rnotion record.s. The values of S.. are plotted as a function of the period, T, of the structure and the percentage of critical darnping. The base shear for any given structure is given by

2 n S V e =

, " = x W ( 4 )

which is of the forrn K x W. It can be shown that the fundarnental period of a building will increase with the nurnber of storeys, and the following expression is known to be quite accurate for tall buildings ( > 2 5 s t o r e y s ) ,

T = 0 . 1 N .

It is then possible to plot values of the earthquake load pararneter,K, of equation (4) as a function of N. This has been done in Fig. 4 for t w o c a s e s o f z e r o a n d I 0 p e r c e n t o f c r i t i c a l d a r n p i n g .

NBC EARTHQUAKE LOAD PARAMETER, K

The earthquake load pararneter, K, of the 1965 National B u i l d i n g C o d e i s e x p r e s s e d a s f o l l o w s ,

K = R x C x I x F x S .

where R, C, I and F are four of the factors included in Table I; R is a regional seisrnic factor, C depends on the type of structure, I depends on the use of the building, and F is a function of the foundation soil conditions. S is a function of the nurnber of storeys in the building

o ? ^ 1S =

ffi), attd in Fig. 5 the rnaxirnurn (Case A) and rninirnurn (Case B) values of K for zor'e 3 (R = 4l are plotted as a function of the nurnber of

3

-F. = '!!-.

t t

vn

( 3 )

( 5 )

4

s t o r e y s , N . _{A c o r n p a r i s o n o f F i g s . 4 a n d 5 s h o w s t h a t t h e y h a v e} the sarne general forrn, although the values of K in Fig. 5 are s rnalle r .

T h e S f a c t o r i n t h e I 9 6 5 N a t i o n a l B u i l d i n g C o d e is there-f o r e b a s e d o n t h e o b s e r v a t i o n t h a t t h e b a s e shear caused by an e a r t h q u a k e a p p e a r s t o b e i n v e r s e l y p r o p o r t i o n a l to the period of natural vibration of the structure. In the I95l Uniforrn Building code (5) trru period is explicitly included in the pararneter K, but because this is a field of continuing research it was not attempted i n t h e 1 9 6 5 N a t i o n a l B u i l d i n g C o d e .

Nine of the building codes have earthquake load parameters based on the concept of a flexible structure, although in each instance the values are srnaller by a tactor of 4 or 5 than those calculated by Housner for large earthquakes. _{This is probably due to the fact that} past experience has shown that buildings designed for these srnaller values have successfully withstood earthquakes (for exarnple the value of K for t}',e 43-storey Torre Latino Arnericana in Mexico citv was r n e a s u r e d a t 0 . 0 3 3 d u r i n g a n e a r t h q u a k e in l9 57 $)y

The distribution _{of base shear through the height of a}

building is accornplished in the 1965 National Building Code by rneans of equation (3). rt is assurned that the building vibrates in the

fundarnental _{rnode and that it is a linear function of height. It appears} that both these assurnptions are realistic; _{although it is possible that} the higher rnodes of tall buildings will have a significant effect on the earthquake loads because the frequencies of these higher rnodes will be nearer to the predorninant frequencies of strong-rnotion

earthquakes (1 to 5 cps). The Soviet, French and Rurnanian building codes allow for consideration of the higher rnodes of vibration, but the application of these codes requires a deterrnination of the natural

rnodes and frequencies of vibration of the building.

If the structural _{designer wishes to allow for the higher rnodes} of vibration, _{he is perrnitted to do so by the National Building Code;} c l a u s e 4 . I . 3 . 1 5 ( l ) _{s t a t e s I ' T h e d e s i g n l o a d i n g d u e t o e a r t h q u a k e r n o t i o n} rnay be deterrnined by a sirnple statical analysis as provided for in Sentence (4) (equation l), or rnay be deterrnined by a dynarnic analysis, w h e r e s u c h a n a n a l y s i s i s c a r r i e d o u t b y a person conrpetent in this field of work. rr The National Building Code, however, does not provide any guide to the rnethod of dynarnic analysis to be used.

5

-Figure 6 shows a corrlparison of the earthquake load p a r a r n e t e r , K , a c c o r d i n g t o t h e I 9 6 0 a n d 1 9 6 5 N a t i o n a l B u i l d i n g C o d e s f o r z o n e 3 . T h e C a s e B c u r v e o f t h e 1 9 6 5 c o d e r e p r e s e n t s the minirnurn K value, and the Case C curve represents the rnost probable design curve for buildings on a good foundation rnaterial. This shows that the base shear, as calculated by the two editions of the code, will not be too different. The forces acting at different levels of the building will be different,however, because the method of distributing the base shear has been changed to that shown in e q u a t i o n ( 3 ) .

