Snow loads on roofs 1957-58: second progress report

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Snow loads on roofs 1957-58: second progress report Allen, D. E.; Turkstra, C. J.

N A T I O N A L R E S E A R C I I COITNC I L O F CANADA D I V I S I O N OF h T I L D I N G H E S E A R C H SNOW LOADS ON R O O F S

1957-58

S E C O N D P R O G R E S S R E P O R T by D . E . A l l e n and C . J . T u r k s t r a R e p o r t N o .

163

of t h e D i v i s i o n o f B u i l d i n g Research O t t a w a November

1958

PREF 4CE

The c r i t i c a l l o a d f o r which t h e r o o f s o f b u i l d i n g s and houses i n Canada g e n e r a l l y must be designed i s t h e load

imposed by snow, Thus t h e magnitude of t h e d e s i g n snow l o a d

has a c o n s i d e r a b l e e f f e c t on t h e c o s t of roof c o n s t r u c t i o n s . Snow l o a d s t o be expected a c r o s s Canada a r e g i v e n i n t h e N a t i o n a l B u i l d i n g Code (1953) i n a map from which t h e d e s i g n snow l o a d can b e o b t a i n e d f o r any r e g i o n , The l o a d s

shown on t h i s map were based on measurements of maximum snow d e p t h s on t h e ground from r e c o r d s t a k e n over a t e n - y c a r p e r i o d a t a number o f p o i n t s a c r o s s t h e c o u n t r y . Opinion has b e e n widely expressed t h a t measurements of snow d e p t h s on t h e ground cannot b e a p p l i e d d i r e c t l y t o t h e d e t e r m i n a t i o n of d e s i g n snow l o a d s f o r r o o f s and t h a t t h e snow l o a d v a l u e s g i v e n i n t h e 1953 Code a r e t o o h i g h f o r some r e g i o n s .

The A s s o c i a t e Committee on t h e N a t i o n a l B u i l d i n g Code i s r e s p o n s i b l e f o r t h e p r e p a r a t i o n and r e v i s i o n of t h e Code. As a s e r v i c e t o t h i s Committee, t h e D i v i s i o n of B u i l d i n g Research of t h e N a t i o n a l Research Council undertook t o study a c t u a l snow l o a d s a s t h e y occur on r o o f s . T h i s s t u d y , because of r e g i o n a l and y e a r l y c l i m a t i c v a r i a t i o n s , must extend over

s e v e r a l y e a r s and must t a k e i n t o account t h e whole of Canada,

T h i s r e p o r t i s t h e second p r o g r e s s r e p o r t on t h e survey

of snow l o a d s on r o o f s , t h e f i r s t , (DBR Report No,

134),

having

covered only t h e p i l o t survey which was c a r r i e d o u t d u r i n g t h e f i r s t w i n t e r of o b s e r v a t i o n s , T h i s , t h e r e f o r e , i s t h e f i r s t r e p o r t of t h e f u l l survey, g i v i n g t h e r e s u l t s o b t a i n e d by t h e t h r e e t y p e s of s t a t i o n s t a k i n g p a r t

i n

t h e work ( A , B and C

S t a t i o n s ) , The r e c o r d s a r e shown mainly i n g r a p h i c a l form. The r e p o r t a l s o d i s c u s s e s t r e n d s and u n u s u a l f e a t u r e s of t h e o b s e r v a t i o n s of t h e w i n t e r . An appendix t e l l s o f t h e h a n d l i n g o f snow l o a d s i n Russia, i l l u s t r a t i n g t h a t e n g i n e e r s i n a n o t h e r c o u n t r y w i t h a c o l d c l i m a t e a r e e n c o u n t e r i n g s i m i l a r problems. Ottawa

SNOW LOADS ON ROOFS 1957-58

SECOND PROGRESS REPORT

D.EI A l l e n and C * J I T u r k s t r a

I n

1956

t h e D i v i s i o n of B u i l d i n g R e s e a r c h began a s u r v e y of snow l o a d s on r o o f s a c r o s s Canada f o r the A s s o c i a t e Committee of t h e N a t i o n a l B u i l d i n g Code. The purpose of t h i s s u r v e y i s t o o b t a i n r e c o r d s o v e r a number of y e a r s of a c t u a l r o o f l o a d s i n o r d e r t o p r o v i d e a b a s i s f o r more r a t i o n a l d e s i g n snow l o a d s i n

th N a t i o n a l B u i l d i n g Code i n t h e f u t u r e . A t p r e s e n t t h e d e s i g n snow l o a d s a r e based on snow measurements on t h e ground only.

A more complete d e s c r i p t i o n of t h e background f o r t h i s survey

and t h e r e s u l t s of a p i l o t s u r v e y t a k e n i n t h e f i r s t y e a r a r e p r e s e n t e d i n

DBR

R e p o r t s 106 and

134.

The purpose of t h i s and f u t u r e y e a r l y p r o g r e s s r e p o r t e i s t o summarize t h e f i n d i n g s of one w i n t e r ! 8 o b s e r v a t i o n s and t o p o i n t o u t a n y t r e n d s and u n u e u a l f e a t u r e s of the observations. I n a d d i t i o n , t h i s r e p o r t s u g g e s t 8 a method of p r e s e n t a t i o n of t h e y e a r ' a o b s e r v a t i o n s and d i s c u s s e s t h e problem o f l o a d i n g p a t t e r n by comparing s p e c i f i c a t i o n s w i t h a c t u a l o c c u r r e n c e s .

A s an example of the use of a non-uniform l o a d i n g p a t t e r n

i n a b u i l d i n g code a n e x c e r p t from t h e Russian ' ' C o n s t r u c t i o n S t a n d a r d s and ~ e g u l a t i o n s " i s g i v e n i n an appendix. I n c l u d e d a l s o i n t h e a p p e n d i x a r e a b s t r a c t s from 2 Russian a r t i c l e s

d i s c u s s i n g t h e snow l o a d s i n the Russian s t a n d a r d s . These may

i l l u s t r a t e t h a t e n g i n e e r s i n a n o t h e r c o u n t r y w i t h c o l d w i n t e r s a r e e n c o u n t e r i n g s i m i l a r code problems.

Swnmary of

1957-58

O b s e r v a t i o n s

Few of t h e s t a t i o n s a t which snow l o a d s o r d e p t h s were

measured had a v e r a g e o r above a v e r a g e y e a r l y s n o w f a l l . A number of

s t a t i o n s had a l m o s t n e g l i g i b l e snow a c c u m u l a t i o n s on r o o f s . The

maximum r e c o r d e d a v e r a g e ground and roof l o a d s were 120 p s f and 82

p s f b o t h observed a t t h e A S t a t i o n , G l a c i e r , B.C.

