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Snow loads on roofs: the present requirements and a proposal for a survey of snow loads on roofs
NATlONAL 3ESEARCH COUNCIL
CANADA
D I V I S I O N OF BUILDING RESEARCH
SNOW
LOADS ONROOFS
The P r e s e n t Requirements and A Proposal F o r A
Survey of Snow Loads on Roofs
by D . E . A l l e n ANALYZED Report No.
106
of t h e D i v i s i o n of B u i l d i n g Research Ottawa September, 1956T h e ' t l c a v i e s t l o a d t h a t - r o ~ f s of b u i l d . i n g s a n d h o u s e s i n Canada c e n e r a l l y h a v e t o c a r r y i s t h e l o o d imposccl b y snobl,
The c h o i c e of t h e desip:n smw lon.ci t h e r e f o r e h a s c o n s i d e r a b l e i n f l u e n c e on t h e c o n s t r u c t i o n c o s t o f b u i 1 d i . n ~ ~ ~ and o f h o u s e s i n n : : r t i c u l a r , a n d t h . ~ ~ on khe ecornony of t h e c o n s t r u c t i o n i n d u s t r y ,
Snow l o a d s t o b e e x p e c t e d i n Canada, w t t h i t s many c l i m a t i c r e c i o n s , a r e gi'ven i n t h e T f a t i o n a l B u i l d i n g Code ( 1 9 5 3 ) i n t h e
f o r m of a map Tram w h i c h t h e d e s i g n snow l o a d c a n be o b t a i n e d f o r a n y l o c a l i t y , The 1oac:s sllown o n t h i s map were b a s e d on m e a s u r e - m e n t s of snow on the p o u n d talcen o v e r rnany g e a r s a t a number of' p o i n t s a c r o s s t h e c o u n t r y ,
The o p i n i o n h a s b e e n w i d e l y e x p r e s s e d t h a t m e a s u r e m e n t s o f snow on t h e g r o u n d c a n n o t d i r e c t l y h e a p p l i e d 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 :,nd 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 1 9 5 3 Code a r e t o o h i g h f o r some r o g i o n s .
The P , s s o c i a t a Committee on t h e N a t i o n a l B u i l d i n g Code o f t l i e N a t i o n a l R e s e a r c h C o u n c i l 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 c o n t i n u e d r e v i s i o n o f t h e Code and i s e n d e a v o u r i n g t o i m p r o v e t h e Code t o k e e p i t ic l f n e w i t h new d e v e l o p m e n t s and w i t h t h e
a d v a n c e o f k n o w l e d e e Lr! c ons t r u c t i on.
A s a s e r v i c e t a t h i s Committee, t h e r e f o r e , t h e D i v i s i o n o f B u i l d i n s R e s e a r c h o f t h e N a t i o n a l R e s e a r c h C o u n c i l h a s d e c i d e d t o s t u d y a c t u a l snorr Eoac" a s t h e y o c c u r on r o o f s . T h i s s t u d y m u s t , d u e t~ t h e v a r i a t i o n i n s n o w f a l l f r o m y e a r t o y e a r , e x t e n d o v e r s e v e r a l y e a r s and m u s t t a k e i n t o a c c o u n t t h e whole o f Canada. B e f o r e s t a r t i n g :3uch a w i d e s p r e a d s t u d y , i t seemed a d v i s a b l e t o c o n d u c t a p i l o t s u r v e y , i n o r d e r t o t e s t t h e p r o c e d u r e s p r o p o s e d f o r t h i s work, T h i s r e p o r t g i v e s d e t a i l s o f t h e p r o p o s e d p i l o t s u r v e y , t o b e c o n d u c t e d d u r i n g t h e w i n t e r o f 1 9 5 6
-
57.
C o p i e s w i l l , t h e r e f o r e , b e d i s t r f b u t e d t o t h e o b s e r v e r s p a r t i c i p a t i n g i n t h i s s u r v e y a n d t o othe~.-s i n t e r e s t e d , The r e p o r t w a s p r e p a r , e d by n a v i d E. A l l e n , a f o u r t h - y e a r c i v i l e n g i n e e r i n g s t u d e n t o f Q u e e n ' s U n i v e r s i t y , d u r i n g h i s work w i t h 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 i n t h e summer o f 1 9 5 6 w i t h t h e B u i l d i n g D e s i g n S e c t i o n !which 5. s h e a d e d b y W O ii, S c h r i e v e r , O t t a w a O c t o b e r 1 9 5 6 R a F a L e g g e t D i r e c t o rTABLE O F CONTENTS
-.-
Page FOREWORD
PART I DISCUSSION OF PRESENT SNOW LOAD REQUIREMENTS
1. Past Approach t o Design Snow Loads 1
2 . B a s i s f o r Chart
8.
Computed Maximum Snow Load-
H o r i z o n t a l Surf a c e
3
3 . D i s c u s s i o n of t h e Accuracy of Chart
8
44 . A p p l i c a t i o n of Chart 8 t o Load Hequirements on Roofs 6
PART 11 A SURVEY OF SNOW, LOA.DS ON ROOFS
1. G e n e r a l Purpose of t h e I n v e s t f g a t i o n
9
2 , Snow Loads on Roofs 1 03.
P r o p o s a l f o r a Survey o f Snow Loads on Roof's 114.
Data Required from Observers 115.
Data A v a i l a b l e from t h e Climate S e r v i c e 126 .
P r o c e d u r a l I n s t r u c t i o n s13
A . P r e l i m i n a r y I n s t r u c t i o n s 1 3 B. General I n s t r u c t i o n s
13
C . F i e l d Measurements 15
D . F i l l i n g o u t Form A-2 and when t o t a k e
Measurements
17
REFERFNCES
18
FOHEW ORD
S i n c e snow l o a d s i n Canada r e p r e s e n t t h e main d e s i g n l o a d f o r r o o f s , t h e magnitude of t h i s d e s i g n l o a d i s of g r e a t economic importance t o t h e b u i l d i n g i n d u s t r y . It i s , t h e r e - f o r e , important t h a t t h e r e be some confidence i n t h e snow l o a d s used t o d e s i g n b u i l d i n g s such a s t h o s e used i n t h e N a t i o n a l B u i l d i n g Code of Canada. A s t h e o p i n i o n h a s been e x p r e s s e d t h a t v a l u e s 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) a r e t o o h i g h i n many l o c a t i o n s i t was d e c i d e d t o s t u d y snow l o a d s f u r t h e r .
T h i s r e p o r t c o n s i s t s of two p a r t s ; t h e f i r s t reviews and d i s c u s s e s t h e b a s i s f o r t h e snow l o a d r e q u i r e m e n t s i n t h e 1953 N a t i o n a l B u i l d i n g Code; t h e second proposes a s u r v e y of snow l o a d s on r o o f s
.
An appendix w i l l be added, c o n t a i n i n g e x t r a c t s from snow l o a d r e q u i r e m e n t s of o t h e r c o u n t r i e s and, on how snow l o a d s were a r r i v e d a t i n t h o s e c o u n t r i e s .SNOW LOADS ON ROOFS
by
D.E. ALLEN
PART I
-
DISCUSSION OF PRESENT SNOW LOAD REQUIREM.ENTS1. PAST APPROACH TO DESIGN SNON LOADS
F i r s t of a l l , t h e background of t h e p r e s e n t approach t o d e s i g n snox l o a d s i n t h e ' 1 9 5 . 3 N a t i o n a l B u i l d i n g Code w i l l be d e s c r i b e d t o z e t h e r w i t h t h e development from t h e 1941 N a t i o n a l B u i l d i n g Code. The f o l l o w i n g e x t r a c t s a r e t a k e n from t h e
1941
N a t i o n a l B u i l d i n g Code:"3.6.4.1.