The difference is shown in Tab1e l[, where the value of the force acting at the top of a building of N storeys in zone 3 is given. The results in this table are based on the assurnption that each storey height and the weight of each floor, 'W, are equal. The 1961 Uniforrn Building Code reguires that l0 per cent of the base shear be placed at the top floor if the height to depth ratio of a lateral force resisting systern is equal to or greater than five to one. This i d e a p r o b a b l y a r o s e b e c a u s e t h e r e i s a t e n d e n c y f o r t h e u p p e r f l o o r s of tall buildings to be quite flexible and for heavy service equiprnent t o b e p l a c e d t h e r e . T h i s c o n c e p t h a s n o t b e e n i n c l u d e d i n t h e 1 9 6 5 National Buitding Code, but as more research and inforrnation becorne available such a provision rnay be justified.

SEISMIC REGIONALIZATION AND T'OUNDATION SOIL CONDITIONS

The earthquake load pararneter, K, of the 1965 National Building Code contains a seismic regionalization factor, R, and a foundation soil factor, F. Because their influences overlap they will b e c o n s i d e r e d t o g e t h e r .

In a recent paper, Hod.gson (7) has reviewed the earthquake risks associated with Canada. He points out that the Soviet Union has pioneered the production of seisrnic regionalization rnaps; these are based on seisrnic inforrnation gathered by such scientific disciplines as geophysics, geology, seisrnology and geodetic survey. Sirnilar work is being carried out by the Canadian Departrnent of Mines and Technical Surveys, and eventually the National Building Code may include such inforrnation. In the rneantirne, the earthquake probability rnap of the Clirnatological Atlas (8) ptonides the basis for the seisrnic regional-ization factor, R, in equation (6). As pointed out by Hodgson, the rnap is based on what is known about past earthquakes and on what geological t h e o r y s u g g e s t s a b o u t t h e p r o b a b l e o c c u r r e n c e o f e a r t h q u a k e s . T h e

6

-earthguake probability _{rnap divides Canada into four zones nurnbered} 0 , l , 2 a n d 3 , a n d t h e c o r r e s p o n d i n g v a l u e s f o r R a r e 0 , l , ? a r r d 4 . The sarne zor.es and range of values for R are ernployed by the l96l Uniforrn Building Code.

One of the striking tessons dernonstrated tirne and again by the occurrence _{of earthquakes is the effect of the foundation soil on} t h e d a m a g e e x p e r i e n c e d b y b u i l d i n g s . I n t h e 1 9 0 6 S a n F r a n c i s c o earthquake the greatest proportion of earthquake darnage occurred to buildings placed on fill rnaterial. During the 1957 Mexican earth-quake l9l 99 per cent of the darnaged buildings were founded on the highfy compressible volcanic clay strata of the old lake bed, and only

I per cent on the incornpressible rnaterial of the Sierra.

The indications, therefore, are that it is better to found a building on an incornpressible rnaterial than on a cornpressible one if the structure is in an earthquake zor.e. The I965 National Building Code includes a foundation soil factor, 4_r that is equal to 1.5 for .highly cornpressible soil and has the vffi I.O for oth." sffiffiitions.

The 50 per cent increase in loading for buildings on cornpressible rnaterial rnay not be conservative, however; six of the l2 codes that take account of foundation soil conditions require a 100 per cent increase z in loading in changing frorn an incornpressible to a cornpressible soil.

As rnore research and inforrnation becorne available, it will be possible to bring the F factor in the National Building Code up to date, but in its present sirnple forrn it probably provides a first order approxirnation of the effect of soil conditions on earthquake loads.

The Chilean, Japanese and Mexican building codes include foundation factors that depend on the type of structure and the soil conditions. For exarnple, in the Japanese code a wood structure p l a c e d o n r o c k i s d e s i g n e d f o r a v a l u e o f F = 0 . 5 , b u t i f t h e s t r u c t u r e i s p l a c e d o n a s o f t f o u n d a t i o n , F = 1 . 5 . I n e f f e c t , t h e J a p a n e s e c o d e indicates that it is best to put a flexible structure on a rigid soil and a rigid structure on a corrrpressible soil. A considerable arnount of research is being done in the field on the interaction of structures and soil and eventually this rnaterial should be incorporated in building codes, probably in a manner sirnilar to that of the Japanese code.

T h e t y p e o f f o u n d a t i o n ( s p r e a d f o o t i n g , p i l e s , e t c . ) is also known to influence the arnount of darnage a building may suffer during an earthquake. _{Experience frorn past earthquakes has shorrrn that the} foundations of a building should act in an integral rnanner if darnage is

- t

to be rninirnized. _{Thus, rnany building codes, including the National} Building Code, recornrnend that foundation units be tied together. Only the French building code provides soil factors that depend upon the type of foundation construction.

USE OF BUILDING

The use of a structure should be an important consideration in the earthquake load provisions of a building code. One of the rnain objects of such provisions is to rninirnize loss of life in the event of an earthquake and to ensure that essential facilities are available to contend with any emergency rrreasures stemrning frorn earthquakes.