(4,100

f t above

s e a l e v e l ) . The y e a r ' s measurements confirmed t h e i m p r e s s i o n

t h a t t h e r e i s a n a p p r e c i a b l e d i f f e r e n c e between r o o f a n d ground snow l o a d s . I n a l l b u t a few s p e c i a l c a s e s roof l o a d s were l i g h t e r . A s e x p e c t e d , however, l a r g e snow c o n c e n t r a t i o n s o c c u r r e d on some

r o c f s , p a r t i c u l a r l y a l o n g the p a r a p e t s of f l a t r o o f s , on t h e lower p o r t i o n s of s t e p p e d r o o f s and i n r o o f v a l l e y s . These l o c a l i z e d d r i f t s were o f t e n deeper and h e a v i e r than t h e snow l o a d on t h e ground.

D e s c r i p t i o n of S t a t i o n s

F i f t y - f i v e s t a t i o n s r e p o r t e d i n t h e

1957-58

surveys

(15;

A,

32 B and

8

C S t a t i o n s ) . The t h r e e t y p e s of s t a t i o n s and t h e

procedures used were d e s c r i b e d i n DBR Report No.

13b0

I n b r i e f , a t

t h e A S t a t i o n s d e t a i l e d roof and ground d e p t h s and d e n s i t i e ~ were r e c o r d e d weekly and a f t e r snowstorms on r e l a t i v e l y c o n v e n t i o n a l 8loped and f l a t r o o f s a c c o r d i n g t o t h e f i e l d i n s t r u c t i o n s i s s u e d i n

DBR Technical Note No. 233. A t B S t a t i o n s o n l y roof and ground

d e p t h s were measured, weekly and a f t e r l a r g e s n o w f a l l s . B S t a t i o n

o b s e r v e r s r e c o r d e d the d e e p e s t r o o f d r i f t and a number of r e p r e s e n - t a t i v e snow d e p t h s on a b u i l d i n g ( a v o i d i n g d r i f t s and b a r e s p o t s ) .

I n a d d i t i o n , o b s e r v e r s were asked t o r e c o r d some g e n e r a l i n f o r m a t i o n on wind and c l i m a t e a l o n g w i t h o b s e r v a t i o n s of extreme snow accumu- l a t i o n s i n t h e a r e a .

A t C S t a t i o n s t h e R.C.A.F. made measurements s i m i l a r t o

t h o s e of t h e A S t a t i o n s on l a r g e r o o f s a t

9

b a s e s a c r o s s t h e country. Where p o s s i b l e f o u r s t a n d a r d b u i l d i n g s were used a t each s t a t i o n

-

a c a n t i l e v e r hangar ( f l a t r o o f ) , a n a r c h hangar ( c u r v e d r o o f ), a

mechanical equipment garage ( f l a t r o o f ) and a supply depot ( f l a t r o o f )

( F i g - 12). For e a c h type of r o o f a number of p o i n t s had been

chosen a t which snow d e p t h s were measured weekly by means of

a

yard-

s t i c k . These were accompanied by d e n s i t y and ground d e p t h measurements

P r e s e n t a t i o n of R e s u l t s

A t t h e A and C S t a t i o n s snow l o a d s c o u l d be r e a d i l y d o t e r -

mined because b o t h d e p t h and d e n s i t y measurement s were a v a i l a b l e .

For e a c h b u i l d i n g t h e maximum, average and minimum r o o f l o a d s and t h e average ground l o a d were p l o t t e d f o r e a c h day of o b s e r v a t i o n . The l o c a t i o n s of the extreme l o a d s a r e n o t e d because t h e maximum l o a d

d i d n o t always occur a t t h e same gauge. These p l o t s a r e g i v e n i n

F i g s . 1 t o

I S .

Only s t a t i o n s having more t h a n

5

in.

of accumulated

snow d u r i n g he w i n t e r a r e shown, R e s u l t s f o r a l l s t a t i o n r o o f s

a r e summarized i n Table I ( A S t a t i o n s ) and Table I1 ( C S t a t i o n e )

which g i v e t h e maximum average r o o f and ground l o a d s , t h e maximum

observed roof l o a d ( d e e p e s t d r i f t ) and t h e N.B.C. d e s i g n snow l o a d

f o r comparison.

F i g u r e s 1 t o

15

a l s o show some d e t a i l of t h e b u i l d i n g s

observed t o g e t h e r ~ i . t h the maximum d a i l y wj.nd speed and d i r e c t i o n ,

average d a i l y t e m p e r a t u r e and t h e t o t a l h o u r s of b r i g h t sunshine which were t a k e n from t h e Department of T r a n s p o r t M e t e o r o l o g i c a l Sumnlaries f o r e a c h s t a t i o n .

A t t h e I3 S t a t i o n s maximum roof and ground l o a d s were

e s t i m a t e d assuming a d e n s i t y of 0.2. To s m a r i z e t h o s e r e s u l t 8 t h e l o a d s were r e p r e s e n t e d on a map of Canada ( F i g . 16). As a f i n a l

summary of t h e o b s e r v a t i o n s f o r t h e A and C S t a t i o n s a s i m i l a r map was p r e p a r e d ( ~ i g .

17).

D i s c u s s i o n of 1957-58 O b s e r v a t i o n s

A s p r e v i o u s l y n o t e d average roof l o a d s were l e s s t h a n

were average ground l o a d s . The l o a d s were u s u a l l y not d l s t r i b u t e d uniformly over t h e r o o f a r e a . I n t h e f o l l o w i n g n o t e s some s p e c i a l f e a t u r e s of a few of t h e o b s e r v a t i o n s a r e d i s c u s s e d .

, Aklavikt ( ~ n example of a l o n g c o n t i n u o u s w i n t e r w i t h low w i n d s ) This i s one of the few s t a t i o n s a t which s n o w f a l l was

above average f o r t h e wintor. Because of the very low t e m p e r a t u r e , t h e l a c k of sunshine, and t h e comparatively low wind v e l o c i t i e s ,

snow accumulated on t h e r o o f s f o r s e v e r a l months. This i s v e r y unusual f o r Aklavik. The average roof l o a d , however, was alwaya a good d e a l l e s s t h a n the ground l o a d . The snow d i s t r i b u t i o n wag f a i r l y even over b o t h t h e r o o f s observed a t t h i s s t a t i o n ( F i g s .

1

and 18).

G l a c i e r : ( A n example of very deep snow t y p i c a l of some mountain r e g i o n s i n B.C. )

On most of t h e s e s l o ~ e d r o o f s t h e snow d i d n o t acmwrmlate f o r l o n g p e r i o d s b u t s l i d of? q u l c k l y ( F i g s . 2 and

3 ) .

The

Alpine Club Lodge shown i n Fig.

I9

i s a c a s e or' a c o m ~ l e t e l y s h e l t e r e d , unheated gable r o o f . A roof l o a d of 82 psf was r e a a h e d on t h i a

b u i l d i n g b e f o r e t h e snow was removed, compared t o a measured ground l o a d of 120 p s f . I f t h i a measured ground l o a d i s compared w i t h t h e r e d u c t i o n f a c t o r of t h e N a t i o n a l B u i l d i n g Code f o r a

4s0

p i t c h t h e r e i s a d e s i g n l o a d of

69

p s f . This s e r v e s t o show t h e extremes which a r e p o s s i b l e under s p e c i a l c o n d i t i o n s p a r t i u u l a r l y i n

mountain a r e a s ( f o r a d i s c u s s i o n of snow l o a d s i n mountains r e f e r t o

DBR Report No. 162). I n G l a c i e r l a r g e accumulations of i c e and o r e e p i n g snow were observed a l o n g t h e e a v e s and overhanging t h e eaves. No o t h e r s t a t i o n r e c o r d e d t h i s phenomenon.