Snow-
Roofs...
s h a l l be c a p a b l e of s u p p o r t i n g a v e r t i c a l l i v e l o a d of from 20 t o40
pounds p e r s q u a r e f o o t of roof s u r f a c e , t h e v a l u e t o be assumed depending upon t h e p r o b a b l e s n o w f a l l and r a i n f a l l i n t h e l o c a l i t y i n which t h e b u i l d i n g i s t o be e r e c t e d . *F o o t n o t e : * A s a guide t o t h e s e l e c t i o n of t h e l i v e l o a d t o be assumed t h e f o l l o w i n g formula s h a l l be used:
where S i s t h e sum of t h e a v e r a g e s n o w f a l l s i n
J a n u a m , F e b r u a r y , and March, i n I n c h e s , o v e r a n u ~ b e s of y e a r s ;
R is t h e swn of t h e a v e r a g e r a i n f a l l s i n J a n u a r y , February, and March i n i n c h e s o v e r a number of y e a r s g,
and L i s r e l a t e d t o t h e l i v e l o a d by t h e f o l l o w i n g t a b l e :
Live Load
Less t h a n 20 20 pounds per. s q u a r e f o o t 20
-
30 30 p o u ~ ~ d s p e r s q u a r e f o o tThe f o l l o w i n g i s from t h e 1953 N a t i o n a l B u i l d i n g Code :
"4.1.2.1. ( a ) ( i ) When t h e s l o p e of t h e s u r f a c e i s l e s s t h a n
20 d e g r e e s t o t h e h o r i z o n t a l , t h e v a l u e of t h e uniformly d i s t r i b u t e d load s h a l l be t h e value f o r t h a t l o c a t i o n on Chart 8, Part 2 : - Climate;" ( F i g . l), ( 2 ) ,
Since snow l o a d s depend on l o c a l c l i m a t e a n a t i o n a l code must a p p l y l o c a l c l i m a t e t o i t s snow l o a d requirements.
I n t h e 1941 Code t h e l o c a l c l i m a t e was asaumed t o be t h e t o t a l s n o w f a l l and t h e t o t a l r a i n f a l l (except t h a t l i m i t s of
20 and 40 pounds p e r square f o o t were s e t ) , When a new
arrangement was decided on f o r t h e
1953
Code, c l i m a t o l o g i c a l d a t a were s e p a r a t e d from t h e o t h e r code reQuirements because e f f e c t s of c l i m a t e on b u i l d i n g s a r e n o t caused by t h e b u i l d i n g o r l.ts use but by n a t u r e , The c l i m a t o l o g i c a l d a t a were t h e r e - f o r e d e a l t w i t h s e p a r a t e l y and t h u s gained c o n s i d e r a b l e a t t e n t i o nA s a r e s u l t of reviewing snow d e p t h d a t a
( 3 ) ,
Chart8
of t h e Climate S e c t i o n of t h e1953
Code was drawn up ( F i g . 1 ) .Some improvements made by t h i s new approach a r e :
( 1 ) The c h a r t c l o s e l y r e p r e s e n t s a c t u a l l o a d s t h a t d i d occur because it i s based on measurements of snow d e p t h s .
( 2 ) The c h a r t can be r e v i s e d when more d a t a have been c o l l e c t e d without a f f e c t i n g t h e b a s i c r e q u i r e m e n t s .
( 3 )
The c h a r t p r o v i d e s a b a s i s of i n f o r m a t i o n f o rpurposes o t h e r t h a n snow l o a d s on roof's of b u i l d i n g s , Table 1 l i s t s twenty-six of t h e most populous c i t i e s of Canada and t h r e e of t h e more remote e s t a b l i s h m e n t s , w i t h
a comparison of snow l o a d s a c c o r d i n g t o ( a ) N a t i o n a l B u i l d i n g Code (1941), a s d e r i v e d from t h e formula L = S
+
R , ( b )National B u i l d i n g Code
(1953)
and ( e ) t h e i r own code, Valuesc P L = S
+
R determined from t h e s t a n d a r d 30 - y e a r normals (1921 t o 1950) are g i v e n by D.W. Boyd, C l i m a t o l o g i s t of t h e D i v ? s i o n of B u i l d i n g Research ( 4 ) . The v a l u e s l i s t e d f o r t h e Nat5onal B u i l d i n g Code (1941) a r e based on t h e s e v a l u e s ofL
-
S+
R . The l o a d s f o r t h e N a t i o n a l B u i l d i n g Code(1953)
a r e from t h e a c t u a l v a l u e s used t o draw up Chart
8.
The v a l u e s for t h e 1953 Code a r e somewhat d i f f e r e n t from t h o s e i n t e r p r e t e d d i r e c t l y from Chart
8.
The r e a s o n s a r e (1) p r i n t i n g i n a c c u r a c i e s i n t h e c h a r t which d i s p l a c e t h e i s o l l n e s ( 2 ) smoothing of t h e contour l i n e s which over- looks d i f f e r e n c e s of' a l o c a l n a t u r e( 3 )
l a r g e d i f f e r e n c e sThe ba.sis f o r t h e c h a r t i s completely d e s c r i b e d by
M . K . Thomas ( 4 ) . Much t h a t f o l l o w s i n Past I of t h i s r e p o r t i s t a k e n f ~ o m h i s . p a p e r .
2 . BASIS FOR CHART 8 - COMPUTED MAXIMUM SNOW LOAD
-
HORIZONTALm E
A t weather s t a t i o n s i n Canada, t h e d e b t h of snow on f l a t ground has been recorded d a i l y o r weekly. The d e p t h s e l e c t e d i s c o n s i d e r e d t h e average d e p t h of snow on a
h o r i z o n t a l s u r f a c e around each o b s e r v a t i o n s t a t i o n . Areas which have been blown h a r e , o r have been h e a v i l y d r i f t e d a r e avoided as f a r a s p o s s i b l e i n observing snow d e p t h s ,
h he
d e p t h d e s c r i b e d above w i l l be c a l l e d t h e t y p i c a l d e p t h ) . The maximum d e p t h s recorded d u r i n g 1941-50 were p l o t t e d on a map of Canada and i s o l i n e s were drawn. I f t h e maximum d e p t h s of' two a d j a c e n t s t a t i o n s d i d n o t a g r e e , t h e l a r g e r f i g u r e was a d o p t e d .A s p e c i f i c g r a v i t y of 0 , 2 was a r b i t r a r i l y chosen a s average f o r t h e snow pack when measured a t i t s maximum d e p t h . S i n c e maximum snow d e p t h u s u a l l y o c c u r s immediately a f t e r a
heavy s n o w f a l l , t h e average s p e c i f i c g r a v i t y should be some- where between t h a t of O , l f o r f r e s h l y f a l l e n snow and 0 , 2 t o
0 , 4
f o ~ an o l d snow pack. T h i s was used t o compute t h e weight of t h e maximum snow cover.The maximum 24-hour r a i n f a l l over t h e c r i t i c a l w i n t e r s e a s o n , e s t i m a t e d from d a t a i n 1941-50, was added t o t h e weight of t h e snow. F r e q u e n t l y , r a i n f a l l s i n t o t h e snow cover and i n c r e a s e s t h e a c t i n g l o a d f o r a s h o r t t i m e .
The r e s u l t i n g weight was expressed i n i n c h e s of w a t e r and was m u l t i p l i e d by an approximate f a c t o r of
5
t o o b t a i n pounds p e r square f o o t.
The snow l o a d s were p l o t t e d on a map of Canada and i s o l f n e s were drawn.T h i s procedure g i v e s a maximum l o a d o b t a i n e d from 1 0 y e a r s of o b s e r v a t i o n s on t h e gsow.d. Wheri p r e d i c t i n g snow l o a d s i t s h z u l d be remembered t h m s l o a d i s based on t h e f o l l o w i n g assumptions ( t h e v a l f d i t y of t h e a p p l i c a t i o n of snow l o a d on t h e ground t o snow l o a d on t h e roof i s d i s c u s s e d l a t e r ) :
( 1 ) The d e p t h i s t y p i c a l and t h u s r e p r e s e n t s a mean d e p t h
of snow on t h e ground. That i s , t h e volume of scow i n t h e ayea t r i b u t a r y t o t h e s t a t i o n l a e q u a l t o t h e t y p i c a l d e p t h t i m e s t h e t r i h u t . a r y a r e a .
( 2 ) The over a l l s p e c i f i c g r ~ a v i t y of t h e snow i s 0.2 when t h e snow i s a t i t s maximim recorded d e p t h .
( 3 ) The maxl.mum snow l o a d occurs when t h e maximum d e p t h of snow i s reached and when, a t t h e same time t h i s load i s i n c r e ~ s e d by t h e weight of t h e 10-year maximum 24-haus r a i n - f a l l , which would fa3 1 f ~ t o t h e snow covers.
( 4 )
This obser~li-,-i -1-0-year maximum d e p t h i s t h e t r u e probable 10-year maximum.3.
DISCUSSION OF THE ACCURACY OF CHART 8. COMPUTED MAXIMUMSNOW
LOAD - HORIZONTAL SURFACE (GROUND)( 1 ) Recorded Snow Depth
Maximum ELepth may be I n a c c u r a t e i n some a r e a s because i t i s d i f f i c u l t t o f i n d
a
t y p i c a l d e p t h . I n uaasheltered a r e a s where most of t h e d a t a f c r t h e c h a r t were o b t a i n e d , t h e problemof d r i f t i n g a f f e c t s t h e a c c u r a c y of' r e c o r d e d d e p t h s . I n mountainous a r e a s t h e d e p t h of snow cannot be a c c u r a t e l y
r e p r e s e n t e d l o c a l l y by a map of Canada (e.g.,Vancouver). Thus, t h e snow l o a d c h a ~ t should be c o n s i d e r e d i n t h e l i g h t of l o c a l d a t a wherever a v a i l a b l e . I n most a r e a s , however, maximum d e p t h s r e f l e c t e d by t h e snow l o a d c h a r t a r e r e p r e s e n t a t i v e s i n c e t h e y a r e based on a c t u a l pure snow d e p t h d a t a .