This sort of consideration led to the inclusion in the 1965 edition of the National Building Code of a factor designated I, the value o f w h i c h d e p e n d s o n t h e u s e o f t h e b u i l d i n g . I h a s t h e v a l u e 1 . 3 f o r buildings in which large nurnbers of people assemble, or which are irnportant for public well-being, and 1.0 for other buildings. rn the building codes that consider the use of the building as a design factor, t h e i n c r e a s e i n d e s i g n l o a d d u e to the I factor varies frorn 30 to 100 p e r c e n t .

T Y P E O F S T R U C T U R E

The ability to withstand Iarge deforrnations is an ixnportant asset in a structure that has to withstand earthquakes. The energy absorbed in plastic deforrnations reduces the dynarnic response of the building and in so doing lirnits the lateral forces developed in the

s t r u c t u r e . _{o n t h e o t h e r h a n d , i f t h e s t r u c t u r e does not possess this} capability, _{it rnust be designed for a cornparatively larger load.} Steinbrugge and Bush (rul have indicated that frarned buildings

i n c o r p o r a t i n g _{s h e a r w a l l s o r m o n o l i t h i c r e i n f o r c e d c o n c r e t e s t r u c r u r e s ,} t h a t a r e p r o p e r l y d e s i g n e d and constructed, have shown thernselves to be

satisfactory _{in earthquake zones. Braced structures or buildings that} d o n o t p o s s e s s a s t r u c t u r a l s e c o n d l i n e of defence (structural redundancies) a r e n o t s o s a t i s f a c t o r v .

T h e I 9 6 5 N a t i o n a l B u i l d i n g c o d e includes a factor, c, that is dependent on the type of construction. For buildings that can withstand iarge deforrnations (frarnes with rnornent-resistant connections) or types of construqtion that have proven themselves in past earthquakes ( s h e a r w a l l c o n s t r u c t i o n ) the value of c is 0.75: for other types of

s t r u c t u r e s i t i s L . 2 5 . T h e 1 9 6 1 U B C c o n s i d e r s f i v e t y p e s o f c o n s t r u c t i o n w i t h v a l u e s o f C t h a t v a r y f r o m 0 . 6 7 , f o r mornent-resistant frarnes, to

8

-1 . 5 0 f o r a t y p e o f c o n s t r u c t i o n t h a t i s n o t d e s c r i b e d e x p l i c i t l y i n t h e b u i l d i n g c o d e . W h e n r e v i s i o n s w e r e b e i n g c o n s i d e r e d f o r t h e 1 9 6 5 edition of the National Building Code, the division of C into rrrore than two values was conternplated, but the necessary information was not available to justify such a fine division. This may be forth-corning as rnore rnaterial is accurnulated about the behaviour of buildings during earthquake s.

VERTICAL SEISMIC FORCES AND FORCES FOR ATTACHED STRUCTURES M o s t b u i l d i n g c o d e s c o n s i d e r o n l y t h e l a t e r a l f o r c e s t h a t a r e

c r e a t e d b y e a r t h q u a k e s , b u t i n a c t u a l f a c t l a r g e v e r t i c a l a c c e l e r a t i o n s c a n a l s o o c c u r , p a r t i c u l a r l y i n e p i c e n t r a l r e g i o n s . T h e r e a s o n f o r neglecting this aspect of earthquake loading is generally attributed to t h e r e s e r v e v e r t i c a l s t r e n g t h p o s s e . s s e d b y r n o s t s t r u c t u r e s . F i v e o f the building codes that have been studied consider the effect of vertical I o a d s . F o u r o f t h e s e c o d e s c a l l f o r i n c r e a s e s t o t h e d e a d l o a d f r o r n 7 to 40 per cent; the Rurnanian code considers the effects of vertical

l o a d s o n s t r u t s o n l y , i n w h i c h c a s e t h e l o a d i s i n c r e a s e d b y 1 0 0 p e r c e n t . The National Building Code does not consider vertical loading caused by earthquakes, but this problern is now under review.

Quite frequently, it has been found that structures attached to the rnain body of a building constitute a danger to life and lirnb during a n e a r t h q u a k e . F o r e x a r n p l e , t h e r e a r e m a n y i n s t a n c e s w h e r e p a r a p e t or curtain walI sections have been torn frorn their supports and caused loss of life" In view of this situation, it has becorne cornmon practice in building codes to specify high design earthquake loads for such parts of buildings. The National Building Code specifies a K value that can b e a s l a r g e a s 0 . 8 i n z o n e 3 ; t h e l a r g e s t K V a l u e s p e c i f i e d b y a n y c o d e for such structures is that of the Rurnanian code where the value is l. 4. The approach of all building codes to this problern is sirnilar in that they specif.y a higher design load than that for the rnain structure. TORSIONAL FORCES AND OVERTURNING MOMENT

Darnage frorn torsional loads caused by earthquakes has been observed in rnany buildings. Such darnage can be attributed to under-estirnating the eccentricity between the centre of rnass and the centre of rigidity. Six of the building codes incorporate provisions that

require the calculation of torsional forces, thus indicating an awareness of the irnportance of such loads in earthquake resistant design. There is a scarcity of data on the actual torsional loads exerted during earth-quakes and the building code approaches tend to be rather arbitrary.