Montreal: (An example of a low roof between two h i g h b u i l d i n g s ) I n Montreal d u r i n g t h i s w i n t e r , h i g h winds ( a v e r a g e h o u r l y speeds up t o 40 mph) and r e l a t i v e l y h i g h t e m p e r a t u r e s r e s u l t e d i n v e r y s h o r t r e t e n t i o n p e r i o d s f o r t h e snow on t h e v a r i o u s r o o f s

( F i g .

4).

For t h e f l a t roof s i t u a t e d between t w o h i g h e r b u i l d i n g s a heavy snow a c c w n u l a t i o n o c c u r r e d a t t h e r e e r p a r a p e t , probably caused by wind f u n n e l l i n g between the b u i l d i n g s ( F i g . 20). Once a g a i n the average r o o f l o a d wae always l e s s t h a n t h e average ground load.

Ottawa: ( A comparison o f

5

d i f f e r e n t r o o f s i n a s h o r t w i n t e r ) I n t h e Ottawa a r e a o b s e r v a t i o n s were c a r r i e d o u t on f i v e r o o f s . From Fig.

5

i t c a n be s e e n t h a t t h e two g a b l e r o o f s had s m a l l l o a d s and b o t h e x h i b i t e d s i m i l a r l o a d d i s t r i b u t i o n s . T l l i s

was n o t t r u e f o r t h e t h r e e f l a t r o o f s , On t h e r o o f w i t h a

p a r a p e t , a l a r g e snow c o n c e n t r a t i o n o c c u r r e d a l o n g t h e p a r a p e t s . The snow was f a i r l y e v e n l y d i s t r i b u t e d on t h e r o o f s w i t h o u t a p a r a p e t . Snow tended t o p i l e up a d j a c e n t t o t h e h i g h e r s e c t i o n on t h e .roof w i t h a r a i s e d c e n t r a l p o r t i o n , , T h i s r o o f which was c o m p a r a t i v o l y s h e l t e r e d ( t r e e s ) had g r e a t e r l o a d s t h a n a n y of t h e o t h e r s . I n a l l c a s e s t h e a v e r a g e r o o f l o a d was l e s s t h a n t h e a v e r a g e ground l o a d .

F o r t C h u r c h i l l : ( A n example o f extreme d r i f t i n g )

Although t h e ground l o a d i n t h i s a r e a was up t o 30 p s f t h e r o o f l o a d s were n e a r l y z e r o f o r b o t h a f l a t and a p i t c h e d r o o f ( F i g .

6 ) .

Very l a r g e l o a d s were measured on t h e s m a l l p o r c h

r o o f a t t a c h e d t o t h e s i d e of t h e f l a t - r o o f e d b u i l d i n g . P h o t o g r a p h s of t h i s a c c u m u l a t i o n a r e shown i n F i g s .

21

a n d 22. S e v e r a l f e a t u r e s of t h e l o c a l c l i m a t e p r e v e n t e d l a r g e snow a c c u l . ~ u l a t i o n s . Wind s p e e d s were h i g h and t h e r e were many h o u r s of b r i g h t s u n s h i n e i n s p i t e of t h e c o l d w e a t h e r .

C S t a t i o n s : R.C.A.F.

O n a l l t h e l a r g e r o o f s u s e d a t t h e s e s t a t i o n s , wind a c t i o n r e s u l t e d i n d r i f t s a g a i n s t t h e w a l l s o f r a i s e d s e c t i o n s and on lowered r o o f s . It s h o u l d be n o t e d t h a t t h e a v e r a g e l o a d f o r t h e a r c h h a n g a r was t a k e n from d e p t h measurements on t h e f l a t wings a t t h e b a s e of t h e a r o h ( F i g . 12) and d o e s n o t r e p r e s e n t a t r u e a v e r a g e r o o f l o a d s i n c e t h e o b s e r v e d a r e a a c c u m u l a t e s l a r g e l o c a l d r i f t s . A s e x p e c t e d , h i g h a v e r a g e l o a d s were found on t h e s e s e c t i o n s . G e n e r a l D i s c u s s i o n

-

From t h e l i t t l e i n f o r m a t i o n o b t a i n e d s o f a r , it a p p e a r s t h a t t h e a s s u m p t i o n of a u n i f o r m l y d i s t r i b u t e d snow l o a d i s n o t g e n e r a l l y v a l i d . The f o l l o w i n g r e m a r k s a r e i n t e n d e d a s p r e l i m i n a r y comment only.. On f l a t r o o f s , t h e a v e r a g e snow l o a d i s l e s s t h a n t h e l o a d n e a r r o o f p r o j e c t i o n s s u c h a s p a r a p e t s . I n many c a s e s t h e r e f o r e , t h e u s e of a s m a l l e r u n i f o r m l o a d o v e r t h e major p a r t of t h e r o o f w i t h a l a r g e r , perhaps t r i a n g u l a r l o a d b e s i d e roof p r o j e c t i o n s would g i v e a more e c o n o m i c a l and a more a c c u r a t e d e s i g n t h a n t h o p r e s e n t u n i f o r m load. On some r o o f s , t h e g r e a t e r a c c u m u l a t i o n o f snow on

some p o r t i o n s of t h e roof r e s u l t s i n a n unsymmetrical l o a d i n g p a t t e r n which may be c r i t i c a l t o t h e s t r u c t u r e ,

gee.,

on arched and p i t c h e d r o o f s where one s i d e o f t e n shows much more snow t h a n t h e o t h e r .

An e x c e r p t from a Russian b u i l d i n g code ( s e e appendix), g i v e s a n example of a s p e c i f i c a t i o n f o r unbalanced l o a d , I n t h i s c a s e t h e p o s s i b i l i t y of s t r e s s r e v e r s a l i n t r u s s members due t o unbalanced l o a d i s covered by a "shape f a c t o r " of

1,4

f o r one

s i d e ( o r f o r a lowered roof p o r t i o n ) and of

0.6

_{on 1ke o t h e r s i d e} o f t h e span ( o r f o r a p r o j e c t i n g r o o f p o r t i o n ) . Some system of form f a c t o r s f o r v a r i o u s roof t y p e s should be included i n a d e s i g n

snow l o a d s p e c i f i c a t i o n ( a s i a now done i n t h e NBC f o r the roof s l o p e o n l y ) .