( 2 ) Snow D e n s i t y
The v a l u e of 0 . 2 was used a s t h e s p e c i f i c g r a v i t y of' maximum snow cover i n e v e r y p a r t of Canada. T h i s v a l u e i s probably good f o r a n average d e n s i t y of maximum snow pack and l e a d s t o r e a s o n a b l e l o a d s i n most a r e a s .
C l e a r l y , however, t h e average snow d e n s i t y , when maximum y e a r l y d e p t h s a r e c o n s i d e r e d , w i l l v a r y c o n s i d e r a b l y w i t h c l i m a t e c o n d i t i o n s from one p l a c e t o a n o t h e r . High t e r n p e ~ a t u ~ e s and " J can cause c o n s i d e r a b l e packing of t h e
snow d u r i n g i t s accumulation. The f o l l o w i n g a r e a few
recorded average d e n s i t i e s from t h e Canadian Snow Survey ( 5 ) .
Average S t a n d a r d Snow D e n s i t y D e v i a t i o n . . Old Glory
Mtn,
( B . c . )*
0.37 0.07 Winhipeg*
0 . 2 8 0 . 0 5 Malton*
(Toronto) e s t i m a t e d 0 . 3 0 . 1 -pa-*
D e n s i t i e s a r e meas~tsed i n exposed a r e a sThese are average d e n s i t i e s f o r a l l r e a d i n g s throughout t h e c o l d s e a s o n . For t h e maximum d e p t h t h e d e n s i t y probably
w l l l be lower because of l i g h t , new snow. However, t h e previoug v a l u e s show how v a r i a b l e t h e d e n s i t i e s can be a t maximum d e p t h s .
If t h e a c t u a l snow d e n s i t y observed i n t h e maximum y e a r l y snow pack was recorded w i t h t h e d e p t h ( P i g . 2 ) and t h e r e s u l t i n g weight e x p r e s s e d a s w a t e r e q u i v a l e n t (weight of t h e snow pack i n Inches of w a t e r ) , t h e r e s u l t s would g i v e t h e a c t u a l weight of snow pack when a t i t s maximum d e p t h . A
g r e a t e r weight of snow, however, may occur l a t e r , o r e a r l i e r when t h e snow 1.s l e s s deep and more compact. Although much d i f f e r e n c e i n magnitude between both v a l u e s of w a t e r e q u i v a l e n t
i s u n l i k e l y , it could be overcome by a s e a r c h f o r t h e maximum w a t e r e q u i v a l e n t . Water e q u i v a l e n t s a r e now being recorded a t many s t a t i o n s by p r i v a t e and government o r g a n i z a t i o n s .
F u t u ~ e r e v i s i o n of t h e c h a r t might be based on w a t e r e q u i v a l e n t d a t a .
Without r a i n f a l l , t h e snow load i n t h e c h a r t i n pounds p e r s q u a r e f o o t i s t h e maximum 1 0 - y e a r snow d e p t h i n i n c h e s
on
t h e f l a t ground m u l t i p l i e d by 1 (pounds p e r s q u a r e f o o t p e r i n c h ) . I n some a r e a s of Canada w i n t e r r a i n f a l l i s f r e q u e n t and abud~da3t and may f a l l i n t o t h e snow without running o f f immediately, t h u s i n c , r e a s i n g t h e snow pack w e i g h t . The maximum 24-hour r a i n f a l l d u r i n g t h e season when maximum snow cover m i h t ,be expected and i n t h e y e a r s 1941-50, was added, because; (17 sa:n f o r a lo^. e r p e r i o d would most c e r t a i n l y produce a p p r e c i a b l e r u n - o f f and ( 2 7 i t i s t h e o n l y p e r i o d f o r which r a i n f a l l i n t e n s i t y f i g u r e s a r e g e n e r a l l y a v a i l a b l e .
The magnitude of r a i n f a l l t o be added ( ~ i g .
3 )
i s u s u a l l y5
t o 1 0 ' ~ o t u a d s p e r square f o o t a c r o s s t h e i n t e r i o r of' Canada and r e a c h e s a maximum of 1 5 pounds p e r square f o o t a l o n g t h e more s o u t h e r l y c o a s t l i n e s .S i n c e i t i s u n l i k e l y t h a t t h e maximum r a i n f a l l o v e r
10 yeaTs w i l l o c c u r soon a f t e r t h e 10-year maximum snow d e p t h h a s accumulated, t h i s procedure i n v o l v e s some i n c r e a s e of t h e f a c t o r of s a f e t y a p p l i e d i n t h e d e s i g n l o a d ( i f t h e c o r r e c t d e s i g n l o a d i s assumed t o be e q u a l l e d o r exceeded once i n 10
y e a r s ) , The f a c t t h a t some r o o f f a i l u r e s o c c u r r e d when snow accumulation was accompanied by r a i n h a s had c o n s i d e r a b l e
i n f l u e n c e i n b r i n g i n g about an i n c r e a s e of' l o a d t o account f o r r a i n . It i s t r u e t h a t l o a d s based on snow d e p t h s and an
a r b i t r a r y s p e c i f i c g r a v i t y o n l y , would have n e g l e c t e d t h e e f f e c t of' any r a i n f a l l . I f , however, f u t u r e snow l o a d s ape based on w a t e r e q u i v a l e n t d a t a , t h e l o a d
from
r a i n which h a s f a l l e n i n t ot h e snow cover would be automatl.cally' i n c l u d e d by t h e measure- ment of d e n a i t y . I f t h e sa3.n 3.s i n t h e form of s l u s h a t t h e bottom of t h e snow pack its d e n . 3 l t y measurement would i n c l u d e t h e weight of t h e r a ? ? . It i s p o s s i b l e , however, t h a t t h e r a i n might
run
o f f bc:I"ose t h e d e n s i t y measurement was t a k e n , e s p e c i a l . l y if t h e d a t a a r e obtad~zed from per-Lodicmeasurements.
A t thEs time i t seems r e a s o n a b l e t 0 i n c l u d e r a l n f a l l I n t h e p s e s e n t form expecial1.y when t h e s m a l l magnitude of t h e r a i n l o a d r e l a t i v e t o t h e t o t a l snow l o a d i s c o n s i d e r e d ,
( 4 )
Observation Perl.odF i g w e
4
g i v e s t h e v a r i a t i o n Of maximum d e p t h s from y e a r t o y e a s i n a diagram.natIc comparlscn of y e a r l y maximumsnow d e p t h s f o s Ottswa, Shavrbxidge ( E a s t e r n Townships, ~ u e b e c )
and Aklavlk. I n a r e a s w!.th warmer c l i m a t e s t h e v a r i a t i o n
from year t c Year w i l l be much g r e a t e r t h a n i n o t h e r a r e a s . I n w a r m a r e a s wlch a h i g h p r e c i p i t a t i o n t h e occurrence of extreme snow pack l o a d s i s q u i t e p o s s i b l e even thaugh t h e recorc?.~ show verDy l l t t l e snow. These a r e o t h e r r e a s o n s why t h e c h a r t should be c o n s i d e r e d i n t h e l i g h t of l o c a l d a t a .
These e f f e c t s could be c o n s i d e r e d through a s t a t i s t i c a l a n a l y s i s of y e a r l y maximum l o a d s whenever enough d a t a i s compiled ( ~ i g . 5 ) . The d a t a a v a i l a b l e f o r t h e c h a r t were n o t enough t o
warrant any s t a t i s t i c a l approach o t h e r t h a n t h e maximum 10- y e a r observed snow d e p t h .
Although t h e r e a r e some l i m i t a t i o n s t o t h e a c c u r a c y of snow l o a d s r e p r e s e n t e d by t h e c h a r t , i t p r o v i d e s what t h e
a u t h o r b e l i e v e s t o be a r e a s o n a b l e b a s i s f o r d e t e r m i n i n g d e s i g n snow l o a d s on r o o f s .
