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NATIONAL RESEARCH COUNCIL CANADA
D I V I S I O N O F B U I L D I N G RESEARCH
LOADING T E S T S ON J O I S T AND RAFPER ROOF FRAMES
by
H. J. T h o r b u r n and B. G. W. P e t e r
I n t e r n a l R e p o r t N o . 225
of t h e
PREFACE
The investigation of conventional joist and rafter roof frames by the Division of Building Research was initiated for the purpose of establishing performance requirements for trusses for house roofs. Trusses, it was thought, should give performance
equivalent to that of the better conventional frames. With the commencement of the work described in this report the purpose of the investigation of house roof frames has changed. Investigations are now being directed to the improvement of the struc-tural aspects of conventional roof frame construction. Preliminary s t e m are made towards the rationalization of frame design and construction which together with possible future work may lead to the design of frames by normal engineering methods. Such a goal becomes increasingly desirable as the use of performance- rather than
specification-type building codes continues to grow,
The authors of this report are civil engineers and research officers with the Building Structures Section of the Division. One of the projects assigned to them has been the study of the structural performance of frame construction.
Ottawa
TABLE OF CONTENTS TEST PROGRAM e e e e
. . .
a u e.
e. .
e. . .
TEST STRUCTURES TEST ARRANGENEXT. . .
( a ) T e s t Method5
. . .
( b ) T e s t P r o c e d u r e7
TEST RESULTS. . .
( a ) S p e c i f i c R e s u l t s 7. . .
( b ) G e n e r a l R e s u l t s 9. . .
CONCLUSIONS.
,.
l.
.
12. . .
IMPLICATIONS 13. . .
REFERENCES 1 4LOADING TESTS ON J O I S T AND IIIIFTER ROOF P W E S
'by
H. J. T h o r b m and B e G. W, P e t e r
A s p a r t of i t s c o n t i n u i n g s t u d y of t h e s t r e n g t h of house frames, t h e Division of Building Research resumed i n
1959 t h e i n v e s t i g a t i o n of t h e s t r u c t u r a l p r o p e r t i e s of house roof frames which it had c a r r i e d out i n 1957, 1956 and e a r l i e r . The r e s u l t s of t h e s e previous t e s t s have been r e p o r t e d (1 t o 5 ) .
There were two r e l a t e d reasons f o r resuming t h e
i n v e s t i g a t i o n of conventional roof frame c o n s t r u c t i o n , F i r s t , t h e r e i s considerable evidence t h a t many conventional roof frames a r e inadequate i n s t r e n g t h , p a r t i c u l a r l y i n t h e connec- t i o n s , t o support, w i t h a n adequate margin of s a f e t y , common Canadian design snow l o a d s (30 t o 50 psf a r e most common), This evidence c o n s i s t s of t h e following:
( a ) Numerous roof f a i l u r e s , p a r t i c u l a r l y of c o t t a g e s , during t h e w i n t e r of 1958-59 under l o a d s which were o f t e n l e s s t h a n 30 t o 40 p s f .
( b ) Loading t e s t s on conventional roof frames which
showed t h a t a t 16 i n . o.c, some forms of c o n s t r u c t i o n f a i l e d . a t l o a d s a s low a s 15 p s f , although t h e b e t t e r forms of c o n s t r u c t i o n c a r r i e d a s much a s 140 psf ( 3 , 4,519
( c ) The f u l l - s c a l e l o a d i n g t e s t on t h e house on Research Road which showed an i n c i p i e n t f a i l u r e load of 70
psf (1).
The second reason f o r resuming t h e s t u d y i s r e l a t e d t o t h e p r e s e n t bases o r s p e c i f i c a t i o n s f o r t h e c o n s t r u c t i o n and acceptance of roof frames. There a r e t h r e e such bases:
( a ) NBC (1953) Subsection 4.3.6 "Timber Engineering": Prames built- on t h e b a s i s of t h i s sect-ion of t h e Code a r e governed by allowable s t r e s s e s i n t h e members and allowable l a t e r a l l o a d s on t h e n a i l s . For a given span and design l o a d t h e y can be
expected t o c a r r y a f a i l u r e l o a d from f o u r t o f i v e times t h e design l o a d , o r f o r i n s t a n c e , f o r a 50- psf design l o a d a f a i l u r e load o f approximately 200 t o 250 p s f .