To d a t e , t h e o b s e r v a t i o n s of t h e s u r v e y have been made a t t h e ma j o r i t y of s t a t i o n s under below average snowfall c o n d i t i o n s . Years of h i g h s n o w f a l l may l e a d t o d i f f e r e n t c o n c l u s i o n s because

l a r g e l o a d s may be d i s t r i b u t e d d i f f e r e n t l y t h a n small loada. For example, i n a n exposed l o c a t i o n enow d r i f t s on f l a t r o o f s would not be h i g h e r t h a n t h e p a r a p e t s . Thus, i n t h e c a s e of a s n o w f a l l deeper than t h e p a r a p e t s t h e l o a d would p r o b a b l y be more n e a r l y uniform. I n s h e l t e r e d l o c a t i o n s t h e anow accumulation i s indepen- dent of p a r a p e t s , This f a c t o r might be c o n s j d e r e d i n r e g i o n s of deep snowfall.

Assuming t h a t t h e p a s t y e a r ' s r e c o r d s a r e t y p i c a l it would seem p o s s i b l e i n t h e f u t u r e t o reduce t h e b a s i c uniform d e s i g n anow l o a d , The d i s t r i b u t i o n of snow l o a d s depends, however, n o t o n l y on the a c t i o n of t h e wind ( d r i f t i n g ) , t h e temperature (snow c r e e p ) , and t h e amount of s u n ~ l h i n e but a l s o on the shape of t h e s t r u c t u r e i t s e l f . A l l t h e f a c t o r s c a u s i n g uneven accumulation a r e h i g h l y v a r i a b l e even i n one l o c a t i o n . Theref o r e , any c o e f f i c i e n t

o r shape f a c t o r s based on a number of o b s e r v a t i o n s must be c a r e f u l l y i n t e r p r e t e d f o r a d e s i g n l o a d i n g p a t t e r n . I t seems i n e v i t a b l e ,

t h e r e f o r e , t h a t any new improved code p r o v i s i o n f o r snow l o a d s w i l l

be more complicated t h a n a r e p r e s e n t r e g u l a t i o n s . I t i s a l s o p o s s i b l e t h a t r e d u c t i o n s made on account of s h i e l d i n g may be quest.ionable because of the p o s s i b l e removal o r a d d i t i o n of s u r r o u n d i n g o b j e c t s .

For t h e p a s t few y e a r s m e t e o r o l o g i c a l s t a t i o n s a c r o s s Canada have been r e c o r d i n g t h e d e p t h s of snow on t h e ground, When

s e v e r a l more y e a r s of r e c o r d s a r e a v a i l a b l e , i t should be p o s s i b l e t o p r e d i c t t h e r e t u r n p e r i o d of l a r g e snow d e p t h s more a c c u r a t e l y t h a n a t p r e s e n t , Before a d e s i g n l o a d i s s p e c i f i e d thought should be given t o t h e p r o b a b i l i t y of overload. If one assumes t h a t a d e s i g n l o a d should be such t h a t it w i l l occur a few t i m o s d u r i n g t h e l i f e of t h e s t r u c t u r e , t h e l o a d expeoted t o occur once e v e r y t e n y e a r s might be a n a p p r o p r i a t e ground l o a d . T h i s p e r i o d i s

c a l l e d t h e r e t u r n p e r i o d . The choice of t h e r e t u r n p e r i o d should a c t u a l l y be based on t h e expected 1 l . f e of t h e s t r u c t u r e .

When s e v e r a l y e a r s of r e c o r d s a r e a v a i l a b l e , i t should be p o s s i b l e t o s p e c i f y two d e s i g n f e a t u r e s :

(1) What p r o p o r t i o n of the s p e c i f i e d ground snow l o a d should be used a s t h e b a s i c uniform d e s i g n l o a d f o r v a r i o u s c l i m a t e zones?

( 2 ) What l o a d i n g p a t t e r n s should be added t o t h i s b a s i c l o a d t o account f o r l o c a l d r i f t i n g and

snow s l i d i n g ? These s t a ternents a r e n o t p o s s i b l e w i t h any degree of a c c u r a c y however, a f t e r o n l y

one y e a r of complete o b s e r v a t ions.

Ac knowledpment s

Without t h e a c t i v e p a r t i c i p a t i o n and a s s i s t a n c e of a l a r g e number of o b s e r v e r s t h i s e x t e n s i v e survey would n o t have been p o s s i b l e

The D i v i s i o n of B u i l d i n g Research wishes t o e x p r e s s i t s

a p p r e c i a t i o n t o a l l of t h e s e , and i n p a r t i c u l a r t o t h e many b u i l d i n g o f f i c i a l s and p r i v a t e companies t a k i n g B S t a t i o n o b s e r v a t i o n s , t o the R C A F p e r s o n n e l a t t h e C S t a t i o n s and t o

Messrs. M e Huepeden and J. S t a r k i n Ottawa. G r a t e f u l a p p r e c i a t i o n

a l s o goes t o A S t a t i o n o b s e r v e r s of the Neteorolo i c a l Branch o f

t h e Department of T r a n s p o r t (Goose Bay and Gander

f

,

t h e Dept. of

N a t i o n a l Defence ( c h u r c h i l l ) , t h e q e p t . of Northern A f f a i r s and

N a t i o n a l Resources ( ~ k l a v i k )

,

t h e Ecole Polytechnique a t Montreal,

Queents U n i v e r s i t y a t Kingston, t h e U n i v e r s i t y of Toronto, t h e U n i v e r s i t y of Manitoba a t Winnipeg, t h e U n i v e r s i t y of A l b e r t a

a t

Edmonton and t h e s t a f f of the D i v i s i o n of B u i l d i n g Research i n

K o n t r e a l , P.Q. ~ 1 s t : Heated 50 32 2 5 fJ7 I n s u l a t e d I n l e e of hI({her r o o f a F l a t : Heated 50 32 22 Not i n s u l a t e d 30 Oable: Heated 50 9 I n ~ u l ~ t o d 38 TABLE I --

SNOW LOADS FOR A STATIONS

O t i.awa, C n t . Gable : Heated I n s u l a t e d Gable: Hentod I n s u l a t e d F l a t ( n o p a l a e p e t ) Heated I n a u l a Led F l a t ( p a r n p e t ) Hen t e d I n s u l e t e d F l a t ( r e 1 ~ e d s e c t i o n ) I n s u l a t e d H e a t e d F o r t C h u r o h i l l , Gable: Heatod Man. I n s u l a t a d F l a t : Hoatod I n s u l n t e d

Gagetown, N.B. Gable: Heated I n s u l a t e d F l a t : Heated

I n s u l n t e d

K i n g s t o n , Ont. Gable: Heatmd I n a u l e t e d F l a t : Heated

I n s u l a t e d

Gander, Nf l d . Gable : Not h e a t e d I n s u l a t e d F l a t : Not h e a t e d I n s u l a t e d S t a t i o n H a l i f a x , N.S. Ga h l e : Hea t e d N o t J n ~ ~ l a b e d F l ~ t : Hea t e d I n s l l l e t o d F l a t : Heated 1 n . s u l a t e d A k l a v i k , N.W.T. Gable : Exposed 30 2 5 14 2 5 Hooted I n s u l a t e d Gable: S h n l t e r e d 30 2 5 12 16 Heated I n s u l a t e d '0 G l a c i e r , B.C. Gable: Not h e a t e d

-

0 1 0 8 16 26 ( G a r a g e ) I n s u l a t e d o Gable: Not I r ~ s u l a t e d .rl 0 110 30 ( H o u s e ) Heatod 0 50 a Gable: Not i n s u l a t e d ro 111 3 0 ₃₆ (Of f l o e ) H e a t e d w Gable: S h e l t e r e d 0 z 120 82 Not h e n t e d 95 Not i n s u l a t e d N.3.C. ( p a r )

----

Roof

Goose Day, Lab. Gable: Hooted 7 5

I n s u l u t e d 114 F l a t : Heated 90 12 I r . S u l ~ t e d I Winnipeg, Man. A l l r ~ ~ n f s Toronto, Ont

.

