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Snow loads on roofs 1960-61: fifth progress report
NATIONAL R E S E A R C H COUNCIL CANADA DIVISION O F BUILDING R E S E A R C H SNOW LOADS ON R O O F S 1960-61 F I F T H P R O G R E S S R E P O R T b y J . F . Scott and B. G. W. P e t e r I n t e r n a l R e p o r t No. 228 of 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 OTTAWA N o v e m b e r 1961
P R E F A C E T h e h e a v i e s t loat1 t h a t r o o f s i n C a n a d a h a v e t o s u p p o r t i s u s u a l l y t h e loatl i l n p o s e d by snow. It i s o b v i o u s , t h e r e f o r e , t h a t t h e m a g n i t u d e and d i s t r i b ~ i t i o n of t h e d e s i g n s n o w l o a d h a v e a c o n - s i d e r a b l e e f f e c t on t h e s a f e t y of a s t r u c t u r e , a s w e l l as i t s c o s t of c o n s t r u c t i o n . T h i s i s t r u e f o r a n y r o o 1 but p a r t i c u l a r l y f o r r o o f s of s m a l l e r d w e l l i n g s u n d e r o r d i n a r y , r e l a t i v e l y s h e l t e r e d c o n d i t i o n s f o r which wind l o a d s a r e g e n e r a l l y of l e s s i m p o r t a n c e .
Snow l o a d s f o r d e s i g n p u r p o s e s a r e given i n t h e National Building Code of C a n a d a . In t h e 1953 edition of t h e C o d e , t h e l o a d s which w e r e shown on a m a p w e r e d e r i v e d d i r e c t l y f r o m m e a s u r e m e n t s of snow on t h e g r o u n d . It i s f r e q u e n t l y o b s e r v e d , h o w e v e r , t h a t
s n o w d e p t h s on t h e ground cannot b e applied d i r e c t l y t o t h e d e t e r m i - nation of d e s i g n s n o w l o a d s f o r r o o f s . C o n s e q u e n t l y , t h e A s s o c i a t e C o m m i t t e e on t h e N a t i o n a l Building Code, which is 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 a n d continued r e v i s i o n of t h e C o d e , a s k e d t h e D i v i s i o n of Building R e s e a r c h t o c a r r y out a s u r v e y of a c t u a l s n o w l o a d s on r o o f s . T h i s s u r v e y w a s begun i n 1956. B e c a u s e of r e g i o n a l and a n n u a l c l i m a t i c v a r i a t i o n s , s u c h a s t u d y m u s t extend o v e r s e v e r a l y e a r s a n d m u s t t a k e into a c c o u n t t h e whole of C a n a d a . T h i s fifth p r o g r e s s r e p o r t i n t h e continuing s u r v e y g i v e s t h e r e s u l t s of t h e 1960-61 s n o w load o b s e r v a t i o n s and a l s o d i s c u s s e s t h e s i g n i f i c a n c e of t h e f i n d i n g s t o d a t e . T h e Division of Building R e s e a r c h w i s h e s a g a i n t o e x p r e s s i t s s i n c e r e a p p r e c i a t i o n t o t h e m a n y o b s e r v e r s a c r o s s C a n a d a who h a v e c o n t r i b u t e d s o g r e a t l y t o t h e s u c c e s s of t h i s s u r v e y . In p a r t i c u l a r , t h a n k s a r e e x p r e s s e d t o t h e m a n y building i n s p e c t o r s , p r i v a t e c o m p a n i e s , and i n d i v i d u a l s who m a d e B-Station o b s e r v a t i o n s , and a l s o t o t h e R C A F p e r s o n i l e l a t t h e C S t a t i o n s and t h e i r o r g a n i z e r s in O t t a w a . G r a t e f u l a p p r e c i a t i o n i s r e c o r d e d a l s o t o A-Station o b s e r v e r s of t h e I v ~ e t e o r o l o g i c a l B r a n c h of t h e D e p a r t m e n t of T r a n s p o r t a t G o o s e B a y and G a n d e r , t h e D e p a r t m e n t of National Defence a t F o r t C h u r c h i l l , E c o l e P o l y t e c h n i q u e ( M o n t r e a l ) , Macdonald C o l l e g e ( S t e . Anne d e B e l l e v u e ) , Q u e e n r s U n i v e r s i t y ( K i n g s t o n ) , U n i v e r s i t y of T o r o n t o , U n i v e r s i t y of M a n i t o b a (Winnipeg), U n i v e r s i t y of A l b e r t a ( E d n l o n t o n ) , A l u m i n u m Co. of C a n a d a a t A r v i d a , t h e F e d e r a l Day S c h o o l a t Inuvik, and t h e A t l a n t i c , P r a i r i e and B . C. R e g i o n a l S t a t i o n s of t h e Division. T h i s r e p o r t w a s p r e p a r e d by J . F. S c o t t , g r a d u a t e i n c i v i l e n g i n e e r i n g f r o m Q u e e n ' s U n i v e r s i t y , s u m r n e r w o r k e r with t h e Building S t r u c t u r e s S e c t i o n , and B. G.
M
.
P e t e r , R e s e a r c h O f f i c e r with t h e S e c t i o n , u n d e r t h e d i r e c t i o n of W . R . S c h r i e v e r . O t t a w a N o v e m b e r 196 1 R o b e r t F . L e g g e t , D i r e c t o r .T A B L E O F C O N T E N T S
. . .
INTRODUCTION P a g e 1. . .
.
2 SUMMARY O F 1960 -61 OBSERVATIONS 2. . .
.
3 D E S C R P T I O N O F THE 1960 -61 OBSERVATIONS 2 4.
WEATHER. . .
3. . .
.
5 PRESENTATION O F RESULTS 3 6.
RESULTS. . .
4 ( a ) A Stations. . .
5. . .
( b ) B Stations 8. . .
( c ) C Stations 9. . .
.
7 GENERAL DISCUSSION 10 METEOROLOGICAL FACTORS. . .
( a ) Solar Radiation 10. . .
( b ) Density 11 ( c ) Wind and T e m p e r a t u r e. . .
11 SHAPE FACTORS . -. . .
( a ) Snow L o a d s on L a r g e F l a t Roofs 11. . .
( b ) D r i f t s at P a r a p e t Walls 11. . .
( c ) D r i f t s on t h e Lower P o r t i o n of S p l i t - L e v e l Roofs 11 ( d ) Roof Slope. . .
12. . .
( e ) Concentrated L o a d s on P i t c h e d Roofs 12. . .
( f ) Unbalanced Loads on P i t c h e d Roofs 12
. . .
.
8 INTERIM CONCLUSIONS 13
. . .
SNOW LOADS ON R O O F S 1960-61
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F i f t h P r o g r e s s R e p o r tJ . F. Scott and B. G. W. P e t e r
T h e c o u n t r y -wide s u r v e y of snow l o a d s on r o o f s b e i n g c a r r i e d out by t h e Division of Building R e s e a r c h h a s o n c e a g a i n p r o v i d e d a s u b s t a n t i a l a m o u n t of i n f o r m a t i o n . T h i s s u r v e y c o v e r s o b s e r v a t i o n s of snow depth and d e n s i t y on a l a r g e n u m b e r of r o o f s
and on t h e s u r r o u n d i n g ground. In t h i s fifth p r o g r e s s r e p o r t ,
t h e s e o b s e r v a t i o n s h a v e b e e n a n a l y z e d , t h e i r t r e n d s and u n u s u a l f e a t u r e s d i s c u s s e d , and t h e i r r e s u l t s c o r r e l a t e d with i n t e r i m c o n c l u s i o n s of p r e v i o u s r e p o r t s .
T h e s u r v e y , which w a s begun in 1956 a t t h e r e q u e s t of t h e A s s o c i a t e C o m m i t t e e on t h e National Building Code, p r o v i d e s i n f o r m a t i o n on which t o b a s e m o r e r a t i o n a l snow load s p e c i f i c a t i o n s in f u t u r e e d i t i o n s of t h e C o d e . In t h e 1953 edition of t h e Code, t h e roof l o a d s w e r e colmputed b y adding t h e m a x i m u m depth of snow c o v e r on t h e ground and t h e m a x i m u m w i n t e r r a i n t h a t m i g h t f a l l i n t o t h a t snow c o v e r ; i t w a s found, h o w e v e r , t h a t d e p t h s on t h e roof
f r e q u e n t l y did not c o r r e s p o n d t o d e p t h s on t h e ground. T o i n v e s t i - gate t h i s , a pilot s u r v e y w a s c a r r i e d out d u r i n g t h e w i n t e r of 1956- 57 t o a s s e s s t h e method and a p p a r a t u s p r o p o s e d f o r t h e f u l l s u r v e y ;
in 1957-58 t h e s u r v e y p r o p e r was begun. D e n s i t y a s well a s depth
w a s m e a s u r e d , and in o r d e r t o d e t e r m i n e t h e n a t u r e of t h e snow
l o a d s , a gauge n u ~ n b e r i n g s y s t e m w a s d e v i s e d w h e r e b y t h e f o r m a t i o n
of unbalanced loading p a t t e r n s could b e t r a c e d . A full d e s c r i p t i o n of t h e background and r e s u l t s of t h e s u r v e y a r e p r e s e n t e d in s e v e n DBR I n t e r n a l R e p o r t s ( R e f s 1 t o 7 ) .