9

-The National Building Code specifies that the eccentricity used for the calculation of torsional loads shall equal I.5 tirnes the computed eccentricity, _{plus or minus an accidental eccentricity} equal to 5 per cent of the plan dirnensions. These recotnrnendations are based on those of the Mexican code, and these in turn have been b a s e d o n t h e o r e t i c a l w o r k b y B u s t a r n a n t e and Rosenblueth(11).

Figure 7A shows the distribution of torsional loads, calculated according to the National Building Code, that act on a synrnretrical l 0 - s t o r e y _{b u i l d i n g w h e n t h e w e i g h t o f e a c h f l o o r i s 1 0 0 0 K i p s .}

The lateral forces generated by an earthquake produce an overturning _{rnornent that rnust be resisted at the base of the structure.} As a consequence of the Niigata earthquake(12) in 1964 a nurnber of buildings were tilted or cornpletely turned over; this was caused by overturning _{forces in conjunction with the liquefaction of the supporting} soil caused by the earthquake rnotion.

The overturning rnornent, M, is'calculated frorn the National Building Gode by using the formula

M - F h x x

where F; is the lateral force at the level x and h;5 is the height in feet above the base. This overturning _{rnornent is resisted by the stabilizing} rnornent of the buildingrs own weight, and these two moments are shown for a ten-storey building in Fig. ?8. The overturning rnornents as calculated by equation (7) will always be quite large. other building codes that explicitly allow for overturning rnornents allow up to a 50 per cent reduction of the overturning rnornent, as c,alculated by equation (7), and sorrre recent work by Bustarnante (r3) rnay provide the basis for sorne sort of revision of the National Building Code. DRIFT LIMITATION AND SEPARATION OF BUILDINGS

The deflections of buildings in the event of an earthquake g i v e r i s e t o t w o d e s i g n c o n s i d e r a t i o n s . O n e o f t h e s e i s t h e p o s s i b l e collapse of rigid interior partitions or glazing as a result of excessive deforrnations of the frarne. Such collapses could cause loss of life, and both the French and Mexican building codes recognize this by irnposing a drift lirnitation clause. _{In effect, both these codes require that the} deflections at any height, h, due to the earthquake loads rnust be less t h a n 0 . 0 0 4 t o 0 . 0 0 I h , d e p e r . r d i n g o n t h e b u i l d i n g c o d e .

9

-The National Building Code specifies that the eccentricity u s e d f o r t h e c a l c u l a t i o n o f t o r s i o n a l l o a d s shall equal 1.5 tirnes the cornputed eccentricity, _{plus or rninus an accidental eccentricity} equal to 5 per cent of the plan dirnensions. These recomrnendations are based on those of the Mexican code, and these in turn have been b a s e d o n t h e o r e t i c a l w o r k b y B u s t a r n a n t e and Rosenblueth(ll).

Figure 7A shows the distribution of torsional loads, calculated according to the National Building Code, that act on a syrnrnetrical l 0 - s t o r e y b u i l d i n g w h e n t h e w e i g h t of each floor is I000 Kips.

The lateral forces generated by an earthquake produce an overturning _{rnornent that rnust be resisted at the base of the structure.} As a consequence of the Niigata earthquake(12) in 1964 a nurnber of buildings were tilted or cornpletely turned over; this was caused by overturning _{forces in conjunction with the liquefaction of the supporting} soil caused by the earthquake motion.

The overturning _{rnornent, M, is calculated frorn the National} Building _{Code by using the forrnula}

M - F h x x

where F>< is the lateral force at the level x and h3 is the height in feet above the base. This overturning _{rnoment is resisted by the stabilizing} rnornent of the buildingls own weight, _{and these two rnornents are shown} for a ten-storey building in Fig. ?8. The overturning _{rnornents as} calculated by equation (7) will always be quite large. other building codes that explicitly allow for overturning _{rnornents a1low up to a 50} per cent reduction of the overturning rnornent, as calculated by equation (?), and some recent work by Bustarnante(13) rnay provide the basis for sorne sort of revision of the National Building code. DzuFT LIMITATION AND SEPARATION OF BUILDINGS

The deflections of buildings in the event of an earthquake g i v e r i s e t o t w o d e s i g n c o n s i d e r a t i o n s . O n e o f t h e s e i s t h e p o s s i b l e collapse of rigid interior partitions or glazing as a result of excessive deforrnations of the frarne. _{Such collapses could callse loss of life, and} both the French and Mexican building codes recognize this by irnposing a drift lirnitation clause. _{In effect, both these codes require that the} deflections at any height, h, due to the earthquake loads rnust be Iess t h a n 0 . 0 0 4 t o 0 . 0 0 1 h , d e p e n d i n g o n t h e b u i l d i n g code.