A l l r o ~ l f s S a s k a t o o n , Snuk. A l l r o o f o

Max. Av. Ground Max. Av. HOOP

( p s f ) ( p 3 f )

Vancouver, B.C. A l l r o o f rr

Edmr~nton, P.lta. A l l r < ~ o f s

Wax. Observed

G A R A G E

SHEL TERED NOT INSUL A TED

U N V E N T I L A T E D A T T I C SHEL TERED NOT INSUL A T E 0 UNHEATED A T T I C UNVENTILATED A T T I C z + ~ -BLDG HEATED

FIGURE 2

Y E A R : 1957/58 SNOW L O A D O B S E R V A T I O N S ON ROOFS B.C.

0 0 0 0 ROOF D E S C R I P T I O N REMOVED ALPINE L O D G E SHEL TEREO NOT INSUL 4 TEO

0 _1 O F F I C E SHEL TEREO NO INSUL dTlOM FROM ROOF 2 0

NOVEMBER DECEMBER JANUARY FEBRUARY MARCH

-

FIGURE

3

- 40 -- ..- - ~ ~ ~ . . Y E A R : 1957/58 SNOW L O A D O B S E R V A T l O N S O N R O O F S L O C A T , O N : G L A C I E R . B.C.

7561 C A S G R A I N ST.

( S H E L T E R E D , INSULATED , HEATED)

ELEVATION

( EXPOSED

,

H E A T E D , NOT INSULATED)

INSUL A TED

-

-FIGURE

4

SNOW L O A D OBSERVATIONS ON ROOFS Y E A R : l957/58

Y E A R : 1957/58 SNOW L O A D , OBSERVAT I ONS ON ROOFS

L O C A T , O N : O T T A W A 4 0 2 0 0 4 0 4 0 B A R K E R BLVD.%,.

(EXPOSED , HEA T E D , lNSUL A TED 2")

(EXPOSED, HEATED, INSULTED

54, CElL ING INSULATED 147

HEATED EL DG

FA1 RHAVEN

- - - -

fSHELTERED, HEATED, lNSUL A T E D ) 7 2 B I S H O P BLVD. (,. '\ \ \ \. \ \ \ 0 R O O F DESCR l P T l ON 6 p 7 2 B I S H O P BLVD. I I , / _ 0 - -

fi-T,

4 .

fb

_y

;"

L

3 .

(EXPOSED, HEATED, lNSUL A TED 2'7

\

1

1

4 0 BARKER BLVD.

6 0

i R O O F D E S C R I P T I O N

40 !

!

HEATED, INSULATED, NOT V E N T I L A T E D

HEATED, I N S U L A T E D , V E N T I L A T E D

FIGURE

6

Y E A R : 1957/ 5 8

4 6 0 R O O F D E S C R I P T I O N - _{W 0 R K S H 0 P} 0 -E X P O S -E D H E A T E D INSULATED P A R A P E T I ' n 0 O F F I C E R S ' M E S S F - 5

EXPOSED HEA TED INSULA TED

S C C T I O N X - X

FIGURE

7

SNOW LOAD OBSERVATIONS ON ROOFS Y E A R : 1957/58

Y E A R : 1957/58 SNOW LOAD OBSERVAT I ONS ON ROOFS

L O C A T , O N : K I NGSTON ONT. 9 R O O F D E S C R I P T I O N - 6 0 - -- - p R E S I D E N C E 40 - p --'A'--

4

2 0 N /-.---4 * *' I- SHELTERED N I A T T I C NOT HEATED @LOG HEATED INSUL A TED 20 MC N E l L HOUSE 0 H - 4 0 ' - 0 4 0 . Z I 3: a > z _t - 0 a w 20 ---- 0 W 4" a" H 0 L L 4 - o a - - - - 40 z L L W 12 - 0 t - V) I _- 0 3

F I G U R E

8

- - . -- - -- -

DECEMBER JANUARY F E B R U A R Y MARCH A P R I L

I EXPOSED INSUL A TED A T T I C NOT VENTILATED A T T I C NOT HEATED EL DG HE4TED PARAPET 2'6"

Y E A R : 1957/58 SNOW L O A D O B S E R V A T I O N S ON ROOFS LOCATION: GANDER R O O F DESCRIPTION HAWKER CRESCENT

.+-

kg---

/ SHE1 TEREED HEA TED INSUL A TED )

MECH. EQUIP. R O O M

H = 2 4 '

,

+p

150'

---I

( NOT SHELTERED EXCEPT FROM EAST NOT HEATED INSULATED J

% 0 5 p W % - 40 L L W 1 2 - O Z V) I IIV) 3 z ?: o - - - - - - - - - - - - - - - F I G U R E

9

R O O F DESCRIPTION N.S. TECH COLLEGE P A R A P E T TO G R A D E N.R.C. LAB ENGINEERING BLDG.

F I G U R E

10

SNOW LOAD OBSERVATIONS ON R O O F S

Y E A R : 1957/58 L O C A T I O N : HALIFAX,

Y E A R : 1957/58 SNOW LOAD OBSERVATIONS ON ROOFS

L O C A T I O N : GOOSE BAY b 60 ROOF D E S C R I P T I O N NO PARAPET E X P O S E D , HEATED, INSULATED.

U S D E P O T ARCH. HANGER M.E. GARAGE - - - - CANT. HANGER

F I G U R E

12

SNOW L O A D O B S E R V A T I O N S ON R O O F S

Y E A R : 1957/58 L O C A T I O N : R.C.A.F. S T A T I O N C O L D L A K E , A L T A .

R O O F DESCRIPTION

U.S. DEPOT

S E E F I G U R E 12

---

FIGURE 13

SNOW LOAD OBSERVATIONS ON ROOFS Y E A R : 1957/58

LOCATION: R.C. A.F.

S E E F I G U R E 12 ---

FIGURE

14

SNOW L O A D OBSERVATIONS ON ROOFS Y E A R : 1957/58

L O C A T I O N : R.C.A.F. L A N C A S T E R, A L T A .