On t h e b a s i s of i n f o r m a t i o n g a t h e r e d in t h e y e a r s p r e c e d i n g 1960, s e v e r a l r e v i s i o n s w e r e m a d e in t h e 1960 edition of t h e Code. P r o m i n e n t a m o n g t h e r e v i s i o n s r ecomrnerided b y t h e A s s o c i a t e C o m m i t t e e on t h e National Building Code, i s t h a t t h e ground load b e r e d u c e d by 20 p e r cent t o a r r i v e a t a d e s i g n roof l o a d . W h e r e a s t h e 1953 Code m e r e l y pointed out t h a t non -unifor m l y d i s t r i b u t e d l o a d s conceivably in e x c e s s of t h o s e s p e c i f i e d would b e a.nticipated, t h e 1960 Code s p e c i f i e d " s h a p e f a c t o r s " t o b e u s e d f o r a c c u m u l a t e d l o a d s on c a n o p i e s , m a r q u e e s , p o r c h r o o f s , s h e l t e r e d roof p o r t i o n s and p a r t i c u l a r roof s h a p e s .
W h e r e r e f e r e n c e t o PIBC d e s i g n l o a d s h a s b e e n m a d e i n t h i s r e p o r t t h e c o r r e s p o n d i n g f i g u r e s a r e t h o s e s p e c i f i e d in t h e
Clilrlatc I n f o r t n a t i o n Handbook of t h e NDC 1960 ( 8 ) u n l e s s
o t l ~ e r w i s e i n d i c a t e d . T h e 1960 v a l u e s a r e b a s e d o n a l o n g e r s e r i e s
of r e c o r d s and a l o n g e r d e s i g n p e r i o d ( 3 0 y e a r s ) t h a n w e r e t h e
1953 l o a d s , and t h u s t h e new ~ l ~ a x i m u r n ground l o a d f i g u r e s a r e
often s l i g l ~ t l y l a r g e r . T h e d e s i g n r o o f l o a d s of t h e 1960 Code
a r e 8 0 p e r c e n t of t h e ground load and s h o w both a n i n c r e a s e and d e c r e a s e c o m p a r e d with t h e 1953 l o a d s . 2. SUMMARY O F 1960-61 OBSERVATIONS
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Although t h e s n o w f a l l w a s b e l o w n o r m a l i n m o s t p a r t s of C a n a d a , t h e r e s u l t s of t h e s u r v e y , with r e s p e c t t o roof l o a d t o ground l o a d r a t i o s and p o s i t i o n s of d r i f t a c c u m u l a t i o n s , w e r e u s e f u l and c o n f i r m e d p r e v i o u s r e s u l t s . A few r o o f s h a d m a x i m u m a v e r a g e * r o o f l o a d s e x c e e d i n g t h e c o r r e s p o n d i n g g r o u n d l o a d s . A s i n p r e v i o u s y e a r s , c o n c e n t r a t e d l o a d s w e r e q u i t e h i g h and on s e v e r a l r o o f s t h e s e c o n c e n t r a t i o n s e x c e e d e d t h e b a s i c d e s i g n l o a d . On t h e a v e r a g e , roof l o a d s w e r e c o n s i d e r a b l y l e s s t h a n g r o u n d l o a d s , with m a n y r o o f s c o v e r e d with n e g l i g i b l e a m o u n t s of s n o w . C o n c e n t r a t i o n s o c c u r r e d w h e r e e x p e c t e d b u t t h e s i z e of t h e s e w a s e x t r e m e l y v a r i a b l e . Wind, c o m b i n e d with s h e l t e r , s h a p e and o r i e n t a t i o n of t h e r o o f a g a i n p r o v e d t o h a v e t h e g r e a t e s t e f f e c t on s n o w l o a d s . High t e m p e r a t u r e s r e d u c e d r o o f l o a d s r a p i d l y w h i l e s o l a r r a d i a t i o n d u r i n g cold w e a t h e r w a s a r e l a t i v e l y m i n o r f a c t o r i n s n o w load r e d u c t i o n . DESCRIPTION O F T H E 1960 -61 OBSERVATIONS T h e p r o c e d u r e u s e d t o c o l l e c t i n f o r m a t i o n w a s s i m i l a r t o t h a t u s e d in p r e v i o u s w i n t e r s . T h e s u r v e y ( 2 ) m a d e u s e of t h r e e t y p e s of o b s e r v a t i o n s t a t i o n s w h e r e i n e a s u r e m e n t s w e r e recortlc:d a s f o l l o w s : A -Station o b s e r v e r s c o n t i n u e d t o m a k e d e t a i l e d o b s e r v a t i o n s of s n o w d e p t h and d e n s i t y o n o n e f l a t a n d o n e p i t c h e d r o o f of r e s i d e n t i a l s i z e , a n d o n t h e s u r r o u n d i n g g r o u n d . T h e s e m e a s u r e m e n t s , ma.de w e e k l y and a f t e r e v e r y h e a v y s n o w s t o r m , w e r e c a r r i e d out a s d e s c r i b e d in R e f . ( 7 ) . B - S t a t i o n o b s e r v e r s m a d e p e r i o d i c d e p t h m e a s u r e r n e n t s on o n e r o o f and on t h e s u r r o u n d i n g g r o u n d . A d d i t i o n a l m e a s u r e m e n t s w e r e m a d e on o t h e r r o o f s a t t h e t i m e when s n o w l o a d s w e r e a t a m a x i l ~ ~ u n l*
" m a x i m u m a v e r a g e t ' m e a n s t h e a v e r a g e r o o f l o a d a t t h e t i m e of m a x i ~ n u ~ n s n o w d u r i n g t h e w i n t e r c o n s i d e r e d .for tl) c: winti?-r.
C-Sta.tio11 o b s e r v e r s m a d e t h e equivalent of A-Station. o b s e r -
-
v a t i o n s on l a r g e fl;it r o o f s l o c a t e d a t K C A F b a s e s a c r o s s Canada..
E a c h s t a t i o n o b s c r v c t l two o r m o r e of t h e f o u r s t a n t l a r d C -Station
r o o f s , t h e f o ~ i r b e i n g t h e Unit Supply Depot r o o f , t h e c a n t i l e v e r
h a n g a r r o o f , t h e l r ~ e c h a i l i c a l e q u i p m e n t g a r a g e roof and t h e wings
o f t h e a r c h h a n g a r r o o f .
O b s e r v a t i o n r e p o r t s f o r t h e 1960-61 w i n t e r w e r e
r e c e i v e d f r o m 44 s t a t i o n s ; 7 of t h e s e r e p o r t e d t h a t i n e a s u r e i r ~ e n t s
w e r e not talccn tlue t o i n s u f f i c i e n t snowfall. F i f t e e n A S t a t i o n s
r e p o r t e d o b s e r v a t i o n s on 41 r o o f s , 1 5 B S t a t i o n s r e p o r t e d on 19 r o o f s , and 7 C S t a t i o n s r e p o r t e d on 20 r o o f s , f o r a t o t a l of 80 r o o f s . T h e l o c a t i o n s of a l l S t a t i o n s w h e r e r e a d i n g s w e r e t a k e n a r e given i n F i g s . 1 and 2 . WEATHER T h e w i n t e r of 1960-61 f e a t u r e d m a n y d e v i a t i o n s f r o i n n o r m a l w e a t h e r conditions. In B r i t i s h C o l u m b i a p r e c i p i t a t i o n w a s a b o v e n o r m a l , but b e c a u s e of h i g h t e m p e r a t u r e s m o s t of t h i s o c c u r r e d a s r a i n . In t h e P r a i r i e P r o v i r l c e s s n o w f a l l w a s g e n e r a l l y below n o r m a l although F o r t C h u r c h i l l e x p e r i e n c e d a snowfall m o r e t h a n 100 p e r c e n t a b o v e n o r m a l . T e m p e r a t u r e s w e r e 4 t o 1 2 d e g r e e s above
n o r m a l . In O n t a r i o and Q u e b e c snowfall was a s rnuch a s 4 5 p e r c e n t
below n o r m a l , with e x t r e m e l y l i t t l e snow d u r i n g t h e m o n t h s of
J a n u a r y and F e b r u a r y . T e m p e r a t u r e s w e r e a b o v e n o r rnal in N o v e m b e r , F e b r u a r y and M a r c h , and below n o r i n a l in t h e o t h e r w i n t e r imonths.
In t h e M a r i t i m e s snowfall w a s g e n e r a l l y a b o v e n o r m a l and t e m p e r
--
a t u r e s below n o r m a l . At Gander snowfall w a s 40 p e r c e n t above
n o r m a l while G o o s e Bay r e p o r t e d 40 p e r c e n t below n o r m a l . T h r o u g h -
out t h e c o u n t r y , wind, t h e f a c t o r with t h e g r e a t e s t influence on snow l o a d s , did not show a n y s i g n i f i c a n t d e p a r t u r e f r o m n o r m a l .
5. PRESENTATION O F R E S U L T S
A s in p r e v i o u s p r o g r e s s r e p o r t s , A - S t a t i o n r e s u l t s a r e given in d e t a i l f o r e a c h roof by g r a p h s in which t h e a v e r a g e ground l o a d , t h e m a x i r n u m , a v e r a g e and m i n i m u m roof l o a d s , and in s o m e
c a s e s , t h e m a x i ~ n u l n load on a canopy o r l e a n - t o roof a r e p l o t t e d
a g a i n s t t i m e . With t h e s e g r a p h s a r e f u r t h e r g r a p h i c a l r e c o r d s of wind, t e m p e r a t u r e , and a c c u m u l a t e d snowfall a s w e l l a s d r a w i n z s giving
t h e gauge p o s i t i o n s and b a s i c d i m e n s i o n s of e a c h r o o f . T h e s e d e t a i l e d
r e s u l t s a r e given in F i g s . 3 t o 17 i n c l u s i v e , and a r e suillrrlarized in
13-Station r < ~ s t i l t s arc. l ~ r e s e n t e c l in t h e s a m e m a n n e r a s
f o r t h e p r e v i o u s two w i n t e r s . F i g u r e L ;<ivcs t h e B - S t a t i o n
l o c a t i o n s a s well a s t h e " s t i c k " g r a p h s of r n a x i i n u ~ n a v e r a g e roof
a n d ground l o a d s f o r t h e s e l o c a t i o n s , w h i l c T a b l e I 1 p r e s e n t s a
t a b u l a r stllnrnar y in which r o o f cleLails and loacling i n i l u e n c e s a r e noted.