Drift lirnitation and building the National Building Code, but they should consider if they are designing

ALLOWABLE STRESSES

I 0

-The second factor influenced by structural deformations is

the distance by which adjacent buildings should be separated. This

is an irnportant question because there are many examples where darnage has been caused by two buildings corning together during an earthquake. _{Of the building codes that explicitly account for building} s e p a r a t i o n , t h e r e c o r n r n e n d e d v a l u e is of the order 0. 005H, where H is the height of the building.

s e p a r a t i o n a r e n o t c o v e r e d b y a r e f a c t o r s t h a t C a n a d i a n d e s i g n e r s

in an earthquake zone.

M o s t b u i l d i n g c o d e s a l l o w a n i n c r e a s e in the design stress f o r e a r t h q u a k e l o a d s . _{T h i s i n c r e a s e i s p r o b a b l y b a s e d o n the knowledge} that the behaviour of sorne rnaterials _{under dynarnic loading is different} from that of static loading; for exarnple, the yield stress of steel

increases with rate of strain. _{Because there is lirnited inforrnation on} this factor, the question of allowing an increase in design stress for earthguake loads needs careful consideration.

The Japanese building code specifies earthquake loads nearly t w i c e a s l a r g e a s t h o s e o f o t h e r b u i l d i n g codes, but high design stresses

are allowed. _{Except for the National Building Code, the other building}

c o d e s p e r r n i t a n i n c r e a s e i n working stress of the order of.33 per cent. The National Building code does not perrnit an increase in working

s t r e s s f o r e a r t h q u a k e l o a d s a l o n e . It is suggested that a future revision of the National Building Code should specify that earthquake loads be

calculated as follows. _{The loads should first be calculated with dead}

load plu's 25 per cent of the design live load, with no increase in working stress; _{then with dead load plus full design live load and a 33 per cent} i n c r e a s e i n w o r k i n g s t r e s s . _{T h i s w o u l d p l a c e t h e i n c r e a s e i n d e s i g n}

stress on a logical basis, since it is allowing for the unlikely sirnul-taneous occurrence of rnaxirnurn earthquake and live load. The

p r o v i s i o n f o r n o i n c r e a s e in design stress for the earthquake loads alone i s a l s o c o n s i s t e n t b e c a u s e t h e l o a d s are those resulting frorn a srnall earthquake that can be thought of as having a good chance of occurrence, s a y i n a 3 0 - y e a r r e t u r n p e r i o d .

COMPARISON OF FOUR BUILDING CODES

The earthquake load provisions of three building cod.es are cornpared with those of the I965 edition of the National Building Code by considering the earthquake design of four buildings, the detiils of

I l

-which are shown in Table lII. Mexican, _{the Soviet, and the} c h o s e n b e c a u s e t h e y h a v e a l l include the latest information c o u n t r i e s .

The codes used for comparison are the U n i f o r m B u i l d i n g C o d e . T h e s e were b e e n r e v i s e d i n t h e l a s t f e w y e a r s to a v a i l a b l e t o t h e c o d e w r i t e r s o f t h o s e

Table rv shows the base shears as corrlputed by the four building codes for the four buildings. _{Values of base shear are given} in the direction of the two principal axes of the building. It is assumed that the buildings are in zone 3 and that the values for c, F and r in e q u a t i o n ( 6 ) a r e 0 . 7 5 , 1 . 0 a n d 1 . 3 , r e s p e c t i v e l y . _{r t c a n b e s e e n t h a t} the Soviet code gives higher values than either the National Building code or uniforrn Building code for a1l four buildings, and that the Mexican code gives higher values than these for the two taller buildings. The values of the National Building Code and the Uniforrn Building Coae are quite close to each other, however, and since the Uniforrn Building Code has proven itself in Californian and Alaskan earthquakes it is an indication that the application of the National Building Code provisions will provide structures that can withstand earthquakes. A cornparison o f t h e w a y i n w h i c h t h e b a s e shear is distributed in a l9_storey

building according to the National Building Code, the Uniforrn Building Code, and the Soviet code is shown in Fig. 8.

CONCLUSIONS

The earthquake load prowisions of the I965 edition of the

N a t i o n a l B u i l d i n g C o d e c o n s i d e r ten of the thirteen basic factors generally considered to be of irnportance in earthquake engineering design. The r n a n n e r i n w h i c h i t d e a l s w i t h t h e s e factors is in accordance with present knowledge, but there is roorn for further refinernent when the proper inforrnation _{becornes available. Drift lirnitation and building separation} a r e t w o f a c t o r s n o t c o n s i d e r e d b y the National Buiiding code, but

Canadian designers in earthquake zones should give thern careful thought. The third factor not considered by the National Building Code is the

possibility that vertical accelerations _{will act on a building during an} earthquake, _{but this is not too irnportant in most cases because rnanv} buildings have an adequate reserve strength in this direction.