M E . G A R A G E

\

DECEMBER JANUARY FEBRUARY MARCH APRl L

I I

SNOW LOAD OESERVATIONS ON ROOFS

ROOF DESCRIPTION S E E FIGURE 12

FIGURE 15

Y E A R : 1957/58 L O C A T I O N : R.C.A.F. O T T A W A

FIGURE 17 SNOW L O A D S U R V E Y 1957-58

M A X I M U M ROOF AND GROUND LOADS F O R A 8 C S T A T I O N S

C S T A T I O N S OTTAWA COLD L A K E COMOX B.C. DOWNSVIEW GOOSE BAY NORTH BAY WINNIPEG No I 2 3 4 5 6 7 8 9 10 I 1 12 13 14 15 A S T A T I O N S GOOSE BAY H A L I F A X GANDER GLACIER KINGSTON GAGETOWN A K L A V K OTTAWA MONTREAL WINNIPEG T O R O N T O CHURCHILL SASKATOON VANCOUVER EDMONTON

Figure

& h v f k (March 30,

HgS8),

E-3

H o s t e l e Load d l s t ~ i b u % i c n

on

a l a r g e r o o f e

(Photo by Be Engler, Banff F P p r e

19,

G l a c f e r , B e C e [ P e b r u a ~ y

1958)@

Alpine Club Lodge

-

Snow ssvec~~auletion 11s 8 oomp%e t e l y s h e l t e r e d

a r e a

,e

PLAN

E L E V A T I O N

S C A L E : 6'" 220'

Figure 2 B e C h i l ~ c h f I 1 , Man. _{( J a n u a ~ g} 13,

_19581,

Building F-2.

Snow a c c m u % a t f o n on lowez-ed r o o f sectfon ( r n ~ f n

Fggure 23, McHe$ll House, Queen" University, Ringston

(February

19581,

Snow aecumulaticn at a

APPENDIX

SNOW LOAD SPECIFICATIONS

IW

RUSSIA

S p e c i f i e d snow l o a d s i n most c o u n t r i e s a r e g i v e n a s a uniformly d i s t r i b u t e d l o a d , Some c o u n t r i e s s t i p u l a t e i n a d d i t i o n t h a t c o n c e n t r a t i o n s due t o t h e shape of the b u i l d i n g and t h e

e f f e c t of t h e wind s h a l l be c o n s i d e r e d but do n o t s t a t e how l a r g e t h e s e c o n c e n t r a t i o n s a r e , The Russian b u i l d i n g code " ~ o n s t r u c t i o n

Standard8 and Regulat i o n s w

(lqsk),

however, g i v e s q u a n t i t a t i v e non-

uniformly d i s t r i b u t e d s n m l o a d s , I t should be noted t h a t t h o s e

p a r t s of t h e USSR f o r which non-uniform snow l o a d s a r e s p e c i f i e d , have a d r y , c o l d c l i m a t e i n which d r l f t s occur e x t . e n s i v e l y , These

might be compared t o t h e Canadi- n _{p r a i r i e s and l o c a t i o n s such a s}

F o r t C h u r o h i l l , Manitoba where snow on a roof u s u a l l y accumulates o n l y a l o n g v e r t i c a l o b s t r u c t i o n s ,

To i l l u s t r a t e t h e Russian approach t o snow l o a d s , t h e p e r t i n e n t s e c t i o n from t h e Russian " ~ o n s t r u c t i o n S t a n d a r d s and ~ e g u l a t i o n s " and two abstF:acts of a r t i c l e s which appeared i n t h e Russian p e r i o d i c a l " ~ o n s t r u c t i o n industryw and which d i s o u s s t h e

s t a n d a r d s mentioned above a r e given i n t h i s appendix a s t r a n s l a t e d by t h e a u t h o r w i t h t h e h e l p of M r . G.G. Belkov of t h e NRC T r a n s l a t i o n s O f f i c e . It i s hoped t o p u b l i s h t h e f u l l t r a n s l a t i o n s l a t e r i n a

s e p a r a t e r e p o r t .

1, Russian " ~ o n s t r u c t i o n S t a n d a r d 8 and Reaula t i o n s "

,

P a r t 11. S e c t i o n B

-

S t a n d a r d s f o r t h e Design of L o a d - E e a r a E o n s t r u c t i o n a . S e c t i o n on Snow Loads ( p a

4 b

I

48)

( A

1)

he

snow l o a d p e r square meter a r e a of h o r i z o n t a l p r o j e c t i o n of roof i s determined a c c o r d i n g t o t h e formula

where p

=

weight of snow cover i n kg/m 2

depending on t h e r e g i o n of t h e

USSR a c c o r d i n g t o Table

4

c

=

c o e f f i c i e n t depending on t h e p r o f i l e of t h e r o o f a c c o r d i n g t o Table

5.

The c o e f f i c i e n t of o v e r l o a d Itn" f o r snow l o a d s h a l l be t a k e n e q u a l t o

1.4.

The Weight of Snow Cover P

Table

4

Note: I n mountain d i s t r i c t s , a s w e l l a s i n t h e r e g i o n s f extreme Yi5S31 and remote e a s t t h e weight of snow cover p i n kg/ma s h a l l be

t a k e n n u m e r i a a l l y e q u a l t o 2h, where h

=

d e p t h of snow cover i n c m t a k e n from m e t e o r o l o g i c a l o b s e r v a t i o n s a s t h e mean of t h e

maximum

y e a r l y a t a p r o t e c t e d p l a c e over 1 0 years. I n t h e mountain d i s t r i c s t h e weight of snow cover s h a l l be t a k e n n o t l e s s t h a n

60

_{kg/rns (12 p s f ) . "} No. Value of C o e f f i c i e n t Region of USSR ( s e e F i g . A - 1 ) Table

5

1 2

3

4

5

No. 1 I I1 I11 I V 50 100 150 Shape of Roof

I

c

v

2 00 Note 2 Simple r o o f , p e n t r o o f and g a b l e roof s l o p e A L _{2 5 O} A :-r

600

, /10f

Simple a r c h e d r o o f where/: = span of a r c h f

=

r i s e of a r c h The c o e f f i c i e n t c s h a l n o t be g r e a t e r t h a n l,0 nor l e s s t h a n 0.3 1.0 0.0 A t i n t e r m e d i a t e a n g l e s of roof p i t c h t h e c o e f f i c i e n t c i s found by i n t e r p o l a t i o n

3

Complex roof w i t h t r a n s v e r s a l o r l o n g i - t u d i n a l c l e r e s t o r e y s , w i t h unoqual h e i g h t s of s e p a r a t e p a r t s , e t c . I n a c c o r d w i t h Fig. A-2 4

-

The d i f f e r e n c e i n h e i g h t H i s given i n m e t r e s

---

.A

2 . The Problem of Snow Load S p e c i f i c a t i o n b EoDe Kan Khut

-

-+

-

7 m c t f r o m T ~ ~ n s t r u c t i o n

,

-

-

-

I

~ 0 7 1 2 ~ -1

-2% _(A-2-

o a n a s 22

I n r e g i o n s of t h e USSR where t h e r e a r e h i g h winds (e.g.

~ a z a k s t a n ) snow d o e s n o t accumulate on f l a t r o o f s b u t r a t h e r a s l a r g e d r i f t s behind r o o f p r o j e c t i o n s * The p r e s o n t s t a n d a r d s on snow l o a d s do n o t t a k e t h e s e l o c a l c l i m a t i c f e a t u r e s i n t o account.