With t h e e x c e p t i o n of t h e g r a p h i c a l sulmrnar y, C -Station r e s u l t s a r e p r e s e n t e d in t h e s a m e f o r m a s t h o s e of A - S t a t i o n s . T h e l o a d - t i m e c u r v e s f o r e a c h r o o f a r e given in F i g s . 18 t o 2 2 i ~ ~ c l u s i v e and t h e t a b u l a r s u m m a r y i n T a b l e I 1 I. One change h a s b e e n m a d e in t h e p l o t t e d m e t e o r o l o g i c a l d a t a t h a t a c c o m p a n y t h e l o a d - t i m e g r a p h s f o r A and C S t a t i o n s . R e p l a c i n g t h e s u n s h i n e g r a p h u s e d i n p r e v i o u s r e p o r t s i s a new p l o t showing a c c u n ~ u l a t e d s n o w f a l l i n i n c h e s a g a i n s t t i m e i n d a y s . B e c a u s e s o l a r r a d i a t i o n w a s found t o b e a r e l a t i v e l y m i n o r f a c t o r in snow load r e d u c t i o n , i t w a s b e l i e v e d t h a t a r e c o r d of a c c u ~ n u l a t i o n of t o t a l f r e s h s n o w f a l l would b e m o r e u s e f u l t h a n t h e s u n s h i n e . W h e r e a s i n p r e v i o u s p r o g r e s s r e p o r t s i t w a s not p o s s i b l e t o c o r r e l a t e roof l o a d s with snowfall d i s t r i b u t i o n d u r i n g t h e w i n t e r , t h e e f f e c t s o f wind and t e i n p e r a t u r e d u r i n g a n y s p e c i f i c s n o w f a l l c a n now b e c l e a r l y s e e n .
It had b e e n o r i g i n a l l y p l a n n e d t o p l o t t h e accun-lulated s n o w f a l l i n pounds p e r s q u a r e foot s o t h a t a d i r e c t c o n l p a r i s o n could b e m a d e with t h e o b s e r v e d ground l o a d . T h i s n e c e s s i t a t e c l t h e u s e
of a n a s s u m e d d e n s i t y f o r f r e s h l y f a l l e n s n o w ; t h e v a l u e 0. 10 g l n / ~ ~
w a s s e l e c t e d .
h
s o m e locatiorls, h o w e v e r , t h e o b s e r v e d g r o u n d loade x c e e d e d t h e load f r o m t h e a c c u m u l a t e d s n o w f a l l s . B e c a u s e t h i s would h a v e led t o c o n t r a d i c t o r y i n f o r m a t i o n on t h e load - t i m e g r a p h s , it w a s d e c i d e d t o show t h e a c c u m u l a t e d s n o w f a l l in irlches of f r e s h
snow. T h i s s h e d s s o m e light on t h e v a l i d i t y of a n a s s u n l e d d e n s i t y of 0 . 10 g m / c c . P o t t e r ( 8 ) i n d i c a t e s t h a t t h e d e n s i t y b a s e d on a s i n g l e
s n o w f a l l m a y often v a r y f r o m 0. 0 2 t o 0 . L 3 , and although t h e a s s u m e d
d e n s i t y of 0. 10 g m / c c is valid f o r long p e r i o d s a n d i s a n a v e r a g e f o r t h e whole of C a n a d a , d e n s i t i e s in t h e M a r i t i ~ n e s e x c e e d t h i s v a l u e and t h o s e in t h e P r a i r i e s a r e l e s s t h a n 0. 10 p n / c c . 6 . R E S U L T S
h
g e n e r a l , t h e r e s u l t s of t h e 1960-61 s u r v e y w e r e n o t u n l i k e t h o s e r e c o r d e d in p r e v i o u s y e a r s . An u n u s u a l f e a t u r e w a s o b s e r v e d at R e v e l s t o k e , a new s t a t i o n , w h e r e t h e m a x i ~ m u m a v e r a g cr o o f load w a s 66 psf while t h e m a x i ~ n u l n a v e r a g e grouncl load was
parly llut i l l A s11elt~1-c~d rc:g,ioll. 111 d i l - C C ~ c o ~ i t r ; i s t l o t h i s w a s t h c b a r c r o o f o b s e r v e c l a t Garlder w h e r e t h e m a x i l n u i n a v e r a g e grouncl load u a s 6 0 p s f . T h e r e w a s w i d e v a r i a t i o n i n t h e r a t i o o f r o o f l o a d t o grouild l o a d , but i n g e n e r a l t h e a v e r a g e r o o f l o a d s w e r e l o w c o m p a r e d with c u r r e n t d e s i g i l v a l u e s . On 8 r o o f s t h e c o n c e n t r a t e d l o a d s w e r e g r e a t e r t h a n t h e b a s i c N B C d e s i g n v a l u e s , b u t t h e u s e of t h e 1560 C o d e s h a p e f a c t o r s s a t i s f i e d 7 of t h e s e c a s e s . T h e r a t i o of t h e a v e r a g e r o o f l o a d t o g r o u n d l o a d v a r i e d f r o m 1 . 40 t o 0 ; f o r a l l o b s e r v e d r o o f s t h e a v e r a g e w a s 0. 4 7 with o n l y 25 p e r c e n t of t h e r o o f s h a v i n g a r a t i o e x c e e d i n g 0 . 75. F o r ."L a n d C S t a t i o n s w l l e r e detailecl m e a s u r e m e n t s w e r e t a k e n , t h e r a t i o w a s 0 . 4 2 ; f o r B S t a t i o n s i t w a s 0. 61. T h e e x t e n t t o w h i c h a r o o f w a s s h e l t e r e d w a s t h e p r e d o r n i - n a n t f a c t o r p r o d u c i n g h i g h r a t i o s . T h e 1960 -61 r e s u l t s a l s o s h o w e d t h a t u n i f o r m l y d i s t r i b u t e d s n o w l o a d s o c c u r o n l y on r o o f s t h a t a r e w e l l s h e l t e r e d , o r i n r e g i o n s w h e r e w i n d s a r e v e r y l i g h t . A s i n p r e v i o u s y e a r s d r i f t s a n d a c c u r n u - l a t i o n s o c c u r r e d a t t h e j u n c t i o n of s p l i t - l e v e l r o o f s , o n p o r c h r o o f s a n d c a n o p i e s , a l o n g p a r a p e t w a l l s , o n t h e l e e w a r d s i d e of p i t c h e d r o o f s , a n d a r o u n d c h i m n e y s a n d v e n t i l a t o r s . T h e m a g n i t u d e of t h e s e l o a d s r e m a i n s v e r y d i f f i c u l t t o p r e d i c t . T h e f o l l o w i n g c o m m e n t s o n i n d i v i d u a l A - S t a t i o n o b s e r v a t i o n s p o i n t o u t s o m e of t h e v a r i a t i o n s i n t h e r o o f l o a d t o g r o u n d l o a d r a t i o s a n d t h e c o n c e n t r a t i o n s m e n t i o n e d a b o v e . ( a ) A S t a t i o n s h u v i k .
-
O b s e r v a t i o n s o n t h e r o o f of t h e E - 3 s c h o o l w e r e c h a n g e d t o t h e H o m e E c o n o l n i c s A n n e x , a b u i l d i n g of t h e s a m e d e s i g n , a f t e r t h e E - 3 s c h o o l w a s t o r n down. T h e E - 3 R e s i d e n c e w a s relocated o n a n e w s i t e d u r i n g t h e w i n t e r . With n o r m a l a n n u a l s n o w f a l l , t h e m a x i m u m a v e r a g e g r o u n d l o a d r o s e t o 28 p s f , w h e r e a s d u r i n g t h C p r e v i o u s t w o w i n t e r s t h e g r o u n d l o a d h a d e x c e e d c d t h e 1 9 5 3 N B C d e s i g n l o a d of 35 p s f . T h e 1960 N B C d e s i g n l o a d i s 37 p s f . L i g h t w i n d s a n d e x t r e m e l y l o w t e m p e r a t u r e s a l l o w e d t h e s n o w t o accumulate with f a i r u n i f o r m i t y , a l t h o u g h t h e r e w e r e m i n o r d r i f t s a l o n g t h e p e a k s of t h e p i t c h e d r o o f s . T h e m a x i l n u m a v e r a g e r o o f l o a d w a s 11 p s f . R e v e l s t o k e .-
O b s e r v a t i o r l s a t R e v e l s t o k e , a n e w s t a t i o n i n t h e i n o u n t a i n o u s r e g i o n of B. C .,
w e r e begun t o c o i m p e n s a t e f o r t h e t e m p o r a r y d i s c o n t i i l u a t i o n of l n e a s u r e m e n t s a t G l a c i e r . T h r e e r o o f s w e r e o b s e r v e d , two i n t h e town of R e v e l s t o i c et h e parapet. wall wcbre rclocatecl t o 4 Et f r o m t h e p a r a p e t w a l l . T h e s e gallges had p r e v i o u s l y b e e n c l o s e r to t h e p a r a p e t s i n d r i f t s s o t h a t t h e c o m p u t e d a v e r a g e l o a d s w c r e l a r g e r t h a n t h e a v e r a g e s
which had a c t u a l l y o c c u r r erl. G a ~ l g e $ / 7 w a s l e f t i n it . o r i g i n a l
p o s i t i o n s i n c e , i n p r e v i o u s y e a r s , t h e r r l a x i r n ~ ~ m d r i f t s o c c u r r e d
c o n s i s t e n t l y a t t h i s l o c a t i o n .