4. APPLICATION OF CHART
8
TO LOAD REQUIREMENTS OM ROOFSThe maximum l o a d 02 a f l a t roof was assum-ed t o be t h e
same a s t h a t on t h e ground, b e c a ~ ~ s e OR a f l a t , horj.zonta1, u n l ~ e a t e d roof (which j s t h e l.imiting c a s e ) high w?.r!ds may
sweep o f f t h e snow c o v e r o r p i l e up d r i f t s , depending or, t h e p o s i t i o n of l o c a l wind o5ats1;ct loi?.a. The f i r a t c o n d i t i o n p s o b ~ b l y would occur more f r e q u e n t l y .
The probable l o a d , e q u a l l e d o r exceeded once i n 10 y e a r s i s connidered t o f u l f i l d e s i g n l o a d s e q u i r e m e n t s
.
It i s s t a t e d i n a Housing Research paper on how d e s i g n snow l o a d s were o b t a i n e d I n t h e United S t a t e s , t h a t most
a i l t h o r i t i e s a g r e e t h a t t o t a l d ~ s i g r b l o a d should be t h e l o a d t o whl.ch~ t h e s t ~ u c t u r e p r c b a b l y w i l l be s u h J e c t e d s e v e r a l t i m e s durLng I t s u s e f u l l i f e . T h i s t o t a l d e s i g n load should
produce s t l r c s a c : ~ n o t g r e a t e r t h a n 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 a f o r t h e p a r t i c u l a r s t r u c t u r e - s u p p o r t i n g m a t e r i a l uaed ( 6 ) . The weight of snow pack expected t o be e q u a l l e d o r exceeded once i n
10 y e a r s a t l e a s t , i s suggested as a f u l f i l m e n t of t h e c r i t e r i a f o r d e s i g n l o a d f r e q u e n c y . The paper made a s t u d y of an " a l l - time rnaxlmwn" l o a d t h a t could occur i n v a r i o u s p a r t s of t h e United S t a t e s , t o check t h a t t h e c a l c u l a t e d f a i l u r e l o a d of t h e b u i l d i n g exceeds t h i s " a l l - t ime maximum" l o a d . T h i s " a l l - t i m e maxfmwn" l o a d , which was c o n s i d e r e d t o be t h e combination of t h e maximum observed w a t e r e q u i v a l e n t and t h e maximum probable
snowstorm, was found t o be approximately t w i c e t h e w a t e r e q u i v a l e n t , e q u a l l e d o r exceeded once i n 10 y e a r s . The
probable fail^?? l o a d i s a t l e a s t twice a s g r e a t
as
t h e d e s i g n l o a d under p r e s e n t d e s i g n r e q u i r e m e n t s .The s t a t i s t i c a l a n a l y s i s of y e a r l y maximum l o a d s would be u s e f u l when c o n s i d e r i n g t h e r i s k of f a i l u r e ( ~ i g . .
3 ) .
ThereI s more r i s k of f a i l u r e i n Vancouver t h a n on t h e P r a i r i e s when u s i n g t h e 10-year maximum snow l o a d , because extreme snow l o a d s a r e more l i k e l y t o occur t h e r e . Also, depending on t h e i r use and occupancy, some b u i l d i n g s should be designed f o r a s m a l l e r f a i l u r e r i s k t h a n o t h e r s .
A r e d u c t i o n of maxirnw l o a d f o r r o o f s w i t h a s l o p e of o v e r 20" was g i v e n by
where L i s t h e a p p r o p r i a t e value f o r a f l a t s u r f a c e and x i s
t h e s l o p e of t h e roof i n d e g r e e s ( 2 ) ( 1 ) .
T h i s formula i m p l i e s t h a t t h e maximum l o a d on a roof of 20" s l o p e i s t h e same a s on a f l a t roof and t h a t t h e r e i s
no a c c ~ u l a t i o n on a r o o f of s l o p e 63O o r g r e a t e r . The formula f i r s t appeared i n t h e 1941 N a t i o n a l B u i l d i n g Code. It a p p a r e n t l y r e p l a c e d a t a b l e of r e d u c t i o n f a c t o r s f o r d i f f e r e n t r o o f s l o p e s which had been .Cirst c c ~ n s i d e r e d by t h e committee r e s p o n s i b l e f o r t h a t p a r t o.!' t n e Code
.
The b a s i s f o r t h e formula i s n o t a p p a r e n t but 1 ':, a p p e a r s r a t i o n a l . Trae f i r s t driri't (7'' ~ 4 6 - -oh? BL-Llding C O C - 1 : ?1~-e;l?;ll;e3 r e d u c ' z i ~ n Z c l ~ u o r s which r e s e m b ~ e a th o s ei n t h e
1g2E
. n t o B u i l d i n g Code. These r e d u c t i o n f a c t o r s f o r t h e e f f e c t of ''roof s l o p e seem t o be based mainly on e x p e r i e n c e .Other c o n s i d e r a t i o n s have been provided f o r but it i s
n o t shown how t h e y should be made. These a r e :
"4.1.2.1. ( b ) The non-uniformly d i s t r i b u t e d l o a d s h a l l be based on c o n c e n t r a t i o n s which a r e l i a b l e t o occur due t o t h e e f f e c t of wind and t h e shape of t h e s t r u c t u r e .
4.1.2.1. ( c ) With r e s p e c t t o s p e c i a l s t ~ u c t u r e s ouch a s greenhouses, t h e a p p r o p r i a t e snow l o a d value from S e c t i o n 2 . 8
may be reduced i f , i n t h e o p i n i o n of t h e administrative o f f i c i a l ,
it can be shown by e x p e r i e n c e o r c a l c u l a t i o n t h a t t h e s t a t e d l o a d
i s never obtained"(2 )
.
F a c t o r s such as s i z e , ahape, exposure ( t o wind and sun), and h e a t l o s s have n o t been provided f o r i n t h e N a t l o n a l B u i l d i n g Code o t h e r t h a n by t h e assumption t h a t t h e l i m i t i n g c a s e i s t h e unheated h o r i z o n t a l f l a t r o o f . T h e i r e f f e c t s a r e considered t o o complex t o r e f i n e f u r t h e r and no d a t a were a v a i l a b l e from which t o c o n s i d e r th~zrn.
PART I1
A SURVEY OF SNOW LOADS ON ROOFS
1. GENERAL PURPOSE OF THE INVESTIGATION
I n view of t h e many r e s e r v a t i o n s about t h e a p p l i c a t i o n t o r o o f s of t h e p r e s e n t d e s i g n snow l o a d s , a d i s c u s s i o n of t h e whole snow load problem was h e l d i n May 1956, a t which members
of t h e BuildingaDesign, National Building CbdeJ Building P r a c t i c e and Climate S e c t i o n s of t h e D i v i s i o n of Building Research were p r e s e n t . It was decided t o undertake an i n v e s t i g a t i o n i n t o a c t u a l a c c ~ m u l a t i o n of snow on r o o f s .
The o b j e c t i v e of t h e i n v e s t i g a t i o n i s t o study snow l o a d s which occur under c o n d i t i o n s which p r e v a i l not on t h e ground, but on d i f f e r e a i t t y p e s of r o o f s o r s t r u c t u r e s .
S p e c i f i c a l l y it i s hoped t h a t t h e i n v e s t i g a t i o n w i l l y i e l d information on t h e following:
( 1 ) The d i f f e r e n c e between snow accumulation on v a r i o u s f l a t r o o f s and t h e accumulation on t h e ground a s used f o r Chart
8
of t h e Code. A comparison between r o o f s and t h e ground of maximum o c c u r r i n g l o a d s would be u s e f u l t o i n t e r p r e t Chart8
f o r d e s i g n l o a d s on r o o f s .
( 2 ) The f a c t o r s which a f f e c t snow accumulation on r o o f s such a s shapeJ s l o p e , s i z e l o c a t i o n . Although d r i f t i n g and m e l t i h g a r e extremely complex f a c t o r s c o n s i d e r a t i o n w i l l be d i r e c t e d .towards snow accumulation on r o o f s ahd t h e f a c t o r s which cause d i f f e r e n t a c c w ~ l u l a t i o n s on r o o f s under d i f f e r e n t
c o n d i t i o n s . Data and o b s e r v a t i o n s c o l l e c t e d i n t h e survey w i l l be used t o check t h e v a l i d i t y of t h e s l o p e r e d u c t i o n formula.
( 3 )
A secondary purpose of t h e study i s t o o b t a i n d a t a t h a tw i l l a s s i s t t h e d e s i g n e r t o a r r i v e a t a more a c c u r a t e e s t i m a t i o n o r a b e t t e r understanding of d e s i g n loading f o r t h e c o n d i t i o n s t o which h i s roof' i s s u b j e c t e d . This could be published a s a !separate r e p o r t o r paper o r perhaps a n appendix t o t h e Code.