( a ) F i g u r e

A-3

i l l u s t r a t e s t h e d i f f e r e n c e between s p e c i f i e d and a c t u a l snow l o a d ( t a k e n i n t h e

Karaganda p r o v i n c e )

.

The s p e c i f i e d l o a d for

f l a t r o o f s i s 70 kg/m2

(14

p s f ) .

Snow i s n o t r e t a f n e d on f l a t r o o f s , even when t h e r e i s s n o w f a l l w i t h o u t wind t h e l o a d s on f l a t r o o f s never a p p r o a c h t h o s e s p e c i f i e d . For t h i s l o c a t i o n , o n l y once i n

8

y e a r s d i d t h e

monthly s n o w f a l l exceed 1 0 cm

( 4

i n . ) which w i t h

a d e n s i y of 0.2 f o r l o o s e snow g i v e s a l o a d of

t

20 kg/m

(4

p s f ). Since t h e r e a r e no l o n g p e r i o d s w i t h o u t h i g h w i d s i n t h e Karaganda a maximum l o a d o f 20 kg/m9

(4

p s f ) c a n be used f o r low-pitched r o o f s up t o

l o 0

s l o p e . ( b ) On roof s u r f a c e s n e a r roof p r o j e c t i o n s t h e s p e c i f i e d l o a d i s 112 kg/rn2 ( 2 3 p s f ) which w i t h a n o v e r l o a d c o e f f i c i e n t of

1.4

becomes 157 kg/m2 (32 p s f ) .

The a c t u a l snow l o a d depends f i r s t of a l l on t h e p r o j e c t i o n h e i g h t H. U p t o c e r t a i n v a l u e s of H (which ~ $ 1 1 d i f f e r f o r d i f f e r e n t l o c a l i t i e s ) , a

maximum h e i g h t of

O.9H

f o r snow d r i f t and a n

a v e r a g e of 0.7H a r e found ( F i g .

A - 3 ) .

The measured

s p e c i f i c g r a v i t y of d r i f t e d snow bohlnd r o o f p r o j e c t i o n s r e a c h e s 0.3500 For example, t a k i n g a p r o j e c t i o n h e i g h t H

=

2.0 m, which i s n o t v e r y l a r g e , an a c t u a l l o a d of 0*7

x

2.0 x 350 =

490

kg/m (100 p s f ) i s r e a c h e d , a b o u t t h r e e t i m e s t h o s p e c i f i e d load. P e r i o d i c snow removal c a n n o t be c o n s i d e r o d r e l i a b l e s i n c e

snow c a n n o t be removed d u r i n g b l i z z a r d s w i t h h i g h wind speeds.

( c ) On i n c l i n e d s o o f s t h e s p o c i f i c d snow l o a d d o o r e a s e s

from 70 kg/m

(111

p s f ) t o z e r o botvrcen 2 S U a n d

60°.

A c t u a l l y on t h e leeward sido of r c o f s w i t h n

s l o p e of

3 5 O

t o 4 5 O onow accumula ti.on.:r, up t o 1",0

metre have been o b a e ~ v o d (Fig* A-!L), ra loricl. ranching

The d i f f e r o r l c e between t h e s p e c i f i e d and a c t u a l snow l e a d e

i n windy r e g i o n s o c c u r s because t h e s p e c i f i e d l o a d s a r e based

on

t h e amount of s n o w f a l l o n l y , whereas t h e d i s t r i b u t i o n of snow i d

a l s o a f u n c t i o n of wind s p e e d s * The s p e c i f i e d l o a d i s the same f o r Minak and Karaganda, b u t t h e d i s 1 ; r i b u t i o n of snow a t Karaganda, where t h e r e a r e h i g h winds d i f f e r s w i d e l y from Minska

The f o l l o w i n g e r r o r s i n t h e s p e c i f i e d l o a d s a r e t h e r e f o r e a p p a r e n t : (1) A l l r o o f s w i t h a s l o p e l e s s t h a n 10' and w i t h o u t p r o j e c t i o n s have s p e c i f i e d l o a d s which a r e t o o l a r g e and n e v e r e n c o u n t e r e d i n p r a c t i c e . A r e d u c t i o n would p r o v i d e a g r e a t economical s a v i n g . ( 2 ) A l l r o o f s w i t h s t e e p s l o p e s o r p r o j e c t i o n s have much g r e a t e r a c t u a l snow l o a d s t h a n t h o s e s p e c i f i e d c a u s i n g damage and even c o l l a p s e i n some c a s e s .

It i s d e s i r a b l e t h a t t h e s p e c i f i c a t i o n o ornmittee o r g a n i z e s f i e l d o b s e r v a t i o n and d e r i v e s snow l o a d s p e c i f i c a t i o n s which t a k e a c c o u n t of a c t u a l a c c u m u l a t i o n i n r e l a t i o n t o t h e wind v e l o c i t y .

The R e s e a r c h I n s t i t u t e of I n d u s t r i a l C o n s t r u c t i o n h a s examined b o t h t h e d i s t r i b u t i o n and magnitude of snow l o a d s i n a l l p a r t s of t h e USSR and t h e i n f l u e n c e of snow l o a d s on s t r u c t u r a l damages and f a i l u r e s a f t e r heavy s n o w f a l l s .

The magnitude of d e s i n r o o f snow l o a d depends on t h e amount of snow p r e c i p i t a t i o n

f

m e t e o r o l o g i c a l r e c o r d s ) , s i z e and

ahape of r o o f and t h e r e g u l a r i t y of r o o f maintenance. O t h e r f a c t o r s

which d e t e r m i n e snow l o a d s a r e wind speed and d i r e c t i o n , a b i l i t y of snow t o be d r i f t e d and t h e p r e s e n c e of v e n t i l a t i o n d u c t s , p i p e s , p a r a p e t s , e t c . The c a l c u l a t i o n of the e f f e c t s of combining a l l t h e s e f a c t o r s i s v e r y d i f f i c u l t and u n d o u b t e d l y l e a d s t o complex r e l a t i o n s h i p s .

The problem may be approached by i n v e s t i g a t i n g o n l y t h e p r i n c i p a l c a u s e s a n d drawing c o n o l u s i o n s which a r e v e r i f i e d i n p r a c t i c e . M e t e o r o l o g i c a l r e c o r d s and t h e shape and s i z e of r o o f a r e t h e most i m p o r t a n t i n f l u e n c e s .

The i n f l u e n c e of r o ~ f maintenance (snow c l e a r i n g ) c a n be c o n s i d e r e d from two p o i n t s of views Because of t h e d i f f i c u l t y and c o s t of snow removal snow l o a d s should n o t be r e d u c e d because

o f a n t i c i p a t e d snow removal, Also, t h e r e d u c e d c o s t of t h e

b u i l d i n g i s s u f f i c i e n t t o make snow removal d e s i r a b l e . The snow

l o a d s i n " S t r u c t u r a l S t a n d a r d s R e g u l a t i o n s " t a k e i n t o a c c o u n t removal of snow from t h e r o o f .