On t h e f l a t roof of McNeill H o u s e t h e rnaxiinuin load of
30 psf o c c ~ ~ r r e d a t t h e intersectioiz of two p a r a p e t w a l l s , while
l e s s e r d r i f t s o c c i i r r e d on t h e l o w e r windward edge of t h e g a b l e r o o f . L i t t l e snow f e l l d u r i n g t h e m o n t h s of J a n u a r y and F e b r u a r y , and t h e roof l o a d s d e c r e a s e d v e r y s l i g h t l y , i n d i c a t i n g t h a t s o l a r r a d i a t i o n i s a m i n o r f a c t o r in snow load r e d u c t i o n . High winds and f r e q u e n t
t h a w s p r e v e n t e d t h e a c c u ~ n u l a t i o n of l o a d s t o a n y a p p r e c i a b l e
extent.
Ottawa..
-
With snowfall 22 p e r c e n t below n o r m a l , t h em a x i m u i n a v e r a g e roof load was only 21 p e r c e n t of t h e NBC d e s i g n load. M a x i m u m c o n c e n t r a t i o n of 21 psf o c c u r r e d a t t h e i n t e r s e c t i o n
of two p a r a p e t w a l l s on t h e of t h e Building R e s e a r c h C e n t r e . T h e
two gable r o o f s r e m a i n e d c o m p l e t e l y b a r e d u r i n g m o s t of t h e w i n t e r . S o l a r r a d i a t i o n did not a p p r e c i a b l y a f f e c t t h e snow l o a d s d u r i n g t h e long p e r i o d without snowfall.
Ste. Anne de B e l l e v u e .
-
Although t h e silowfall was 1 1 p e r c e n tbelow n o r m a l , t h e a v e r a g e ground load r o s e t o 35 p s f , t h e s a m e a s
t h e p r e v i o u s y e a r when t h e snowfall w a s 40 p e r c e n t a b o v e n o r r n a l .
T h a w s d u r i n g t h e w i n t e r of 1959-60 r e d u c e d t h e l o a d s on s e v e r a l o c c a s i o n s . T h e m a x i m u m a v e r a g e roof l o a d s w e r e quite s i n a l l ,
h o w e v e r , with 8 psf on t h e f l a t r o o f and 5 psf on t h e gable r o o f . S u b l i -
m a t i o n did not s i g n i f i c a n t l y r e d u c e r o o f l o a d s . S t r o n g winds ( 5 0
m p h a c c o r d i n g t o m e t e o r o l o g i c a l r e c o r d s ) following t h e long snow -fr e e
p e r i o d f a i l e d t o c l e a r t h e r o o f s , i n d i c a t i n g t h a t d e n s i f i c a t i o n and bonding with t h e roof s u r f a c e had t a k e n p l a c e . A s in p r e v i o u s y e a r s
c o n c e n t r a t i o n s o c c u r r e d on t h e e a s t end of t h e hip r o o f , and on t h e f l a t r o o f i m m e d i a t e l y b e s i d e t h e s l o p i n g p o r t i o n of t h e h i g h e r r o o f . A r v i d a .
-
On t h e l a r g e f l a t roof t h e m a x i m u m a v e r a g e load w a s 8 psf i. e. 35 p e r c e n t of t h e ground l o a d , while t h e m a x i m u m d r i f t o c c u r r i n g next t o t h e h i g h e r p o r t i o n of t h e roof w a s 17 p s f . C o n s i d e r i n g t h e t o t a l snowfall of 106 i n . , t h e s e l o a d s a r e s m a l l i n d i c a t i n g t h e effect of wind on a l a r g e exposed r o o f .Halifax.
-
T h e s n o w f a l l was 23 p e r c e n t a b o v e n o r m a l butt h e m i l d w e a t h e r , a s in p r e v i o u s y e a r s , p r e v e n t e d a n y s u s t a i n e d
a c c u m u l a t i o n of l o a d s . T h e r o o f - t o -ground load r a t i o w a s 0 .
90
whicha c c o i n p a n i e d by winds of 30 r-nph, r c s u l t e d in a 44-psf a c c u m u l a t i o n
( b a s i c NBC t l e s i g t ~ load = 36 p s f ) i n t h e l e e of a 1 4 - f t p e n t h o u s e
while t h e exposecl p o r t i o n of t h e roof w a s swept c o m p l e t e l y b a r e .
Using t h e s h a p e f a c t o r of 1 . 5, t h e d e s i g n load would h a v e been. 54 p s f .
On 4 J a n u a r y , a 1 9 - i n . snowfall with high wind s p e e d s p r o d u c e d
m a x i m u m load of 9 psf o n t h e s a m e r o o f , indic.atirlg t h e e x t r e m e l y v a r i a b l e effect of wind.
G a n d e r .
-
With a snowfall39
p e r c e n t a b o v e n o r m a l , t h em a x i m u m a v e r a g e g r o u n d load r e a c h e d 80 p s f , e x c e e d i n g t h e NBC
ground load by 2 8 p e r c e n t . T h i s high ground load o c c u r r e d on
29 M a r c h when m e a s u r e m e n t s showed 24 i n . of snow with a d e n s i t y
of 0. 65. Although t h e d e n s i t y a p p e a r s e x t r e m e l y high, t h e w e a t h e r
could h a v e b e e n t h e c a u s e . In M a r c h , G a n d e r had 42 in. of snow
( n o r m a l i s 18 in. ) and 6. 2 i n . of r a i n . T h e t o t a l snowfall up u n t i l 29 M a r c h w a s 142 i n . On 22 M a r c h t h e r e w a s a m e a s u r e d depth of
32 i n . , and on 25 M a r c h 5 in. of snow f e l l . T h e s n o w was c o m p a c t e d
u n d e r v e r y s l u s h y c o n d i t i o n s . D e s p i t e t h e h e a v y s n o w f a l l , t h e l a r g e
f l a t roof of t h e
M.
E. R o o m r e t a i n e d v e r y l i t t l e silow, and t h e r o o f -t o - g r o u n d load r a t i o f o r t h e pitched roof w a s o n l y 0. 14. T h e m a x i m u m
a c c u m u l a t i o n o f 34 psf o c c u r r e d n e a r t h e p e a k on t h e l e e w a r d s i d e of
t h e g a b l e r o o f .
Goose B a y .
-
T h e roof load w a s o n l y 1 1 p e r c e n t of t h ed e s i g n l o a d , and t h e roof -to -ground load r a t i o w a s 0 . 1 7 . T h e s e low v a l u e s m a y b e p a r t i a l l y explained by t h e below n o r m a l s n o w f a l l , but i t a p p e a r s t h a t , i n s p i t e of s h e l t e r , t h e high winds d e c r e a s e d t h e roof l o a d s . A f t e r four y e a r s of o b s e r v a t i o n t h e i n a x i m u r n a v e r a g e roof load h a s n e v e r e x c e e d e d 10 psf on t h e s e A-Station r o o f s , and yet
G o o s e B a y i s i n a n 8 2 - p s f ;.,one a c c o r d i n g t o t h e 1960 National Building
Gode. In c o m p a r i s o n t o t h e s e A-Station roof loacls, t h e roof l o a d s a t
t h e G o o s e B a y C Station w e r e q u i t e high. T h i s i n d i c a t e s t h a t a s m a l l
n u m b e r of roof o b s e r v a t i o n s m u s t be a n a l y z e d with c a u t i o n i n o r d e r t o d e t e r m i n e " r e p r e s e n t a t i v e " l o a d s f o r a n y s p e c i f i c g e o g r a p h i c a r e a .
O t h e r A S t a t i o n s .
-
L o a d s o b s e r v e d at A S t a t i o n s not m e n t i o n e da b o v e showed t h a t a v e r a g e roof loacls a r e u s u a l l y l e s s t h a n t h e c o r r e s p o n d i n g ground l o a d . No u n u s u a l f e a t u r e s w e r e o b s e r v e d ; t h e r e s u l t s given in t h e g r a p h s ( F i g s . 3 to 17) a r e t h e r e f o r e self
-
e x p l a n a t o r y . ( b ) B S t a t i o n s A s in p r e v i o u s y e a r s , t h e i n f o r m a t i o n r e c e i v e d f r o m B S t a t i o n s s u b s t a n t i a t e d t h a t roof l o a d s a r e g e n e r a l l y l e s s t h a n ground l o a d s and t h a t a c c u m u l a . t i o n s a r e found on t h e l o w e r l e v e l s of s p l i t - l e v e lr o o f s , a l o n g p a r a p e t w a l l s , on t h e l e e w a r d s l o p e s of pitched r o o f s ,
and on m a r q u e e s and c a n o p i e s . S t r o n g winds and m i l d w e a t h e r
w e r e a g a i n t h e m a i n f a c t o r s in snow load r e d u c t i o n .