A cornparison _{of the National Building Code and the Uniforrn}

Building Code shows that they give sirnilar earthquake loads. This rrleans that the application of the National Building Code will provide structures with an earthquake resistance cornparable to those in California and A l a s k a , a n d t h e s e h a v e w i t h s t o o d severe earthquakes.

L Z

-REFERENCES

(l) _{Earthquake Resistant Regulations. A World Liet 1963. Compiled} by the International Association for Earthquake Engineering.

(Z) _{Biot, M. A. Analytical and Experirnental Methods in Engineering} S e i e m o l o g y , T r a n s . A S C E 1 0 8 , 1 t 4 3 , p . 3 6 5 - 4 0 8 .

( 3 ) _{C h e r r y , S . B a s i c D l m a m i c P r i n c i p l e e of Response of Linear} Structures to Earthquake Ground Motions. Presented at the Syrnposiurrr on earthquake engineering, Vancouver, B. C., I - l l S e p t e m b e r 1 9 6 5 .

( 4 ) _{H o u s n e r , G . W . L i r n i t D e s i g n o f S t r u c t u r e s to Resist Earthquakes.} Proceedings of the first \f orld Conference on Earthquake

E n g i n e e r i n g , S a n F r a n c i s c o , Galifornia, 1960. ( 5 ) -, _Unifsrm Fuilding Code, l96l Edition.

(6) _{Zeevaert, L. Base Shear in Tall Buildings during Earthquake} J u I y 2 8 , 1 9 5 7 , i n M e x i c o C i t y . P r o c e e d i n g s o f the Second World C o n f e r e n c e o n E a r t h q u a k e E n g i n e e r i n g , J a p a n r l 9 6 0 , V o l . Z, p . 9 8 3 - 9 9 6 "

H o d g s o n , J . H . T h e r e a r e e a r t h q u a k e r i s k s i n C a n a d a , G a n a d i a n C o n s u l t i n g E n g i n e e r , J u l y 1 9 6 5 , p . 4 2 - 5 L .

Clirnate Inforrnation for Building Design in Canada 1965.

Supplernent No. I to the National Building Code of Canada, Chart 1 0 .

(9) Rosenblueth, E. The earthquake of 28 July, 195? in Mexico City. P r o c e e d i n g s o f t h e S e c o n d W o r l d C o n f e r e n c e o n E a r t h q u a k e E n g i n e e r i n g , J a p a n , 1 9 6 0 , V o l . l , p . 3 5 9 - 3 7 9 . ( 1 0 ) _{S t e i n b r u g g e , K . V . , a n d V . R . B u s h . E a r t h q u a k e E x p e r i e n c e i n} N o r t h A r n e r i c a , 1 9 5 0 - L 9 5 9 . P r o c e e d i n g s o f t h e S e c o n d l t r o r l d C o n f e r e n c e o n E a r t h q u a k e E n g i n e e r i n g , J a p a n r I 9 6 0 , V o l . l , p . 3 8 1 - 3 9 6 .

( I I ) Bustarnante , J. I. , and E. Rosenblueth. Building Code Provisions o n T o r s i o n a l O s c i l l a t i o n s . P r o c e e d i n g s o f t h e S e c o n d W o r l d Conference on Earthquake Engineering.

( 7 )

t 3

-(IZl _{The Niigata Earthquake, I6 June, 1964, and Resulting}

Darnage to Reinforced concrete Buildings. rnternational rnstitute of seisrnology and Earthquake Engineering Report N o . I , J a p a n , February 1965.

(13) Bustarnante, J.r. seisrnic shears and overturning Mornents in

Buildings. _{Proceedings of the Third world conference on}

FACTORS

THAT ARE CONSIDERED

tN

THE EARTHQUAKE

LOAD REGULATIONS

OF NINETEEN

BUILDING CODES

NUMBER

OF CODES

THAT CONS

IDER

TI.I

E FA

CTO

R

Calbulation of Base Shear

19

Distribution of Base Shear

19

Use of the Building

7

Regional Seismic Zoning

l9

Foundation Soil Conditions

T2

Type of Structu re

6

Vertical Seismic Force

5

Seismic

Structu

towers)

Fo

rces fo r Attac h ed

res (parapets,

chimneys,

l4

0verturning Moment

7

I

Torsional Forces

6

Drift Limitation

2

Separation of Bui ldings

7

Allowable Stresses

l8

TABLE I

FACTORS

THAT ARE GENERALLY

EARTHQUAKE

REGULATIONS

OF

CONSIDERED

IN THE

BUILDING CODES

!

I tr\ r0 \ o ; o. it F { d o a

z

o \o o. lJ.l

-o

C9

z.