S t a n d a r d snow l o a d s a r e d e t e r m i n e d a s t h e mean of maximum y e a r l y snow c o v e r from 1 0 y e a r s of o b s e r v a t i o n s a t a p r o t e c t e d l o c a t i o n , u s i n g a snow d e n s i t y of 0.2,

I n d e r i v i n g more p r e c i s e v a l u e s of snow l o a d , t h e new s p e c i f i c a t i o n " S t r u c t u r a l S t a n d a r d s and R e g u l a t i o n s " r a i s e d l o d s

i n t h e r e g i o n s of N o v o s i b i r s k , Kemenof and Tobalsk t o 150 kg/m

9

( 3 1

p s f ) from 1 0 0 kg/rn2 ( 2 0 p s f ) i n t h e e a r l i e r s t a n d a r d . To a s s e s s t h e e f f e c t of snow l o a d on d e f o r m a t i o n and f a i l u r e of s t r u c t u r e s comparisons of s t r e s s e s were made between t h e c o n d i t i o n of a c t u a l ( o b s e r v e d ) snow l o a d s and t h e c o n d i t i o n of snow l o a d s from t h e e a r l i e r s t a n d a r d .

F i g u r e

A-5

shows a t r u s s - s u p p o r t e d r o o f of a l a r g e

i n d u s t r i a l b u i l d i n g and t h e t r i a n g u l a r - s h a p e d snow a c c u m u l a t i o n

whioh oaused s t r u c t u r a l collapse. A comparison of t h e t r u s s -

member s t r e s s e s f r o m dead l o a d p l u s snow l o a d i n t h e e a r l i e r s t a n d a r d t o dead p l u s a c t u a l snow l o a d i s shown i n Fig.

A-6.

The a c t u a l snow l o a d s t r e s s e s g r e a t l y e x c e e d t h e a l l o w a b l e d e s i g n s t r e s s e s i n most members e s p e c i a l l y i n t h o s e h a v i n g s m a l l s t r e s s e s and t h o s e d i r e c t l y l o a d e d (e.g. b a r

V 5

where t h e i n c r e a s e i s from

688

t o 1,822 kg/crn2 19,800 t o 25,900 p s i t o r

1.65%).

The i n v e s t i g a t i o n of s t r e s s c o n d i t i o n s i n r o o f s h a v i n g e x c e s s i v e d e f l e c t i o n s e s t a b l i s h e d t h a t ( i ) r o o f s t r u c t , ~ - ~ e s ~ i h i c h

deformed e x c e s s i v e l y u n d e r t h e weight of snow were found t o have d e f e c t s ; ( i i ) t h e d e n s i t y of snow c a u s i n g def orlna t i o n exceeded t h e a v e r a g e of newly f a l l e n snow showing t h a t snow was n o t c l e a r e d from t h e r o o f and t h u s became compressed.; ( i i i ) e x c e s s i v a d c f o r - m e t i o n s a f t e r s n o v ~ f a l l s o c c u r r e d i n d l v e r s e geographic regi0n.s and;

( i v ) t h e snow on r o o f s w a s d i s t r i b u t e d i r r e g u l a r l y and accumulated i n lower s e c t i o n s .

F i e l d o b s e r v a t i o n s of snow a c c u m u l n t l o n were t a k e n on

some i n d u s t r i a l b u i l d i n g s d u r i n g t h e w i n t e r of

1954-55

n e a r losc scow,

A p l a n view of a n i n d u s t r i a l b u i l d i n g w i t h a two s t o r y l e a n - t o

( F i g . A-7) shows i s o l i n e s o f snow l o a d which i n d 3 c a t e how i r r e g u l a r l y t h e snow i s d i s t r i b u t e d a c c u m u l a t i n g a t t h e w a l l s i n t h e shape of a tr a n g l e . The snow l o a d i n t h e t r i a n g u l a r d r i f t r e a c h e d 350-b00 kdm3 ( 7 2 t o 82 p s f ) e x c e e d i n g the s t a n d a r d s p e o j f ' i e d l o a d by

3.5

t o

4

t i m e s whereas t h e t o t a l r o o f snow l o a d d i d n o t exceed t h e t o t a l s t a n d a r d l o a d .

References

A . 1

Construction Standards ~ n d

Regulations. Part I1

-

Standarda

of Construction Planning. Gosu.darstvennoe Izda

tell stvo

Litera

tury po Stroitell

stvu i Arkhitecture, Moskva,

1954.

A.2

Kan Khut,

E.D.

The problem of snow load specification.

Stroitellnafa Promyshlennost, No. 12,

1954,

pp 23-23.

A.3

Go11 denblat,

I.I.,

B.G.

Korenev and A.M.

Sizov. Snow loads

according to "~onstruction

Standards and ~egulationa".

PC = 2 0 0 H ( M E T R E S ) B U T N O T L E S S T H A N q

NOR M O R E T H A N 4 q a = 2 H BUT NOT LESS THAN 5 . 0 M NOR M O R E

T H A N 10.0 M

F I G U R E A - 2

C O E F F I C I E N T S F O R D E S I G N SNOW LOADS FOR R O O F S W I T H C O M P L E X P R O F I L E S

- -

0.9

H W l N D H

?--

m ; / / / / / / / / / / / / / / / / / / / / /

F I G U R E

A - 3

SHAPE O F SNOWDRIFT

ON

A

R O O F L E D G E

W l N D

-

F I G U R E

A - 4

W I N D WATERPROOFED 1 2 . 2 0 PREFAB R/C SLABS

FIGURE A - 5

CROSS - S E C T I O N OF AN I N D U S T R I A L B U I L D I N G

AND A TRIANGULAR SNOW A C C U M U L A T I O N

A O V E R S T R E S S F R O M A C T U A L SNOW L O A D S U S I N G S T E E L T Y P E NO S.T 3

A L L O W A B L E S T R E S S )

UPPER BARS LOWER B A R S S L A N T I N G B A R S COUNTERS

'1 '2 '3 '4 '5 '6 Ui '2 "3 '4 '5 'I '2 '3 '4 '5 '6 7' 8' '9 ' 1 0 ~ l l '12 "2 V 3 "4 "5 "6

FIGURE A - 6

THE INFLUENCE OF SNOW OVERLOAD ON THE CEILING TRUSS SHOWN I N FIG. A - 5

A - S T R E S S E S I N THE M E M B E R S OF T H E T R U S S F O R D I F F E R E N T T Y P E S OF L O A D I N G ; a - FROM T H E P E R M A N E N T L O A D ACCORDING TO T H E D E S I G N AND SNOW LOADS ACCORDING TO O S T 9 0 0 5 8 - 4 0 ; b - F R O M THE P E R M A N E N T L O A D A C C O R D I N G TO T H E D E S I G N AND A C T U A L L O A D S OF SNOW;

0 - T H E DEVIATION O F FORCES I N THE MEMBERS OF T H E T R U S S W I T H D E S I G N L O A D FORCES T A K E N A T 100 O/o

A - A