Comparison of the probabilities of wind and earthquake loads in the NBC 1970

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Comparison of the probabilities of wind and earthquake loads in the

NBC 1970

Ferahian, R. H.

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C A N A D A Ser T H l

B92

no- 72 c. 2

BrnG

COMPARISON

OF

THE

PROBABILITIES

OF

WIND AND

EARTHQUAKE LOADS IN THE

NBC

1976

3 3 4 ] y 7

-

, ~

(3)

COMPAREON

O F

TlXE PROBABILITIES O F WIND AND

EARTHQUAKE LOADS

I

N

_THE _NBC _{1 9 7 0}

h

Canada the majority of buildings are designed for wind loads corresponding to the "30-year" maximum hourly wind, i. e . the wind that has an annual probability of exceedance of 1 in 30 (1, 2 ) . The

d e s i g n p a r m e t e r used f o r earthquake loads is ground acceleration,

A = ,

,

with an annual probability of exceedance of 1 in 100 (2,

3).

It is the purpose of this Note t o show that despite the apparent in- consistency of elastic design parameters, the safety factors in the

t w o c a s e s

{i.

e. the probabilities of structural failure f r m wind and

from earthquake) are comparable.

Available information on the distribution of winds in Canada (4)

provides an estimate of qR, the wind pressure with an annual probability

of exceedance of

I/R,

f o r a range of values of

R.

Using

R

=

1 0 0 as a

convenient reference value,

a

relation q

_{R I ~}

o o as a function of

R

may

be deduced (5). A similar relation may be deduced f o r the seismic

acceleration ratio A ~ / A ~ as a function of

R

f o r different localities

in Canada using the statistical parameters f o r the seismic sample of Canada (3,

6 ) .

These two curves are shown

in

_{Figure 1.}

Considering t h e wind forces, it may be seen that %,/qloo

=

8 2 per cent; i f a building w e r e designed f o r the wind o c c u r r i n g once in 30 years, the d e s i g n loads would be only 2 2 per cent below design values

f o r winds occurring once i n 100 years. F o r earthquakes, however,

A , ~ J A ~

=

40 per cent,

i.

e . the ground acceleration for

the

100-year

earthquake is 2.5 that of the 30-year earthquake. This d i f f e r e n c e in

statistical distribution indicates an important difference between wind

and seismic forces that must be considered when comparing the two.

T o

compare actual s d e t y of buildings under wind and seismic f o r c e s it will be necessary to consider loads close to the collapse

load rather than close to design loads.

It

is difficult to predict collapse,

but 70 per cent overload is a plausible criterion.

It

is consistent with

the load f a c t o r s needed in ultimate strength design. Thus, by extrapo- lation of the curves in Figure

I

the mean recurrence period for this

(4)

years, respectively.

These

figures for wind and earthquake are

comparable, again indicating consistent factors of safety.

It

can

be concluded, therefore, that t h e choice of the 100-year earthquake acceleration as the parameter far seismic zoning and f o r the earth-

quake

resistant elastic design of the structure is consistent with the

factors of safety used for wind resistant design.

References

1. National Building Code of Canada 197 0. A s sociate C m r n i t t e e

on the N a t i o n a l Building

C

ode, National R e s e a r c h Council of Canada, Ottawa, 1970.

2 .

NBC

Supplement No. 4, Canadian Structural Design Manual

1970. Associate Committee on the N a t i o n a l Building

G o d e , N a t i o n a l Research Council of Canada, Ottawa

1970.

3. Ferahian,

R.

H. Commentary on Loads Due t o Earthquakes,

in Supplement No.

4

to the National Building Code of

-

Canada 1970, Assaciate Committee on the National

Building G o d e , National R e s e a r c h Council of Canada,

Ottawa, 1970.

4.

B

oyd,

D m

W.

Climatic Information for Building D e sign in

Canada 1970, in Supplement No. 1 to t h e National

Building Code o T ~ a n a d a 1970, A s s o c i a t e Committee

on the N a t i o n a l Building Code

,

National Research

Council of Canada, Ottawa, 1370, NRC 1 1 1 5 3 .

5 . Ferahian,

R,

H. Discussion of 'lProposed American Standard

Building G o d e Requirements f o r Minimum Design of W i n d Loadst' by Edward Cohen, Joseph Vellozzi and

H.

C,

S , Thorn. Proceedings, International Research Seminar on W i n d Effects on Buildings and Structures, Vol. 2,

1968,

p, 405.

6 .

Davenport, A. G. and

W.

G.

Milne, D i s t r i b u t i o n of Earth-

quake Risk in Canada, Fourth W o r l d Conference on

Earthquake Engineering, Santiago, Chile, January

1969.

(See also Bulletin of the Seismological Society

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C i T I E 5 C O N S I D E R E D QUEBEC, M O N T R E A L , O T T A W A , T O R O N T O & V I C T O R I A S C A T T E R FOR T H E C I T I E S C O N S I D E R E D It It 0 R MEAN RECURRENCE P E R I O D w P R O B A B I L I T Y O F A N N U A L E X C E E D A N C E R F I G U R E 1 P R O B A B I L I S T I C V A R I A T I O N O F T H E W I N D P R E S S U R E S A N D E A R T H Q U A K E A C C E L E R A T I O N S