6 E O O (J E lrj V\ lrl .tr 6

o

z.

I

m e, tt, a C) l4l O

f z .

r r u r

o :

-z' ls O 6 Lrr

- ur,

E Z .

< o

i-l _o -; = F F (.) u.J |fr (Y\ = ff,l FA o = .sf o o rJ^\ AI |'r\ o (Yt = (Y\ (fl o = sf o o \o (\r \o u\ (\l = or\ on o = sf r.r\ o o \o C\l oo o C\I = ffr (Y\ o = c\l \o o o c\l o. l.c\ = (Yr (rr\ o = O o oo N c\t o = (Y'\ (Y\ c) = O o. o o C\I (n oo rf\ = (r'\ (r'\ o = cf'l (Y\ o sf (Y\ (Y\ = (Y't on o = o. C> o o o Oo r./, t ! E

o

a

o

E lrJ co = = = (J E

z.

A o

\J \o) O o .

o

A- 1-(.)

; o

z

{ o s1 E

c r o

e c.)

6 < J t l { () ca

z.

!! ,t

\J \O O o .

o

A- p-; ( t

z

< o

v t &

c r o

s L )

q> o

tr- { L U &

2 o

;3

O o -t.1 O q 9 F

TE-z. oo u t E Z .

9ffo

- = \ o trl J- O\ O- t/t rr

4z

;3

o o

-rrr O ( 9 F

f!-z. trl t+t & Z.

Efs

n _l/l pr

g\

I O.

a 9

| J t i l A d O o (J (9

z.

6 J

o

UJ <

-o

.t1 t-Ll E

o-=

o

C)

o

a

ul ../l .tr C9

z.

6 6 E =

o

t-f _A -J O F F o

o

o

o

X O sf (Y\

o

\o

st |rr \o a

s

|r\

\o

.if

c)

_o

o

(J C\I Fl

x

oo \o oon sf on \o

s

sf sf o. a (Y1 o_o o

o

€ oo >< o .sf (\_on NI o\ @ GI a

o

o

c) o o sf

x

\c} \o G'I

sr

o

o\ \o o

o\

n

o

O

o

o

(9

z.

a = E v,

c

o

v, E (l)

E

6 G' CL

-c

CD €) -./t c)

o

T',

o

o) c

E

J

z.

(o f (.' E'

c

(D o-q) o - ^ : o ) ' ; -q) _vl

o-

.-x

c <

o crl : - o ; - J l - o 6E f (J E

c

o) o-q)

o - ;

(l) Y v l ; _./, c)

.-o _ x

C,

.9E

o G t c ! t n -Gt

'-.- o

o-\Z o o lJ-(-) (1E' u-l o trtl

c

E, (t, o E q)

E

:J vl rtl

B U I L D I N G B U I L D I N G C O D E B A S E S H E A R I N T H E D I R E C T I O N P E R P E N D I C U L A R T O T H E L O N G A X I S K I P S B A S E S H E A R I N T H E D I R E C T I O N P E R P E N D I C U L A R T O T H E S H O R T A X I S K I P S A I O S T O R E Y S N B C 1 9 6 5

5t3

5 1 3 M e x i c a n I 9 6 3 4 0 0 4 0 0 S o v i e t 1 9 5 7

t020

t260

u B c 1 9 6 1

3 7 9 4 0 0 B I 9 S T O R E Y S N B C 1 9 6 5 6 6 1 _{6 6 1} M e x i c a n 1 9 6 3 7 6 0 _{7 6 0} S o v i e t I 9 5 7 9 6 9 i l 5 9

u B c 1 9 6 1

585 636

c

3 7 S T O R E Y S N B C 1 9 6 5 7 8 3 7 8 3 M e x i c a n I 9 6 3 I 4 8 0 I 4 8 0 S o v i e t 1 9 5 7 1665 1665

u B c l 9 6 t

7 5 5 7 8 4 D 4 7 S T 0 R t Y S N B C 1 9 6 5

8 t 6

8 1 6 M e x i c a n 1 9 6 3 I 8 8 0 _{I 8 8 0} S o v i e t 1 9 5 7

2tt5

2rt5

u B c 1 9 6 1

9 4 0 9 4 0 T A B L E IV C O M P A R I S O N O F T H E B A S E S H E A R F O R C E A S C A L C U L A T E D B Y F O U R B U I L D I N G C O D E S F O R A N E A R T H Q U A K E Z O N E 3 c a - t + b 1 - t c