( 4 )
Another important f u n c t i o n of t h e survey would be t o c o l l e c t opinions and i d e a s on t h e s u b j e c t of d e s i g n snowl o a d s . This i s most important when compiling a s e c t i o n on snow l o a d s i n any b u i l d i n g code.
P r i m a r i l y t h e survey i s an experiment t o . compare t h e maximum o c c u r r i n g snow load on t h e ground w i t h t h e corresponding
2. SNOW LOADS ON ROOFS
T h e f i ~ s t s t e p I s t h e d-l..t':E'erence between snow on t h e ground and snow on a f l a t r o o f . I f we c o n s l d e r a 1 0 - y e a r
m a x i r n ~ ~ n ~ snow l o a d on t h e ground. w.here a t y p l . c a l d e p t h o c c u r s ,
it appea.r.5 t h a t t h e average s!?o~~J d e p t h on 'most r o o f s would be l e s s t.han t h z t 0 x 1 t h e gro~;l.nd beca.use t h e wind blows t h e
snow o f f t h e r o o f . Other efi'i'ects of l o c a l c o n d i t i . o n s o r c l i m a t e may cause a di:Cference between t h e amount of snow on t h e ground and on a f l a t r o o f .
Sescsdly, s n Q w g e n e r a l l y dosa n o t a c c m u l a t e evenly on a r o o f . ~ n s t e a d snow i s a p t t o d r i T t on t h e r o o f a s on t h e ground. I n g a b l e roofs snow accumulates more h e a v i l y on one
s i d e of t h e ~ o c i ' t h a n t h e o t h e r . R e v e r s a l of s t r e s s e s might occur under extreme d r i f t i n g c o n d i t i o n s .
T h i r d l y , t h e e f f e c t of d i f f e r e n t roof t y p e s i s c o n s i d e r a b l e . A f l a t roof w i t h a p a r a p e t p r o b a b l y would
accumulate more snow t h a n ona w i t h o u t because t h e p a r a p e t a c t d t o block t h e snow and prevent the wind from blowing o f f t h e snow. C e r t a i n shapes of r o o f s may accumulate v e r y l i t t l e snow.
F a c t o r s which a f f e c t snow l o a d s can be broken down a s
f 0ll.ows :
Snow and Rain
Accmu.l.ated snow and r a i n h e l d by snow
Wind
General d l r e c t i o n and speed of winds d u r i n g snow s t o r m and p o s s i b l y between storms ( c l i m a t i c d a t a ) Height, s i z e , shape and s l o p e of roof (aerodynamfc p r o p e r t f e s )
0 ; " 2 e r ? t ~ t i o n of b u l l d i n g ( b e a r i n g )
Topography of s ~ x r r o m d j . n g a r e a and r e l a t i v e s i z e and shape of nearby s t r u c t u r e s o r t r e e s
T e m ~ e r a t u r e and Sun
L a t i t u d e and a l t i t u d e
Temperature and d u r a t i o n of sunshine ( c l l r n a t f c d a t a )
O r i e n t a t i o n o r b e a r i n g o f b u i l d l n g ( t h e s t r i k e of t h e roof su.rfaces )
C o l o u * and dc s c r i p t i o n of roof sux*face m a t e r i a l Meat l o s s through roof:
3. PR,OPOSAL FOR A SURVEY OF SNOIJ LOADS ON ROOFS
I n view of t h e many f a c t o r s involved t h a t cannot be s e p a r a t e d , all ex.peri:nental s u r v e y of snow c o v e r on r o o f s i s t o be made 91?_ a number of t y p i c a . 1 l o c a t i o n s i n Canada. The s u r v e y
w i l l be c o ~ c e r n e d w i t h t h e a c t u a l weight of snow and i c e on
r c o f a.~;.d. ground. S i n c e t h e weight of snow on t h e roof i s di.?f i c u . l . t t o measure d i r e c t l y 9 a p r o p o s a l was made t o observe
t h e d e p t h s and measure a ~ e r k g e d e n s i t i e s of snow on p a r t s of t b ? r o o f and on t h e ground,
Already much of t h e c l i m a t i c d a t a h a s been and i s b e i n g r e c o r d e d . Wind, snow, snow d e n s i t y and w a t e r e q u i v a l e n t s a r e
beil;g r e c o r d e d i n many s t a t i o n s throughout Canada. T h i s
Ir,fozmation of c o u r s e , h a s been used t o draw up Chart 8 of t h e Code.
It h a s been decided t h a t a p i l o t s u r v e y w i l l be c a r r i e d out in t h e f i r s t w i n t e r t o check t h e method of i n v e s t i g a t i o n and review t h e whole snow l o a d problem. T h i s s u r v e y a l s o w i l l be used t o f i n d e f f i c i e n t methods of c a r ~ y i n g o u t t h e s u r v e y s i n c e
i t i s e x t r e m e l y d e s i r a b l e t o make t h e o b s e r v a t i o n s as e a s y a s p o s s i b l e and. y e t o b t a i n complete i n f o r m a t i o n . The p i l o t s u r v e y
w i l l be c a r s f e d o u t by one competent p e r s o n I n major d i s t r i c t s cf Canada, i.2
.,
EIalifax, F r e d e r i c t o n , Ottawa, Kingston, Toronto, C i ~ ~ . r c h i l l , Saskatoon and Vancouver.One purpose of t h i s r e p o r t i s t o e x p l a i n t o t h e o b s e r v e r s t a k i n g p a r t i n t h i a s u r v e y how t o t a k e and r e c o r d t h e d e s i r e d i ~ f o r ~ a t i o n . It w i l l be a p p a r e n t t h a t o b s e r v a t i o n s made i n t h e f i e l d a r e e s s e r ? . t i a l i n r e a c h i n g ths o b j e c t i v e of t h i s s t u d y . To
be ~ e 1 i a b l . e ~ t h e s e o b s e r v a t i o n s must be made w i t h some thought and i n t e r e s t as t h e y foran t h e b a s i s of t h e whole p r o j e c t .
4 DATA REQUIRED FROM OBSERVERS
Form A - 1 (two s h e e t s f o r e a c h b u i l d l . n g ) ; Lo'zat i o n
Photograph o r d e s c r l p t , io n of b u l l d i n g and s u r r o m d f n g s
,
I.ncluding r o o f dimensions and sl.opes,
o r i e n t a t i o n , col.our and m a t e r i a l of roof surface.. Also ad e s c r i p t i o n of the p r c v a i l l . n g c o n d i t i o n s under t h e roof s u r f a c e such a s i n a u l a t i o n , v e n t i l a t i o n , e t c .
Arrangement and numbering of gauges
Form A - 2 (one s h e e t a f t e r e a c h snowstorm, but a t l e a s t one a week);
De t h of snow af t e s snowstorm. Average d e n s i t y of snow ( a 7 on t h e ground, ( b ) on t h e roof
D e s c r i p t i o n , of snow on roof
Descrl.ption of ~ I l ~ : ~ a % o r i f l i n c l u d i n g wind speed and and d i r e c t i o n , t y p e and amount sf new snow
General remarks and o b s e r v a t i o n s which might be of importance ( e s p e c i a l l y f o r t h e p i l o t . s u r v e y )
5. DATA AVAILABLE FROM THE CLIMATE SERVICE
T y p i c a l d e p t h of accumulated snow on t h e ground Recorded w a t e r e q u i v a l e n t d u r i n g season ( t h e w a t e r
e q u i v a l e n t i s t h e weight of snowpack i n i n c h e s of w a t e r ) Recorded r a i n f a l l and s n o w f a l l d u r i n g season
Recorded temperature d u r i n g season
Recorded h o i ~ r s of d l r e ' c t sil.nshine each day of season Observed storms arid '.depth of new snow
6
.
PROCEDURAL INSTRUCTIONSObservers w i l l be supplied w i t h snow survey equipment i n c l u d i n g t h e f o l l o v ~ i n g items:
( a ) Report and One Set of Data Sheets
A f t e r t h e observer ha8 read t h e r e p o r t he should choose t h e roof o r r o o f s t h a t he v11shes t o study. He should t h e n
deteriaine a s u i t a b l e arrcngencnt of snow gauges f o r each roof and f i l l out one nhect of f o m A - l f o r each roof and save one f o r each r o o f . Me E J L U then send t h e completed forms f o r each roof t o t h e Buildlng Dcsfgn S e c t i o n , Division of Building
Research, EJ2.t iosnal Plcszarch Council,
Ottawa,
and r e q u e s t bynumber t h e f ollov:in= equipaent
.