( b ) NBC (1953) Subsection 4.3.7 "Frame Construction" (which i s t h e same a s i n "Housing Standards"
Frames b u i l t on t h i s e s of all-owable spans of r a f t e r s an5 j o i s t s and by a schedule of the-number
h e e l j o i n t of a conventional roof frame t h e number of n a i l s r e q u i r e d i s about 1/6 of t h a t s p e c i f i e d under ( a ) . Thus i s it n o t s u r p r i s i n g t h a t Hansents l o a d i n g t e s t s on v a r i o u s conventional roof frames b u i l t on t h i s b a s i s showed f a i l u r e l o a d s varying from approximately 1 5 t o 1 4 0 p s f . If we assume t h a t t h e s e roof frames a r e used a t 1 6 i n , O . C . i n a 50-psf snow load a r e a , t h e l o a d s a f e t y f a c t o r would vary from 0 . 3 t o 2 . 8 1
( c ) CMHC B u i l d e r s f B u l l e t i n on Nailed Roof Trusses:
me -t;hird b a s i s f o r t l i e c o n s t r u c t i o n and a c c e ~ t a n c e *
of roof frames is given i n CMHC B u l l e t i n No. 94 which g i v e s s t a n d a r d d e s i g n s o f IN-trusses f o r use
i n house c o n s t r u c t i o n . These s t a n d a r d d e s i g n s
were prepared t o ensure t h a t t r u s s e d r o o f s w i l l have l o a d - c a r r y i n g c a p a c i t i e s of a t l e a s t 100 p s f , This requirement i s based on t h e f a i l u r e l o a d s o f roughly 100 psf o r b e t t e r t h a t were observed f o r t h e b e t t e r t y p e s o f conventional roof frames. A s it was
judged t h a t t h e s e t y p e s had given s a t i s f a c t o r y performance i n a r e a s of up t o 50-psf design snow l o a d (NBC 1 9 5 3 ) , a minimum load f a c t o r of 2 was a r r i v e d a t and used t o determine t h e r e q u i r e d spacing of t h e t r u s s e s f o r a r e a s where t h e snow load was o t h e r t h a n 50 p s f .
There e x i s t t h e r e f o r e t h r e e d i f f e r e n t bases f o r t h e a c c e p t a b i l i t y o f roof frames. One provides f o r a load s a f e t y f a c t o r of 4 t o
5 ,
a n o t h e r f o r a f a c t o r of 0.3 t o 2.8 anda n o t h e r f o r a f a c t o r of 2. Thus we have b a s e s which, i n t h e extreme, allow v a r i a t i o n s i n s t r e n g t h of over 1500 p e r c e n t .
I t should be e v i d e n t , t h e r e f o r e , t h a t t h e r e i s
c o n s i d e r a b l e need f o r f u r t h e r i n v e s t i g a t i o n o f conventional roof-frame c o n s t r u c t i o n . The f i r s t and more immediate aim of such i n v e s t i g a t i o n would be t o prevent a r e c u r r e n c e of numerous roof f a i l u r e s a s i n t h e 1958-59 w i n t e r . Secondly, i n v e s t i g a t i o n i s r e q u i r e d t o provide r a t i o n a l information on which t o base any r e s o l u t i o n of t h e t h r e e c o n f l i c t i n g b a s e s
of acceptance. The need f o r t h i s i n v e s t i g a t i o n was confirmed by t h e 1960 e d i t i o n of t h e NBC which was i n t h e p r e l i m i n a r y s t a g e s of p r e p a r a t i o n . Although it was r e a l i z e d t h a t t h e c o n f l i c t could n o t be e l i m i n a t e d i n one s t e p , it was thought t h a t a n a p p r o p r i a t e f i r s t s t e p would be t o reduce t h e s t r e n g t h v a r i a t i o n by modifying some o f t h e o l d requirements f o r t h e 1960 e d i t i o n of t h e NBC. I t was f u r t h e r thought t h a t t h e s e modifications should be aimed a t t h o s e requirements which allowed roof c o n s t r u c t i o n s w i t h "substandard" s t r e n g t h , i . e , l o a d s a f e t y f a c t o r s o f , s a y , l e s s t h a n 2.