0riginal Position of

Structure

Position

at an lnstant

Duri ng an Earthquake

t-I

I

I

I

I

I

I

I

I

I

FIGUR

BASE

xf

X1

VB

E IA

SHEAR

CALCULATION

FOR

Fou

ndation Di splacement

Fou

ndation Acceleration

Total Mass x Acceleration

w

i1

*x

_g'g

X+=:xW=KxW

Acceleration

Due to Gravity

A RIGID BO DY

FIGURE 1B

LATERAL

FORCES

ON

INEUCED

BY GROUN

D MOTION

A RIGID STRUCTU.RE,

xf

x

0 r i g i n a l P o s i t i o n o f

S t r u c t u r e

P o s i t i o n a t a n I n s t a n t

D u r i n g a n E a r t h q u a k e

= Foundation

Dis-p l a c e m e n t

= Displacement

o f

S t r u c t u r e w i t h

R e s p e c t

t o t h e

F o u n d a t i o n

F I G U R E 2 A

F L E X I

B L E S T R U C T U R E

f w2

x.a

w2xr9+

-rL W' 2

g w

-= { 9*'*i

i = r Y

f *'a',

w ' d , h ,

VB

F I G U R E 2 8

L A T E R A L

F O R C E S

O N A

I N DUCE

D BY GROUN

D

d h ,

F L E X

I B L E S T R U C T U R E ,

M O T

I O N

e t 7 1 a g - G

(J lrJ

a

5

at'l I l r l

Lrr

z.

O

o-tt1 L!.1 E F (J O u-l

5.0

4.0

3.0

2.0

1.0

Base

shear = 1IJ , x w

_gr

l3= o

l5 = ?To

P= lo%

p =eercent of Critical Damping

2.0

1.0

3.0

PERIOD

T (SECONDS}

FIGURE 3

VELOCITY

REPONSE

SPECTRA

AFTER

HOUSNER

OF EARTHQUAKES(,)

Z e r o D a m p i n g

l 0 % C r i t i c a l Damping

B a s e S h e a r = K x W

o

o

a t

\z

E 1 . 6

lrl F lrl =

eL2

o-o

o

-t

rrr o' 8

\z

g

= 0 . 4

E |JJ a

n

N U M B E R

O F S T O R E Y S

- N

F I G U R E 4

EA RTHQU

A KE

R E S U L T S

A N D

L O A D P A R A M E T E R

B A S E D

O N

T H E A S S U I f I P T I O N

T = ( ) . IN

H O U S N E R ' S

V E L O C

I T Y S P E C T R U M

E w 3 4 6 1 - A

\z

I

E 0 . 2

lJ-l t-]JJ = &, o-o

o

-J

'*r 0. I

V O F E lII

c C a s e A

C a s e

c

c

t.2r, I

0 . 7 5 , I

1 . 3 , F

I . O . F

1 . 5

1 . 0

n

N U M B E R

O F S T O R E Y S . N

F I G U R E 5

N B C E A R T H Q U A K E

L O A D P A R A M E T E R

F O R Z O N E

3

s o - r + a 1 - j

\Z E, I r l F UJ = E o-a

o

J laJ V a F E. lrJ

0. 12

0. I0

0 . 0 8

0 . 0 6

0.04

0 . 0 2

1 9 6 0 N a t i o n a l B u i l d i n g C o d e

C a s e B 1 9 6 5 N a t i o n a l B u i l d i n g C o d e

C a s e C 1 9 6 5 N a t i o n a l B u i l d i n g C o d e

( c = 0 . 7 5 , l = 1 . 3 , F = 1 . 0 )

B a s e S h e a r = K x W

20

N U M B E R

O F S T O R E Y S

- N

F I G U R E 6

C O M P A R

I S O N O F T H E 1 9 6 0 A N D

R E Q U

I R E M E N T S

F O R Z O N E 3

I 9 6 5 N B C E A R T H Q U A K E

L O A D

b a - , 4 o 1 - r o

10

9

tt1 IJ.J e.

o

a Lr-O E IJ.l dl = =

8

7

6

5

4

3

2

t

500

T o r s i o n a l

C

Plan

Dimensions

1000

M o m e n t s (K i p Ft)

Overturning

Moment

Due

to Earthquake

52,783

Kip Ft

Stabilizing

Moment

o f the Building

370,000

Kip Ft

( A l

lOth Floo

Ground

(B) 0verturning Moment

C a J 1 4 6 ? - t l

F I G U R E 7

T O R S I O N A L

A N D OVERTURNING

M O M E N T

_{F O R A TEN STOREY}

B U I L D I N G ACCORDING

_{T O T H E 1 9 6 5 NBC}

t9

15

rr-l lJ.J E

o

tt1 IJ- T't O I U E u.t oo =

z.

5

100

80

20

0

40

60

LATERAL

FORCE

(KIPS)

FIGURE 8

DISTRIBUTION

OF

FORCES

FOR A 19

EACH

FLOOR

LOAD

ba- v+on- t2

EARTHQUAKE

LATERAL

STOREY

BUILDING

WHEN

ts 1000

Kt Ps

I

/

Uniform

Building

Code,

1961

National

Building

Code,

1965

Soviet

Building

Code,

1957

,/ ,/ ,/

r

r

7

t/

//

r

, t