( b ) Snowa Density K i t (one f o r each o b s e r v e r ) including;
B-1 Snow Sample C u t t e r
B-2 Back P l a t e ( t o snow sample c u t t e r )
B-3
Container graduated i n c cB-5 Yardstick D-6 Knife
( c ) Snow Depth Q a w e s f o r Roofs
D-1 Dspth gauge f o r shlngled p i t c h e d r o o f s
D-2 Depth game f o r f l a t r o o f s (with m a s t i c ) ( d ) Ivlore data s h e e t s (one s e t f o r each e x t r a r o o f )
(a)
Choo3ing tkz RoofSince t h e study should concern i t s e l f w i t h t h e prevalent t y p e s of r o o f s , t h e observer should confine h i s primary choice t o p i t c h e d r o o f s toith
a
g e n t l e s l o p e . Ventlllated r o o f s o r unheated a t t i c r o o f s f o u ~ d on ncwsr b u i l d i n g s , would p o s s i b l y be most s u i t a b l e . The observer, f o r h i s second choice, could include af l a t r o o f ( e s p e c l a l l y one t h a t i s w e l l v e n t i l a t e d ) , o r he c o u l d choose a f l a t roof with p a r a p e t s , found o f t e n on o f f i c e and i n d u s t r i s l buildings. The c h o i c e of s u i t a b l e r o o f s i s l e f t t o t h e o b s e r v e s , but he s h o u l d c o n f i n e h i s d i r e c t s t u d y t o two o r p o s s i b l y t h r e e ~ o o f s . Tn a d d i t i o n , he c o u l d make g e n e r a l v i s u a l o b s e r v a t i o n s of snow l o a d s i n h i s l o c a l i t y , and r e c o r d extreme c a s e s , ( b ) F l . l l l n g o u t Form ---- A - 1 D e s c r i p t l . o n of t h e House and S u r r o u n d i n g s A seals ' p l a n of t h e b u i l d i n g and s u r r o u n d i n g a r e a t o g e t h e r w i t h a p l a n , f r o n t acd s i d e e l e v a t i o n , and a s e c t i o n of t h e b u i l d i n g would be most u s e f u l . If t h i s i s n o t a v a i l a b l e ? p h o t o g r a p h s showing a c l o s e - u p m f t h e b u i l d i n g and a p i c t u r e of t h e b u i l d i n g and s u r r o u n d i n g a r e a would be s u f f i c i e n t . The f o l l o w i n g s h o u l d be l n c l u d e d i n t h e d e s c r i p t i o n : t h e s u r r o u n d i n g s t r u c t u r e s and Loyography up t o
a
d i s t a n c e of about. -200 f e e t f o r l a r g e r b u i l d i n g s a n d - 100 f e e t f o r s m a l l e r b u i l d i n g s t h e o r i e n t a t i o n o r b e a r i n g by a n o r t h arrow t h e Iscj-glr~t of t h e b u i l d i n g t okhe
e a v e s C o n d i t i o n s Under Roof The a t t i c s h o u l d be d e s c r i b e d as b e i n g V e n t i l a t e d , i n s u l a t e d , and h e s t e d c r u n h e a t e d .P o s i t I o n and Nu.mbe.rinq of Gause w .-
The gauges on \:he roof p l a n s h o u l d be numbered and t h e l e n g t h and w i d t h of t h e buf l d i n g s h o u l d be shown.
( c ) Use of S9ow Gauges
Snow gauges a r e s u p p l l e d Lo i ~ c i 3 . v i d u s l o b s e r v e r s t o f a c i l i t a t e t h e measurement of snow d e p t h on t h e r o o f . There i s one t y p e f o r s h i n g l e d r o o f s and one t y p e f o r f l a t r o o f s . The gauges a r e 3 f e e t l o n g and marked i n f e e t and i n c h e s .
. Snow gauges on t h e roo? s h o u l d be a r r a n g e d t o g i v e t h e g e n e r a l p a t t e r n of snow s71r.ce i t i s i m p o r t a n t t o h o w how t h e l o a d s a r e d i s t r i b u t e d u n d e r g i v e n c o n . d i t i o n s . A minimum number of gauges sho1:ld be used and the example i n F i g . 6 may he h e l p -
f u l when a s r a n g i n g t h e gauges: The ~ u r n b e r i ~ g of t h e gauges must be shown on t h e d a t a s h e e t t o i n t e r p r e t t h e d a t a . The
gauge : < h c ~ ~ l i i be k c p t 3 f e e t f r c m t h e edge of t h e r o o f .
The gauge f o r s h i n g l e d . r o o f s i s h e l d on by t h e s h i n g l e s ,
the bottom p l a t e Is p!zced 1mde;y a l a y e r of s h i n g l e s and t h e s t u d i s s t - ~ x c k 'Pet,ween t h e t uo s h i n g l e t a b s ( F i g . 7 ) . The upper p l a t e i s p u t o v e r t . 1 ~ s h i n g l e s and t h e gauge a r m i s screwed on t o clamp t h e s h i n g 2 . e ~ bet.ween t h e p l a t e s .
The gauge f o r t h e f l a t roof w i l l be f a s t e n e d d i r e c t l y on t h e r o o f , and m a s t i c a p p l i e d t o t h e bottom of t h e gauge f o r p r o p e r adherence
.
The d s p t h of snow on a p i t c h e d r o o f s h o u l d be a p p a r e n t from t h e ground s i n c e i t i s n e c e s s a r y 0nl.y t o g e t a d e p t h t o t h e n e a r e s t i n c h . On f l a t r o o f s it may be d i f f i c u l t t o t a k e r e a d i n g s because of o b s t r u c t i o n of view, and t h e o b s e r v e r may have t o use a l a d d e r o r some o t h e r method t o r a i s e h i s eye l e v e l above t h e r o o f .
C
.
FIELD
MEASTJREMENTSMeasurements on t h e Ground
A y a r d s t 1 c . k ?.s s u p p l i e d f o r t a k i n g measurements of snow d e p t h OR t h 2 gso~and.
The o b s e r v e r w l l l t a k e a number of measurements of snow d e p t h or\_ t k ~ e ground a r e a around t h e house simlxltaneous t o t h e snow gauge r e a d i n g s . A measurement could be t a k e n n e a r e a c h c o r r ~ e s of t h e ho:ase,
Dens it.??
Each o b s 2 r v e r i s s u p p l i e d w i t h a s e t of i n s t r u m e n t s t o measure deazslty. These i n s t r : u r ~ e n t s a r e a s i m p l i f i c a t i o n of t h o s e used f o r t h e Snow S u r l e y of tkaa A s s o c i a t e Committee on S o i l and Snow Mechanics
( 7 ) .
Snow Sa.mple C L . ~ t e r
The c u t t e r shown i n F i g .
8
h a s a t h i n saw-tooth c u t t i n g edge f o r u s e I n b o t h h a r d and s o f t snowF: The removable back p l a t e B-2 h a s hooks which engage p i n s D on t h ec y l i n d e r . The i n t e r n a l volume of e a c h c u t t e r i s 2 5 0 c c . betw2en t h e p l a n e s X-X and Y-Y.
The c u t t e r i s always used h o r i z o n t a l l y ; i t i s pushed i n t o t.he snow w h i l e t h e c y l i n d e r i s r o t a t e d around i t s
t o t r i m t h e sample l e v e l w i t h t h e fan-~asd and rear
edges of t h e c y l i n d e r . The back coves r 2 y be u ~ c d t o prcvcnt v e r y a l ?p;m.-,;;r s.?~.plc s C:>.le:-~ s l 2 d i r n ~ out of t h c c u t t c s after t h e sz.!:plc hza becn i s o l a t e d .
Bucket and G r a d u t c d Contelncr
2~1~23, b:.lcI:i3 t ( con~;,?,,~nin:: t :?o z n w zzcple a f r o n one
loc2.t i o a only) r b % b l h;.? cove:~t.d babellcd and ta:c,rcn
i s d o o r s t o rllt the Ealw::. T . k vo.Bun~~ of tt7.2 ?,rates
s::?,RZ t h c n be ficau~mcd
i n
t h graduated c o n t a i n e r .As osnz cc of ;..later :-:i:cb~ha on2 gra,n. tL32 n u ~ , b e r o r C C
ob'czii2cd c..n bc d l r c c t l y used t o &%,?.In t h e avesage
Gcnl;lCt:j fpc:s T3.blc 2 . It is important t o remembzr
ho-;i lily zamples are I n t h e bucket whan i t 1 3 weighed. D m s i t y cc~~zt?scr?,ant~,nt E chould be made I n a p p r o x i ~ a t e l y
4-inch i n t e r v a l s . I.? kitc d e p t h 2s s r a l l nor2 tkan onc s c z d l n g
in cach 12yzr zk-iould bz ;;?2cIc t o g i v e C o w o r c o r e s r ~ p l c a t o t h e buclce't. There s u a t bc th-.,~ ae?~:.3 ~LL:;>CF of r92c\ing3 I n each layer of
tha ancx covcr t o give a ts::c c?vepr:So.