Two of t h e l a r g e s t l o a d s for E S t a t i o n s w e r e o b s e r v e d
a t A m h e r s t , N. S. and S a r n i a , Ont. At A l n h e r s t a 4 - f t ( 5 0
t r i a n g u l a r d r i f t o c c u r r e d a t t h e s p l i t l e v e l of two f l a t r o o f s . T h e top roof w a s f r e e of snow. At S a r n i a a n a v e r a g e snow depth of 3 f t
(30 p s f ) o c c u r r e d on a f i r s t - s t o r y r o o f which w a s s h e l t e r e d on t h r e e
s i d e s b y t h e second s t o r y of t h e s a m e building. T h e b a s i c d e s i g n
snow load for S a r n i a i s 24 p s f ; by applying t h e s h a p e fa.ctor of 1 . 5 , h o w e v e r , t h e d e s i g n load would h a v e b c e n 36 p s f .
It h a s b e e n decided t o modify t h e c h a r a c t e r of t h e I3 S t a t i o n s .
During t h e 1961 -62 winter t h e B Station s u r v e y will b e extended,
and a p p r o x i m a t e l y 450 buil-ding i n s p e c t o r s will b e a s k e d t o r e c o r d
e x t r e m e and u n u s u a l s ~ l o w load conditions on r o o f s . T h e r e q u e s t
f o r r e g u l a r weekly o b s e r v a t i o n s , h o w e v e r , will b e d r o p p e d . T h i s i n f o r m a t i o n will b e u s e d t o c o m p i l e a " c a s e r e c o r d " book which e v e n t u a l l y should o f f e r s t a t i s t i c a l e v i d e n c e t o t h e d e s i g n e r i n d i c a t i n g t h e t y p e and m a g n i t u d e of loading t h a t c a n be a n t i c i p a t e d on a wide v a r i e t y of r o o f s .
( c ) C S t a t i o n s
Goose B a y .
-
D e s p i t e a s ~ l o w f a l l 39 p e r c e n t below n o r m a l ,s o m e h i g h a c c u m u l a t i o n s w e r e o b s e r v e d . A s in p r e v i o u s y e a r s t h e l e a n - t o r o o f s of t h e Unit Supply Depot e x p e r i e n c e d heavy l o a d s ;
t h e m a x i m u m of 7 4 psf w a s 3; t i m e s l a r g e r t h a n t h e m a x i m u r n
d r i f t o n t h e m a i n r o o f . Heavy d r i f t i n g o c c u r r e d n e x t it0 t h e e l e v a t e d
s e c t i o n of t h e M.
E .
G a r a g e ; t h e maxil-num load o i 90 psf e x c e e d e dt h e b a s i c NBC d e s i g n load of 8 2 p s i but mias l e s s t h a n 1 2 3 psf which
would b e t h e d e s i g n load f o r t h i s s e c t i o n of roof. T h i s a c c u l n u l a t i o n
built up f r o m a load of 1 8 psf d u r i n g a p e r i o d of m i n i m u m snowfall, i l l u s t r a t e d t h e e x t r e m e i l n p o r t a n c e of wind i n snow load a n a l y s i s .
With a m a x i m u m a v e r a g e ground load of 77 p s f , t h e g r e a t e s t r o o f - t o -
ground load r a t i o w a s 0. 38.
Other C S t a t i o n s .
-
All a v e r a g e roof l o a d s w e r e l e s s t h a nt h e c o r r e s p o n d i n g ground l o a d s , although i n s e v e r a l c a s e s c o n c e n t r a t e d l o a d s exceeded t h e ground l o a d s . A s in p r e v i o u s y e a r s d r i f t s o c c u r r e d along t h e p a r a p e t w a l l s , on t h e c a n o p i e s of t h e Unit Supply Depot, on
t h e f l a t wings of t h e a r c h h a n g a r s , i ~ n n l e d i a t e l y b e s i d e t h e e l e v a t e d
or ti on
(5. 5-ft d i f f e r e n c e in h e i g h t ) of t h e M e c h a n i c a l E q u i p m e n t G a r a g e , and n e a r t h e door h o u s i n g s on t h e c a n t i l e v e r h a n g a r r o o f s . L a r g e flat r o o f s had low a v e r a g e l o a d s .7 . GENERAL DISCUSSION
-
A f t c r one winter of pilot o b s e r v a t i o r l s and f o u r w i n t e r s of full o b s e r v a t i o n s t h e s u r v e y h a s p r o v i d e d c o n s i d e r a b l e i n f o r m a t i o n of a wide a n d v a r y i n g n a t u r e on t h e s u b j e c t of snow loacls on r o o f s . Although g e n e r a l t r e n d s llave been c l e a r l y i n d i c a t e d , i t i s s t i l l
difficult t o p r e d i c t with c e r t a i n t y t h e roof loacls t h a t could b e expected u n d e r a n y given s e t of c i r c u m s t a n c e s . It i s now p o s s i b l e t o s t a t e t h a t
roof l o a d s a r e g e n e r a l l y c o ~ ~ s i d e r a b l y l e s s t h a n ground l o a d s , and
t h e a s s u r n p t i o n of a uniforlnly d i s t r i b u t e d snow load i s u n n e c e s s a r i l y
c r u d e .
T h e weight of snow accumulating on any given roof s u r f a c e
depends on m a n y f a c t o r s . T h e s e h a v e been roughly divided into
two g r o u p s
-
t h e m e t e o r o l o g i c a l f a c t o r s o v e r which t h e d e s i g n e rh a s no c o n t r o l , and t h e roof f a c t o r s which c o m e to a l a r g e extent
under t h e d e s i g n e r ' s " j u r i s d i c t i o n .
"
T h e r e i s a l a r g e d e g r e e ofi n t e r a c t i o n between t h e s e f a c t o r s and any a n a l y s i s f o r t h e p u r p o s e of r e v i s i n g s p e c i f i c a t i o n s m u s t n e c e s s a r i l y involve a s t u d y of m a n y c o m b i n a t i o n s . B e c a u s e of t h e v a r i a b i l i t y of t h e f a c t o r s c o n c e r n e d , o r i g i n a l h o p e s f o r quantitative a n s w e r s f r o m t h e f i r s t f e w y e a r s of t h e s u r v e y h a v e b e e n d i m m e d ; i t now a p p e a r s t h a t f u t u r e r e f i n e m e n t s of d e s i g n load s p e c i f i c a t i o n s should b e b a s e d on t h e s t a t i s t i c a l s t r e n g t h of m a n y o b s e r v a t i o n s . It i s b e c o m i n g i n c r e a s i r i g l y c l e a r t h a t code s p e c i f i c a t i o n s will n e v e r b e a b l e to c o v e r the m a n y v a r i a t i o n s i n roof s h a p e s and i n o t h e r conditions t h a t will o c c u r in p r a c t i c e s o t h a t t h e code will h a v e t o b e s u p p l e m e n t e d by a c o l l e c t i o n of " c a s e r e c o r d s " f o r s p e c i a l silow load p r o b l e m s . T h e s e " c a s e r e c o r d s 'I, now being g a t h e r e d , will d e a l with e x t r e m e and unusual snow -load conditions on a l a r g e and v a r i e d s e l e c t i o n of r o o f s . E v e n t u a l l y t h e y should o f f e r t h e d e s i g n e r r e c o r d s of s o m e c o m p a r a b l e roof and s t a t i s t i c a l s t r e n g t h t o c o n f i r m t h i s i n f o r m a t i o n . T h e following d i s c u s s i o n p r e s e n t s s o m e of t h e m o r e s a l i e n t f e a t u r e s of t h e snow load p r o b l e m . T h e r e s u l t s of a n a t t e m p t to a n a l y z e t h e o b s e r v a t i o n s to d a t e a r e a l s o shown. M e t e o r o l o g i c a l F a c t o r s ( a ) S o l a r R a d i a t i o n .
-
During t h e winter of 1960-6
1 s e v e r a l .s t a t i o n s a c r o s s Cailacla e x p e r i e n c e d long p e r i o d s d u r i n g which no snow f e l l and r o o f l o a d s w e r e not r e d u c e d t o a n y a p p r e c i a b l e e x t e n t . C o m p a r e d t o t h e e f f e c t s of s t r o n g winds and high t e m p e r a t u r e s , s o l a r r a d i a t i o n
( b ) --- D e n s i t y .
-
Stlow rltxnsity ~ i g ~ + ~ n showed wide v a r i a t i o n f r o m l o c a t i o n t o l o c a t i o n a n d f r o m ~ , > i ) n ~ l i tn m o n t h . T h e g e n e r a l t r e n d i ~ ~ d i c a t e c l in R e f . (41, h o w e v e r , I l ~ l ~ l s t r u e f o r t h e m o s t p a r t ( c ) Wind a n d T e r n p e r a t u r e . - T h e s e t w o m e t e o r o l o g i c a l f a c t o r s p l a y a l e a d i n g r o l e i n t h e a n a l y s i s of s n o w loacls. A s in p r e v i o u s y e a r s high t e m p e r a t u r e s i n v a . r i a b l y r e d u c e d snow l o a d s , while s t r o n g w i n d s recluced o r i n c r e a s e d t h e m . T h e q u a n t i t a t i v e e f f e c t s of wind r e m a i n e x t r e m e l y v a r i a b l e and difficult t o p r e d i c t ,s o t h a t , f o r d e s i g n p u r p o s e s , a wide r a n g e of p o s s i b l e e f f e c t s
( d e c r e a s e s and i n c r e a s e s ) h a v e t o b e t a k e n i n t o a c c o u n t . S h a p e F a c t o r s
( a ) Snow L o a d s o n L a r g e F l a t R o o f s .