If
tz,?
snctl.2 i s cx.tseaely hard i n so;:^ l;1;7cr;l (,?p3:202.ck?-;:3 %ec ),
j;>c c o r d t133 5 h i c : ~ ~ n e 33 oft h e i c e . Jud,zc::,.::rtt cunt be U S G ~ t o talrc t h ~ , sn,asure;-:znta, keep-
i n s i n mind tk2.t o v c k c ~ l l density 1 3 e s t L 1 2 t e d by f i n d i n g t h e averaze d e n s i t y of t h c c r o s s - s e c t i o n a l Psgc1.3.
ti1:~i c u t t d u ~ ; t ! ~ ~ : c ~ ; ) ~ . c I frc-l a VCX~~LCC*CL~ s e c t i o n of snow, c u t f ipst f T C : ~ t h t o g l z ~ c r , ehcn t R 3 ~c::z 12ycr and S O on, t o
t h e r o o f . TkBs could bc done on s p i t c h e d roof near, but not t o o c l o s e t o , t h c edze of tkc ~ 3 0 i ' , ~ : i ~ i h c t h e o b r c r v c r s t z n d s on t h e
l c d d c ~ - , a:?d c z 2.3 z c c c c ~ i ~ b l ~ f l-:,t 2ooF OF the g ~ o r m d by c u t t i n g a
czzP1 t r c i ~ c h .
S o 3 c r ~ c : ~ @c...::?~c:s :;'?.o:-~ld bc c u t cErcf u l l y t o avoid C C : T ~ ~ T ~ S S I ~ ~ t ! 2 2 s n c i , E;;?c;J st :plcsj rkaicla do not c s x p l e t e l y f i l l
t h o intcr~cl v o l u ~ : > 04 tl:;. cesttsr sL>oulQ 32 d k z c z r d ~ d . q y . - , e . , -
.-+-.-.,-
.,
...,...:.,, LIL.; .... .,nt ckmukd k c tzlccn on each l a r 3 p a r t of t h z yoof r.~il;h 'c5a z::~.:c! s2op2, Ci3a
3%1:.3?10 ~ 2 b l c r o o f , fori n s t z n c c , t5'3 d ~ : ~ ~ l t y ~ ? 1 0 u l d be tt>!.:cn on each S ~ G C 2nd 0b3crved
i n t h e d ~ ~ r , s r ;:_r~ov,: 92 * - : " bL,, -! ;aaoP sa:si:::cc, not t o o c l o s e t o t h e
edge of tBc
roo?.
f::azure:.:s:~ts should bz tc!:cn 2t 1czsS
5
f e c t m a y from t h e o n w c 2 q c t o 3 x v c n d O l c t ~ ~ r k x ~ c c of thc snor; a r o u d t h e gauge,It lrould bz r z s s rcl'ablc t o t2l:2 d c : ~ ~ L t y r c s d i i " , ~ ~ P Z C ~ Y ~ IYLICEOC t h e y
S i m u l t a n e o u s l y , on t h e g r o w d I?c:ir 1 I - t I ! i r , one
d e n s i t y measurement should be made i n a s n o w :;~;ci; i o r l with a n a v e r a g e dep'c.h of sncw
.
D. FILI,T?II; OUT FORM A-2 AND WHEN 'TO 'TAKE MEASUREMENTS
The snow d e p t h d a t a s h e e t s should be i ' i l l ? d out a f t e r each heavy s n o w f a l l and a t l e a s t once a week a s i n d i c a t e d i n
form A
-.
I f t h e r e i s a complete thaw of snow on t h e roof d u r i n g t h e w i n t e r , It should be r e c o r d e d on one of t h e weekly o b s e r v a t i o n s h e e t s . Observation should b e g i n w i t h t h e f i r s t heavy s n o w f a l l .(1) Depth measurements on t h e ground and roof should be made once a week and a f t e r each s n o w f a l l over two i n c h e s .
( 2 ) Snow-storm d a t a should be i n c l u d e d f o r e a c h storm o v e r two i n c h e s .
( 3 )
D e n s i t y measurements should be made on t h e roof and t h e ground a f t e r heavy s n o w f a l l s d u r i n g t h e y e a r , e s p e c i a l l y when maximum snow d e p t h o r snow l o a d i s l i k e l y t o o c c u r .A g e n e r a l d e s c r i p t i o n of t h e snow on t h e r o a f t o complete t h e p a t t e r n of snow a c c w n u l a t i o n s h o u l d accompany depLh measurements.
L o c a t i o n of d e n s i t y measurements should be i n d i c a t e d on a p l a n of t h e roof i n form A - 2 .
Snow-Storm Data
Snowatorm d z t a i n d i c a t e d on form A-2 should be o b t a i n e d from t h e l o c a l m e t e o r o l o g i c a l o f f i c e whenever p o s s i b l e .
When d e s c r i b i n g new snow from t h e snow-storm t h e s e d e s c r i p t i v e terms should be used where t h e y a p p l y ; wet, d r y , l i g h t , heavy, d r i f t i n g , n o n - d r i f t i n g , r a i n , h a i l , i c e - p e l l e t s
( s l e e t ), snow, g r a n u l a r snow, ( e .g
.,
heavy wet snow l a t e r mixed w i t h i c e p e l l e t s and r a i n ) .I f m e t e o r o l o g i c a l d a t a cannot be o b t a i n e d , t h e o b s e r v e r , i f p o s s i b l e , should d e s c r i b e wind speed and d i r e c t i o n a s i n d i c a t e d i n T a b l e
3 .
T h i s t a b l e c o n t a i n s t h e Beaufort s c a l e t o h e l p a s s e s s wind s p e e d .It i s hoped t h a t t h i s i n v e s t i g a t i o n w i l l h e l p improve and r e f i n e p r e s e n t d e s i g n snow l o a d r e q u i r e m e n t s and assumptions f o r t h e purpose of b e t t e r d e s i g n and economy i n t h e b u i l d i n g i n d u s t r y .
The a u t h o ~ s w i s h e s t o e x p r e s s h i s a p p r e c i a t i o n t o R.S. F e r g u s o n and S . G . F r o s t of t h e N a t i o n a l B u i l d i n g Code S e c t i o n ; L.W. Gold a ~ d G,P. W i l l i a m s of t h e Snow and I c e S e c t i o n ; 1ll.G.
F'lewes of t h e B u i l d i n g D e s i g n S e c t i o n and o t h e r s 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 , f o r t h e i r v a l u a b l e comments on t h e , f i r s t d r a f t of t h i s r e p o r t .
D.W. Boyd, c l i m a t o l o g i s t , s u p p l i e d t h e a u t h o r w i t h i n f o r m a t i o n and h e l p f u l c r i t i c i s m , and W.R. S c h r i e v e r , head of t h e B u i l d i n g D e s i g n S e c t i o n , d i r e c t e d and a s s i s t e d i n t h e psepesat1.or-t of t h i s work. The a u t h o r a l s o w i s h e s t o express h i s a p p r e c i a t i o n t o M s . R.F. L e g g e t , D i r e c t o r 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 , f o r h i s a s s i s t a n c e and p e r m i s s i o n t o p u b l i s h t h i s r e p o r t . R e f e r e n c e s 1. N a t i o n a l B u i l d i n g Code of Canada ( 1 9 4 1 ) , N a t i o n a l R e s e a r c h C o u n c i l , O t t a w a . 2 . N a t i o n a l B u i l d i n g Code of Canada ( 1 9 5 3 ) ) N a t i o n a l R e s e a r c h C o u n c i l , Ottawa.
3. Thomas, M.K. A method of computing maximum snow l o a d .
, E n g i n e e r i n g J o u r n a l , Vol. 38, No. 2 . F e b r u a r y 1955. pp. 120-123.
4 .
C l i r n a t o l o i c a l a t l a s of Canada p r e p a r e d by M . K . Thomas. December 1953. $A j o i n t p u b l i c a t i o n of t h e M e t e o r o l o g i c a l D i v i s i o n , D e p a ~ t m e n t of T r a n s p o r t and 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 , N a t i o n a l R e s e a r c h C o u n c i l , ~ a n a d a ) . N . R . C . No.3151,
253 p .5 . Gold, T.#.W. and G.P. W i l l i a m s . Some r e s u l t s of t h e Canadian
Snow S u r v e y . 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 , N a t i o n a l R e s e a r c h C o u n c i l . J a n u a r y 1956. P r e p a r e d f o s p r e s e n t a t i o n t o t h e
E a s t e r n Snow Conference Hanover, New Hampshire, F e b r u a r y 9-10, 1956. 6 p .
6 . U n i t e d S t a t e s D i v i s i o n of Housing R e s e a r c h , Housing and Home F i n a n c e Agency. Snow l o a d s t u d i e s . Housing R e s e a r c h Paper No.
19,
Washington, May 1952.19
p .7. K l e i n , G..T.
,
D . C . P e a r c e and L.W. Gold. Method of m e a s u r i n g t h e significant c h a r a c t e r i s t i c s of a snow c o v e r . T e c h n i c a l Memorandum No. 18, A s s o c i a t , e Committee on S o i l and SnowMechanics. N a t i o n a l R e s e a r c h C o u n c i l No. 2269, November 1950.
B i b l i o g r a p h y
A by-law t o z J e g u l a t e t h e e r e c t i o n and p r o v i d e f o r t h e s a f e t y of b u i l d i n g s . Number 9868, C i t y of T o r o n t o . R e v i s e d 1937.
Arne?!-can. Standalad B u i l d i n g -Codhe 17cquLrements f o r minimum d e s i g n l o a d a i n buildS.ngs and o t h e r s t r u c t u r e s . American S t a n d a r d s A s s o c i a t i o n . Approved Septembe;? 3, 1955.
The i n t e r n a t i o n a l c l a s s i f ' i c a t i o x ! f o r snor,r ( w i t h s p e c i a l r e f e r e n c e t o srow on t h e , p o u n d ) . Issusd by Thc Commission on Snow and I c e of t h e Int3e~:j:~ationa1 R s s o c i a . ~ i o n of Hydrology. T e c h n i c a l Memorandum No.
31
of t h e A s s o c i a t e Committee on S o i l and Snow Mechanics, N a t i o n a l R e s e a r c h C o u n c i l , Ottawa, August 1954.19
p . Ketchwn,V.
and J . H . C i s s e l . S t r u c t u r a l l i v e l o a d s . E x t r a c tf r o m B u i l d i n g S t a n d a r d s Monthly, November 1948.
8
p .R i c h t e r , G.D. Snow c o v e r ; i t s format-ion and p r o p e r t i e s . Academy S c i e n c e U . S .S . R e P o p u l a ~ S c i e n c e S e r l e s , p u b l i s h i n g house of t h e Academy S c i e n c e U.S.S.R., Moscow, L e n i n g r a d 1 9 4 5 . 113 p. D m i t r i e v a , N . G . C a l c u l a t i o n of' snow c o v e r d e n s i t y u s i n g
M e t e o r o l o g S c a l d a t a ( R a s c h e t p l o t n a s t i snezhnogo p o k r a po m e t e o r o l o g i s c h e s k i m dannzyh, M e ' c e o r o l o ~ i i a i C - i d r o l o g i i a , 140.
TABLE I
A COIIPARISON OF SNOW LOADS ACCORDING TO DIFFERENT BUILDING CODES (Ponds pel S q u a r e Foot and L = S
+
R i n .inches)P i u n i c i p a l B u i l d i n g h a t i o n a l N a t i o n a l Code i n F o r c e B u i l d i n g Code B u i l d i n g Code C i t y P r o v i n c e 1952 1 9 4 1 1-953 L=S-SR l b / s q .
i"c.
i n , l b / s q . f ' t . l b / s q . ft: M o ~ t r e a l T o r o n t o Vancouver W i ~ n i p e g O t t a v ~ a Qu3bec C i t y Hainilt on Edmonton Windsor C a l g a r y H a l i f a x London V i c t o r i a S a i n t J o h n Regina St,. J o h n ' s S a s k a t oon S h e r b r o o k e T h r e e R i v e r s K i t c h e n e r Sudbury P e t e r b o r o u g h F o r t \ I Z l l i a m K i n g s t on S a u l t S t e.
Marie Sydney file 1 s on Goose Bay A k l a v i k Q ~ ~ e b e c O n t a r i o B. C . P4anit oba O n t a r i o Quebec O n t a r i o A l b e r t a O n t a r i o A l S e r t a N . S. O n t a r i o B. C. N.B.
S a s k . Nfld 0 S a s k . Quebec Quebec O n t a r i o O n t a r i o O m t a ~ f o O n t a r i o O n t a r i o O n t a r i o N o S *B.
C , Nf l d.
N.W.T.TABLE 2 ---. .- --
--
-
- --- -. --
Welght Read on
Scale
i n G r a m sNumber of
Samples
--- --7
39
10 ----140
160
180
200158
180203
225
200 175 220225
250
193
248
275
210240
270
300
228
260
293
325
280
245
263
300
%!
375
350
280
360
320383
400
298
340
405
42 5
315
360
428
450
333
380
475
350
400
450
500
420
368440
495
525
385
403
473
550
460
518
575
420
480
600
438
500
54055.3
625
455
585
520540
608650
473
560
675490
630
700
508
580
653
725
52 5
600
750
543
520
698
560
640
720
800
675775
578
660
743
825
595
680
765
850
613
700808
875
630
720810
648 740833
900
92
566 5
760
855
950
780
878
975
683700
800
900
1000718
829
923
1025
735
840
9k5
1050
753
860
96.61075
770
880
ggb
1100788
900
1013
1125
D e n s i t y- -
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8
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G 0, co 0 ( r l r l L 0 c d C 60 cn( 1 ) Clzoose t h e r o o f s and t h e ai~rnny:ement o f p u g e s on e a c h r o o f . F i l i o u t Form A - 1 a n d =.end f o r t h e s u r v e y equipment,
( 2 ) Wtlon tlze equipment a r r i v e s a t t a c h t h e g a u g e s t o t h e r o o f s , Depth Moas~rrernents
( 1 ) Record, a t l e a s t once a week ( s a y e v e r y Monday), t h e snow d e p t h s a t e a c h gaueo under t h e p r o p e r number f o r t h a t gauge i n Forrn A - I ,
( 2 ) A t t h e same t i m e , r e c o r d a number of r e a d i n g s t a k e n w i t h a y a r d s t i c l c of t h e d e p t h of u n d i s h u r b e d snow on t h e g r o u n d a r o ~ m d t h e h o u s e ,
Snow-Storm D a t a
( 1 ) A f t e r e a c h snowstorm of 2 - i n , snow d e p t h o r more, o b t a i n e i t h e r by e s t i m a t i o n o r t h r o u g h zhe l o c a l m e t e o r o l o g i c a l o f f i c e t h e d a t a r e q u i r e d on Form A-2, D e n s i t y D e n s i t y measurements s h o u l d b e t a k e n a f t e r heavy s n o w f a l l s d u r i n g t h e y e a r and e s p e c i a l l y n e a r t h e t i m e o f e x p e c t e d maximum snow d e p t h . The d e n s i t y s h o u l d b e t a k e n o n e a c h l a r g e p a r t of t h e r o o f and, a t t h e same t i m e , i n a n a v e r a c e l a y e r of snow on t h e ground, S e c o r d t h e d e p t h of t h e snow s e c t i o n where t h e d e n s i t y i 3 measured.
L o c a t e a s u i t a b l e p l a c e f o r t a k i n g a d e n s i t y measurement away f r o m t h e snow gauy;es and n o t t o o n e a r t h e edge of t h e r o o f .
S t a r t c u t t i n g a v e r t i c a l t r e n c h o r w a l l f r o m t h e t o p o f t h e snow l a y e r down and d i v i d e t h e d e p t h f r o m t h e t o p i n t o
a p p r o x i m a t e 4 - i n o L a g e r s ,
Cut one o r more samples f r o m e a c h l a y e r t o o b t a i n a t o t a l of a t l o a s t LL samples. T h e r e must be t h e same number of s a m p l e s i n e a c h l a g e r of t h e v e r t i c a l s e c t i o n , u s e - t h e c u t t e r h o r i z o n t a l l y by s l o r ~ l y p u s h i n g i t i n t o t h e snow w h i l e r o t a t i n g t h e c y l i n d e r around i t s a x i s u n t i l t h e c y l i n d e r i s f u l l , Use t h e b a c k p l a t e f o r removing s l i p p e r y s a m p l e s , I f t h e r e i s i c e r e c o r d i t s t h i c k n e s s ,
P u t e a c h sample i n t o t h e w e i g h i n z pan and, u s i n g T a b l e 2,
f i n d t h e a v e r a g e d e n s i t y o f t h e s e c t i o n lcnowing t h e number of s a m p l e s ,