-
A s b e f o r e xnaximuma v e r a g e roof l o a d s w e r e u s u a l l y c o n s i d e r a b l y l e s s t h a n t h e c o r r e s - ponding ground l o a d s . T h e 1960-61 r e s u l t s a r e shown in T a b l e IV. In o r d e r t o f i n d o u t j u s t how m u c h t h e d i f f e r e n c e w a s , a s t u d y of t h e l a r g e f l a t r o o f s o b s e r v e d a t C S t a t i o n s f o r t h e p a s t f o u r y e a r s w a s c a r r i e d out. It w a s found t h a t t h e r o o f - t o - g r o u n d load r a t i o ,
on t h e a v e r a g e , w a s 0. 40, a n d t h a t
9
o u t of 10 r o o f s h a d a r a t i o l e s s t h a n 0 . 7 5 . A l l a v e r a g e r o o f l o a d s w e r e w e l l below t h e s p e c i f i e d NBC l o a d , i n d i c a t i n g t h a t f o r l a r g e f l a t r o o f s d e s i g n l o a d s m a y b e r e d u c e d f u r t h e r t h a n a s s p e c i f i e d i n t h e 1960 C o d e . D r i f t s in r e g i o n s of l o c a l i z e d s h e l t e r w e r e o f t e n iu e x c e s s of NBC d e s i g n l o a d s , but t h e s e could b e d e a l t with by t h e u s e of a p p r o p r i a t e " s h a p e f a c t o r s . " ( b ) D r i f t s a t P a r a p e t W a l l s . - F o r t h e p a s t f o u r y e a r s o b s e r v a t i o n s i n d i c a t e t h a t t h e i n c r e a s e in t h e h e i g h t of t h e p a r a p e t w a l l i s d i r e c t l y p r o p o r t i o n a l t o t h a t of t h e r a t i o of t h e m a x i m u m c o n c e n t r a t i o n a l o n g t h a t w a l l t o t h e n ~ a x i l ~ ~ u x n a v e r a g e r o o f l o a d . T o e s t a b l i s h a b a s i s f o r c o r n p a r i s o n of p a r a p e t wall d r i f t s t h e g r a p h shown i n F i g . 24 w a s plottec-I. O b s e r v a t i o n s o n a l l r o o f s with p a r a p e tw a l l s at A and I3 S t a t i o n s f o r t h e p a s t f o u r y e a r s w e r e u s e d . TJle p o i n t s o n t h i s g r a p h a r e s c a t t e r e d t.o s u c h a n e x t e n t t h a t no d e f i n i t e q u a n t i t a t i v e c o n c l u s i o n s c a n b e d r a w n . O n l y a l i m i t e d n u m b e r of r o o f s with p a r a p e t w a l l s h a v e b e e n o b s e r v e d t o d a t e ; it i s t h e r e f o r e not p o s s i b l e a s y e t t o s u b d i v i d e t h e g r o u p a n y f u r t h e r b y t a k i n g i n t o a c c o u n t s u c h f a c t o r s a s l o c a t i o n , s h e l t e r and r o o f s i z e . T h i s would i n d i c a t e t h e n e e d of l o c a l c o n c e n t r a t e d o b s e r v a t i o n s . ( c ) D r i f t s o n t h e L o w e r P o r t i o n of S p l i t - L e v e l R o o f s .
-
A s t u d y of d r i f t s o n t h e l o w e r p o r t i o n s of s p l i t - l e v e l r o o f s w a s m a d e f r o m t h e o b s e r v a t i o n s t a k e n o v e r t h e p a s t f o u r y e a r s . 'The c o r r e - l a t i o n b e t w e e n t h e maxirnur-(1 d r i f t l o a d and t h e clifference i n e l e v a t i o nof r o o f p o r t i o i l s i s too v a r i e d t o give a n y d e f i n i t e r e l a t i o n s h i p (Fig,. 2 5 ) .
L to 3 ti111cs the, n v c r ; i G c rovJ' Io,~tl ;111cl tlliit thc, s i x c o f the- d r i f t s
clocs not i l ~ c i - e ; ~ s c ~ 1jt:yoncl ;I cliffcr c n c c i r l c l (:vat i o 1 1 of aljoklt
6
f t .( d ) Roof S l o p e . -
-
Tllc 1960 NCltio~x;~l 13uilclillg; Coclcr e c o g ~ ~ i x e s roof s l o p e a s a f a c t o r which r p t l u c e s tllc lclad on p i t c h e d
1-oofs fronx t h a t which wollltl o c c u r 011 a f l a t r o o f . W h e r c a s t h e 1953
C o d e s p e c i f i e d t h a t r c d u c t i o n s could b c i n a d c OII s l o p e s o v e r 2 0 ° , t h e
1960 Cocle s t a t e s t h a t r c d u c t i o n s stlould not be xliacle i l n l e s s t h e s l o p e i s g r e a t e r t h a n 3 0 " . Bnsetl on r e s u l t s i r o n 1 t h e s u r v e y of tlie p a s t f o u r w i n t e r s , a s t u d y of t h i s f a c t o r h a s bee11 c a r r i e d o u t . T h e
f i n d i n g s a r e shown in F i g . 2 6 , w h e r e t h e r a t i o of t h e rnaxinzum a v e r a g e
load on t h e e n t i r e s u r f a c e of t h e pitchecl roof t o t h e m a x i n ~ u ~ n a v e r a g e
l o a d on a f l a t roof in t h e s a l n e g c o g r a p l l i c a r e a i s p l o t t e d a g a i n s t t h e s l o p e of t h e p i t c h e d r o o f . It w i l l I]? notcd t h a t in s e v e r a l c a s e s t h e l o a d s on p i t c h e d r o o f s exceetlecl t h o s e o n f l a t r o o f s a t t h e s a m e l o c a t i o n . T h r e e r e a s o n s f o r t h i s a p p a r e n t a n o l n a l y a r e o f f e r e d . F i r s t , s o m e of t h e p i t c h e d r o o f s w e r e s h e l t e r e d wllile t h e c o l ~ l p a r a t i v e f l a t r o o f s w e r e not. Second, a f e w of t h e r e s u l t s w e r e t a k e n f r o m r o o f s d u r i n g w i n t e r s of r e l a t i v e l y light s n o w f a l l ancl s o s o m e of t h e r a t i o s m a y not b e r e p r e s e n t a t i v e . T h e t h i r d r e a s o n m a y b e a t t r i b u t e d
t o wind a c t i o n . Although wind will t e n d t o blow snow off a f l a t roof c o n v e r s e l y it w i l l t e n d to p i l e it up on t h e l e e w a r d s i d e of a p i t c h e d r o o f . T h e p e a k in i t s e l f t e n d s t o p r o v i d e l o c a l i z e d s h e l t e r which i s c o n d u c i v e t o s n o w a c c u m u l a t i o n . T h e r e i s s o m e i n d i c a t i o n t h a t desigii l o a d s f o r r o o f s of ~ n e d i u ~ r l p i t c h sllould be g r e a t e r t h a n t h o s e f o r f l a t r o o f s . Although s o n ~ e r o o f s with l e s s t h a n 3 0 " s l o p e h a v e r e d u c e d l o a d s , o t h e r s h a v e i n c r e a s e d l o a d s , i n d i c a t i n g t h a t r e d u c t i o n s on p i t c h e d r o o f s should not b e nlaclc u n l e s s t h c roof s l o p e i s g r e a t e r t h a n 3 0 ° , t h u s substarltiatitxg t h e s p e c i f i c a t i o n s in t h e 1960 C o d e ( F i g . 2 6 ) . ( e ) C o n c e n t r a t e d Lloads o n P i t c h e d R o o f s .
-
A s t u d y of s u r v e y r e s u l t s t o d a t e r e v e a l e d t h a t pitchecl r o o i s a r e s e l d o m u n i f o r m l y l o a d e d , b u t t h a t in t h e l n a j o r i t y of c a s e s c o n c e n t r a t i o n s o c c u r a l o n g t h e r i d g e . T h e a v e r a g e r a t i o of ~ n a x i ~ n u m roof load t o a v e r a g e r o o f load w a s c a l c u l a t e d t o b e 2 . 1 , but it r n u s t b e r e a l i ~ ~ e d t h a t t h i s f i g u r e h a s b e e n a r r i v e d a t without r e g a r d t o r o o f s l o p e , m a g n i t u d e of t h e r o o f l o a d , and t h e difficult a s p e c t of s h e l t e r . R o o f s w e r e n o t s e g r e g a t e d on a b a s i s of t h e i r clegree of s h e l t e r , a s s h e l t e r d e p e n d s t o a l a r g e e x t e n t on t h e d i r e c t i o n of t h e wind, which i s e x t r e m e l y v a r i a b l e . T h e d e g r e e of s h e l t e r , t h e r e f o r e , c a n n o t a l w a y s b e s p e c i f i e d with a n y r e a s o n a b l e a c c u r a c y . ( f ) Unbalanced --L o a d s on P i t c l l e d R o o f s .-
-
T h e 1960 C o d e s t a t e s t h a t s l o p e d o r h i p p e d r o o f s s h a l l b e clcsigned f o r a n u ~ x b a l a n c e dload 1 . 25 t i m e s tlie u n i f o r m Load on o n e s i d e ant1 1x0 loatl on tlxe o t h e r
which, t h e o r e t i c a l l y , co111(1 leacl t o t w i c c t h c clcsi;;11 load on o n e
s i d e and z e r o on t h e o t h c r . Snow i s tilso a p t t o s l i d e o r m e l t off
o n e roof s u r f a c e h e f o r e it d o c s on t h e o t h e r . Unbalanced l o a d s
c a u s e d by d r i f t i n g h a v e 11cc.n o l ~ s e r v e d d u r i n g t h e p a s t w i n t e r s
( F i g . 36) ( 4 ) ; ( F i g s . 2 4 , 25, and 26) ( 5 ) , and although t h e p r e s e n t
s p e c i f i c a t i o n h a s not b c e n d e f i n i t e l y confirmed i t d o e s a p p e a r q u i t e r e a s o n a b l e . 8. INTERIM CONCLUSIONS On t h e whole t h e 1960-61 o b s e r v a t i o n s s u b s t a n t i a t e t h e i n t e r i m c o n c l u s i o n s p r e s e n t e d in p r e v i o u s r e p o r t s . T h e c o n c l u s i o n s c a n now b e r e s t a t e d with m o r e c e r t a i n t y : ( a ) T h e a v e r a g e snow l o a d s on r o o f s a r e s u b s t a n t i a l l y l e s s t h a n t h e load o n t h e s u r r o u n d i n g g r o u n d .
( b ) T h e a m o u n t by which t h e snow load on t h e roof a n d
t h e load o n t h e ground differ d e p e l ~ d s p r . i m a r i l y on
t h e d e g r e e t o which t h e roof i s s h e l t e r e d . ( T h e e f f e c t s of s h e l t e r a r e , h o w e v e r , e x t r e l n e l y difficult t o p r e d i c t q u a n t i t a t i v e l y . ) (c) C o n c e n t r a t i o n s ( d r i f t s ) s o m e t i m e s e x c e e d i n g t h e l o a d on t h e g r o u n d a r e t o b e e x p e c t e d on r o o f s i n a n y a r e a of l o c a l i z e d s h e l t e r , t h e m a g n i t u d e b e i n g d e p e n d e n t on, a m o n g o t h e r t h i n g s , s n o w f a l l and t h e s i z e of t h e roof a r e a which i s " c o n t r i b u t a r y . " A s t h e r e s u l t of a n a t t e m p t t o a n a l y z e t h e s u r v e y i n f o r m a t i o n t o d a t e , s e v e r a l a d d i t i o n a l p o i n t s h a v e b e e n i n d i c a t e d : ( i ) B e c a u s e of t h e c o m p l e x i t y and v a r i a b i l i t y of f a c t o r s i n t h e s n o w load p r o b l e m i t a p p e a r s t h a t a t h e o r e t i c a l a p p r o a c h i s i m p o s s i b l e . ( i i ) "Shape f a c t o r s " in a l l p r o b a b i l i t y w i l l h a v e t o b e a r r i v e d a t e m p i r i c a l l y . I n d i c a t i o n s a r e t h a t a c o n c e n
-
t r a t e d l o c a l i z e d s u r v e y of t h e p r o b l e m s of u n b a l a n c e d l o a d i n g on p i t c h e d r o o f s , p a r a p e t w a l l d r i f t s , d r i f t s on s p l i t - l e v e l h o u s e s , and snow s l i d e - o f f should be c a r r i e d o u t . T h i s woultl not o n l y give m o r e b r e a d t h t o t h e p r e s e n t s t u d y , but a l s o would yield d a t a in a m u c h( i i i ) Building c o d e s will not b e a b l e t o cope with a l l t h e p o s s i b l e roof s h a p e s . T h i s i n d i c a t e s t h e i m p o r t a n c e of a " c a s e r e c o r d " book showing e x t r e m e a n d u n u s u a l snow load conditions on a wide v a r i e t y of r o o f s .
R E F E R E N C E S
A l l e n , D . E . Snow L o a d s on R o o f s . T h e p r e s e n t r e q u i r e m e n t s and a p r o p o s a l f o r a s u r v e y of snow l o a d s on r o o f s . National R e s e a r c h Council, Division of Building R e s e a r c h , I n t e r n a l R e p o r t No. 106, Sept. 1956. 19p.
A l l e n , D. E . Snow L o a d s on R o o f s , 1956-57
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A p r o g r e s s r e p o r t . National R e s e a r c h Council, Division of Building R e s e a r c h ; I n t e r n a l R e p o r t No. 134, J a n . 1958. 7p.A l l e n , D. E . and C. J . T u r k s t r a . Snow L o a d s on R o o f s , 1957-58
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Second p r o g r e s s r e p o r t . National R e s e a r c h C o u n c i l , Division of Building R e s e a r c h , I n t e r n a l R e p o r t No. 163, Nov. 1958. 6 ~ .
T h o r b u r n , H. J . and B. G. W. P e t e r . Snow L o a d s on R o o f s ,
1958 -59
-
T h i r d p r o g r e s s r e p o r t . National R e s e a r c h Council, Division of Building R e s e a r c h , I n t e r n a l R e p o r t No. 184,Nov. 1959. 16p.
P e t e r , B. G. W. and H. J . T h o r b u r n . Snow Load O b s e r v a t i o n s at
B S t a t i o n s , 1958 -59. National R e s e a r c h Council, Division of Building R e s e a r c h , I n t e r n a l R e p o r t No. 189, D e c . 1959. 13.p.
Watt, W . E . and
H.
J . T h o r b u r n . Snow Loads o n R o o f s , 1959-60-
F o u r t h p r o g r e s s r e p o r t . National R e s e a r c h Council, Division of Building R e s e a r c h , I n t e r n a l R e p o r t No. 204, Nov. 1960. l o p .
Allen, D. E . F i e l d I n s t r u c t i o n s f o r O b s e r v e r s a t A S t a t i o n s of Snow L o a d s o n R o o f s . National R e s e a r c h Council, Division of Building R e s e a r c h , T e c h n i c a l Note No. 2 3 3 , O c t . 1957. 1 l p .
C l i m a t e I n f o r m a t i o n Handbook. Supplement No. 1 t o t h e National Building Code 1960. National R e s e a r c h Council, A s s,ociate C o m m i t t e e on t h e National Building Code. T o b e p u b l i s h e d i n J a n u a r y 1962.
P o t t e r , J . G. Density of F r e s h l y F a l l e n Snow. P r o c e e d i n g s of t h e E a s t e r n Snow C o n f e r e n c e , 1959 -60. p . 141
-
147.Station --- I t ~ u v i k , N . W . T . R e v e l s t o k e , B . C . E d ~ ~ l o n t o n , A l t a . S a s katoon, S a s k. Winnipeg, Man. F o r t C h u r c h i l l , Man. T o r o n t o , Ont. K i n g s t o n , Ont. O t t a w a . Ont. Ste. Anne d e B e l l e v u e . P. 0 . M o n t r e a l , P. 0 . A r v i d a , P . Q. H a l i f a x . N. S. G a n d e r , Nfld. G o o s e Bay. Lab. C a b l e : not h e a t e d , i n s u l a t e d , exposed C a b l e : h e a t e d , i n s u l a t e d . e x p o s e d F l a t : h e a t e d , i n s u l a t e d , e x p o s e d C a b l e : h e a t e d , i n s u l a t e d , e x p o s e d C a b l e : not h e a t e d , s h e l t e r e d F l a t : h e a t e d , i n s u l a t e d , e x p o s e d C a b l e : not h e a t e d , s h e l t e r e d F l a t : h e a t e d , i n s u l a t e d , e x p o s e d C a b l e : h e a t e d , i n s u l a t e d , e x p o s e d C a b l e : h e a t e d , i n s u l a t e d , e x p o s e d F l a t : h e a t e d , i n s u l a t e d , exposed F l a t : h e a t e d , i n s u l a t e d , e x p o s e d F l a t : h e a t e d , v e n t i l a t e d , exposed C a b l e : h e a t e d , i n s u l a t e d , e x p o s e d F l a t : h e a t e d , i n s u l a t e d , e x p o s e d C a b l e : h e a t e d , i n s u l a t e d F l a t : h e a t e d , i n s u l a t e d F l a t : ( c o r r i d o r ) h e a t e d , i n s u l a t e d F l a t : ( c o r r i d o r ) h e a t e d , i n s u l a t e d F l a t : ( c o r r i d o r ) h e a t e d , i n s u l a t e d C a b l e : h e a t e d , i n s u l a t e d F l a t : h e a t e d , i n s u l a t e d , s h e l t e r e d C a b l e : h e a t e d , i n s u l a t e d F l a t : h e a t e d , e x p o s e d . 2 . 5' p a r a p e t C a b l e : h e a t e d , i n s u l a t e d , e x p o s e d
1
C a b l e : h e a t e d , i n s u l a t e d , e x p o s e d 1 F l a t : h e a t e d , e x p o s e d , 2' p a r a p e t F l a t : h e a t e d , i n s u l a t e d , s h e l t e r e d1
F l a t : h e a t e d , i n s u l a t e d , exposed C a b l e : h e a t e d , i n s u l a t e d , exposed F l a t : h e a t e d , i n s u l a t e d , e x p o s e d C a b l e : h e a t e d , i n s u l a t e d , e x p o s e d F l a t : h e a t e d , not i n s u l a t e d , e x p o s e d;
F l a t : h e a t e d , i n s u l a t e d , e x p o s e d C a b l e : h e a t e d , not i n s u l a t e d , s h e l t e r e d F l a t : h e a t e d , i n s u l a t e d , exposed F l a t : h e a t e d , s h e l t e r e d , p a r a p e t I C a b l e : h e a t e d , i n s u l a t e d , s h e l t e r e d 1 F l a t : not h e a t e d , i n s u l a t e d Gable: h e a t e d , ins111ated.exposed F l a t : h e a t e d , i n s u l a t e d , no p a r a p e t NI\C roof ( p s f ) 37(30)" 37(10) 37(35) 70( - )-
70( -1 22(25) ZL(25) 2 8 ( 3 3 ) ZR(35) ZR(35) ZR(35) 28(35) 36(32) 3 6 ( 3 5 ) 53(55) 53(55) 53(55) 53(55) 53(55) 26(24) 32(35) 4 0 ( 4 7 ) 40(55) 48(43) 48(43) 48(50) 4H(50) 4H(50) 35(34) 44(50) 43(43) 43(50) 60(55) 36(34) 36(40) 36(40) 50(42) 50(45) 82(7R) 8 2 ( 9 0 )--
M a x a v g ground ( p s f ) 27 2 7 28 45 47 4 0 12 1 3 16 2 6 18 16 17 22 22 36 3 6 3 6 3 6 36 6 8 12 13 13 1 3 1 3 14 1 3 3 5 35 2 7 31 2 5 16 11 11 n o 6 0 5 0 5 2 Max a v g roof ( p s f ) 5 11 5 3 1 6 6 32 4 3 2 2 7 2 7 3 6 4 0 2 5 2 4 14 5 10 5 8 4 4 10 9 6 5 8 3 7 8 5 7 14 I I 0 9 8 Max o b s e r v e d roof (psf)- 9 16 H 33 7 1 39 6 4 7 5 2 3 3 16 11 7 I 2 0, 4oa 56b 5ob 47 6 1 8 1 3 3 0 7 5 2 1 1 3 10 2 5 28 I. 3 9 17 12 2 6 4 4 3 4 0 12 13 T h e f i r s t figure. i n d i c a t e s t h e N R C 1960 roof l o a d ; th,e f i g u r e in ( ) t h e N B C 1953 l o a d .a = on 11~ai11 roof and l e a n - t o b = c o r r i r l o r r u n n i n g NW-SE c = c o r r i d o r r u n n i n g S W - N E
'rAl\l,L.; 1 I
SNOW LOADS F O R 13 S'rA'l'lONS
*
( ) b r a c k e t s i n d i c a t e 1 9 5 3 d e s i g n l o a d**
[3
b r a c k e t s i n d i c a t e b a s i c d e s i g n l o a d s a = d r i f t o n n ~ a r q u e e b = d e s i g n load i n c r e a s e d by s h a p e f a c t o r of 1. 5 1,ocat ion T r a i l , 13. C . N e l s o n , B. C. C a l g a r y , A l t a . L e t h b r i d g e , A l t a . Y o r k t o n , S a s k . T h e P a s . M a n . B r a n d o n , M a n . W a w a , O n t . S a u l t S t e . M a r i e , Ont . S a r n i a , O n t . B e l l e v i l l e , Ont. T h r e e R i v e r s , P. Q. F r e d e r i c t o n , N. B. A m h e r s t , N. S. St. J o h n ' s , Nfld. -o5
g
x x T y p e o l roof G a b l e F l a t G a b l e F l a t F l a t G a b l e G a b l e G a b l e F l a t F l a t G a b l e F l a t G a b l e F l a t G a b l e F l a t G a b l e F l a t -o -o 2 4 * d cg
.* x x x x x x x x x x -o -o,"j
d c . x x x--
2 :2
3 x x x x -.-A:$
4
.$ x x x x x R e n ~ a r k s m i l d , low s n o w f a l l 2 ' p a r a p e t t a l l b u i l d i n g s i n g l e s n o w - f a l l d r i f t a t 4. 5' p a r a p e t s h e l t e r e d d r i f t s o n l e e - w a r d s l o p e s 2 4 " s h e l t e r e d l o w s n o w - f a l l s h e l t e r e d , 2nd s t o r y p a r a p e t d r i f t a t s p l i t I c v e l-
M a j o r I i ~ f l u e n c e vZ r n
x -- N D C r o o f ( p s f ) 50(68)':' 5 0 ( 7 0 ) @(70J*" 1 5 ( 2 5 ) 1 8 ( 2 5 ) 40( 35) 4 7 ( 4 0 ) E ( 3 5 g 40(55) 4 0 ( 5 5 ) 36b(30) 40( 50) 40( 50) 6 7 ( 6 0 ) 6 7 ( 6 0 ) E ~ ( 6 5 3 4 8 ( 5 5 ) 4 7 ( 3 3 ) 58(50) - L o a d s in p s i G r o u n d ( p s f ) 9 7 9 16 4 9 28 10 24 8 19 1 3 13 37 37 35 20 12 2 l O b s e r v e d Roof ( p s f ) R 6 7 9 ' 4 4 8. 6 1 3 0 3 1 3 16 1 4 14 2 1 7 9 l o a d s D r i f t s ( p s f ) - - 15 31.34a 1 8 10 2 5-
- 37 19 19 5 5 5 5 2 3 50 1 6T A B L E 1 1 1
SNOW LOADS FOR C STATIONS ( i n p s f )
Supply Depot A r c h H a n g a r M. E . G a r a g e 9 C a n t . H a n g a r
lMax Avg M a x M a x Avg M a x M a x Avg Max M a x A v g lMax NBC
Ground Roof D r i f t Ground Roof. D r i f t Ground Roof D r i f t Ground Roof D r i f t ( B a s i c )
L a n c a s t e r , Alta. Cold L a k e , Alta. Winnipeg, Man. North B a y , Ont. T o r o n t o , Ont. Goose B a y , L a b .
Note:
-
i n d i c a t e s roof not a v a i l a b l es e c o n d f i p r e i n d i c a t e s m a x i m u m on canopy o r l e a n - t o
T A B L E I V
FIGURE I A
a
C STATION LOCATIONS AND THEIRMAXIMUM AVERAGE ROOF 8 GROUND LOADS ( 1 9 6 0 - 6 1 )
NO A STATIONS I INUVIK 2 REVELSTOKE 3 EDMONTON 4 SASKATOON 5 WINNIPEG 6 FORT CHURCHILL 7 TORONTO B KINGSTON 9 OTTAWA 10 ST€. ANNE DE BELLEVUE I I MONTREAL 12 ARVIDA 13 HALIFAX 14 GANDER 15 GOOSE BAY
No B STATIONS I TRAIL 2 NELSON 3 CALGARY 4 LETHERIDGE 5 YORKTON 6 THE PAS 7 BRANDON B WAWA
9 SAULT STE. MARIE
10 SARNIA I I BELLEVILLE 1 2 TROIS RlVlERES 1 3 FREDERICTON 14 AMHERST I 5 ST JOHN'S
FIGURE
2
B STATION LOCATIONS AND THEIR
E - 3 s c n o o L (SHELTERED. HEATED, IYSUL ATED I
UOME ECONOMICS ANNEX (SHELTERED FROM SOUTH. LOW WEATI
E - 3 RESIOENCE I EXPOSED. HEATED. INSULATED 4.1
FEDERAL B U I L D I N G
I EXPOSED. HEATED, INSUL ATEDl
SNOW LOAD OBSERVATIONS ON ROOFS Y E A R : 1960/61
MT. REVELSTOKE NAT. PARK SHELTER
ISHELTERED, NOT HEATED ELEV 2 8 0 0 F T
RESIDENCE, 2 0 8 g T n ST. E
ISHEL TERED, HEATED, INSULATED) ELEV I 8 5 0 F T
MOUNTAIN VIEW ELEMENTARY SCHOOL
/EXPOSED, HEATED, INSUL ATEDI ELEV I 8 5 0 F T
SNOWFALL
FIGURE 4
SNOW LOAD OBSERVATIONS ON ROOFS
Y E A R : 1960/61R O O F D E S C R I P T I O N I ADMINISTRATION BLDG. H INSULATED SECTION ' & - A ' MINING S T O R E SHED I' SHELTERED, UNHEATED BLDG ) SNOWFALL FIGURE
5
SNOW LOAD OBSERVATIONS ON ROOFS YEAR: 1960/61
SNOW LOAD OBSERVATIONS ON ROOFS
Y E A R : 1960/61 L O C A T I O N : S A S K A T O O N , S A S K . 40 2 0 \ -__..___r_______...--- 5 4 \ 0-
-
-
i 5 4I EXPOSED, HEATED, INSUL A T E D I
I33 - 1 0 7 % S T 8 4 5 ' 6 " I EXPOSED, H E A T E D , INSUL A T E D I O U T D O O R T E S T , S T A T I O N N - 3 .6 .9
(HEATED. INSUl ATED I
N O R M A L 1 9 6 0 / 6 1 S N O W F A L L 3 6 " 2 8 "
FIGURE 6 ( a )
314 LAKE CRES.
ALL UIUIUb ARE ZERO
I---.*
/ \
R O O F D E S C R I P T I O N
N . R . C . L A B
D E T A I L A
I EXPOSED, HEATED, INSUL ATED /
GYMNASIUM
EXPOSED. H E A T E 0 , V E N T I L A T E D . 2' PARAPETS
SNOW LOAD OBSERVATIONS ON ROOFS
YEAR: 1960/61L I B R A R Y S . W I N G
EXPOSED. INSULIP TED, NOT V E N T I L A T E D . U N H E l TED 1 I R SPACE
A G R I C U L T U R A L E N G . BLDG.
SHELTERED, IPTTIC UNHEATED. VENTILIPTED. INSULATED