The inadequate s t r e n g t h of conventional roof frames i s due p r i m a r i l y t o poor connections, M o s t of t h e roof
f a i l u r e s caused by snow l o a d s of t h e 1958-59 w i n t e r were t h e r e s u l t of inadequate o r poor n a i l i n g a t t h e connections. Only a s m a l l percentage of t h e c o l l a p s e s r e s u l t e d from t h e f a i l u r e of t h e members. S i m i l a r l y i n t h e l o a d i n g t e s t s c a r r i e d o u t by A . T. Hansen, most of t h e f a i l u r e s occurred a t t h e connections. The connections a r e a l s o t h e a p p a r e n t source of t h e c o n f l i c t t h a t e x i s t s i n t h e bases of acceptance.
I t should be r e a l i z e d , however, t h a t t h e r e a l source i s t o be found i n t h e choice of load. s a f e t y f a c t o r s . Before t h i s choice can be reconsidered., it i s n e c e s s a r y t o know t h e s t r e n g t h s of t h e frames r e s u l t i n g from t h e e x i s t i n g requirements.
The broad d e s i g n a t i o n of connections a s t h e "weak l i n k " can be narrowed down on t h e b a s i s of t h e evidence
a v a i l a b l e . This evidence shows t h a t t h e most c r i t i c a l j o i n t
i s t h e connection of t h e r a f t e r t o t h e j o i s t , t h e h e e l j o i n t .
It i s a t t h i s j o i n t t h a t t h e f u l l t h r u s t of t h e loaded r a f t e r
i s r e s i s t e d by t h e j o i s t , p a r t l y through f r i c t i o n , b u t mostly through t h e connecting n a i l s . Other j o i n t s have demonstrated weaknesses, t h e most p r e v a l e n t b e i n g i n t h e j o i s t s p l i c e
followed by t h e c o l l a r - r a f t e r connection. There i s no
evidence of any g e n e r a l weakness i n r a f t e r - r i d g e connections. TEST PROGRAM
On t h e b a s i s of t h e evidence c i t e d , it was decided t h a t a s e r i e s of s t r u c t u r a l t e s t s should be c a r r i e d o u t i n which p a r t i c u l a r a t t e n t i o n would be d i r e c t e d t o t h e h e e l
j o i n t s of conventional roof frames. Other j o i n t s would a l s o be s t u d i e d d u r i n g t h e t e s t s because t h e i r s t r e n g t h s
would have t o be matched w i t h t h a t of t h e h e e l j o i n t s . S i m i l a r l y , t h e roof frame members would a l s o r e c e i v e some
a t t e n t i o n . When i n t h e t e s t s e r i e s it was n e c e s s a r y t o assume a d e s i g n load and a l o a d f a c t o r , 50 psf and 2 were used
r e s p e c t i v e l y . I n a d d i t i o n , it was assumed t h a t a l l frames were spaced 16 i n . O . C .
I t was planned t h a t t h e t e s t s e r i e s would be d i v i d e d i n t o f o u r groups, each w i t h a p a r t i c u l a r o b j e c t i v e :
Group A
-
would be t e s t e d t o determine t h e v a r i a t i o n i n roof frame s t r e n g t h r e s u l t i n g from v a r i a t i o ni n t h e number of through n a i l s a t t h e h e e l j o i n t . A c o r o l l a r y t o t h i s would be t h e d e t e r m i n a t i o n of t h e number of through n a i l s r e q u i r e d a t t h e h e e l j o i n t t o r e s i s t a l o a d of 100 p s f .
Group B -
-
would be t e s t e d t o determine t h e e f f e c t of d i f f e r e n t roof s l o p e s on t h e number of through n a i l s r e q u i r e d a t t h e h e e l j o i n t t o r e s i s t 100 p s f .Group C
-
would be t e s t e d t o d e t e r m i n e t h e e f f e c t of l o a d i n g one s i d e o n l y of a r o o f frame, i . e , unbalanced l o a d i n & , Group D-
would be t e s t e d t o d e t e r m i n e t h e e f f e c t of a long-term l o a d i n g of50
p s f on a frame w i t h h e e l j o i n t n a i l i n g t o r e s i s t 100 p s f . TEST STRUCTURESThe t y p e of r o o f frame t e s t e d i n a l l groups was one of t h e most common t y p e s of c o n v e n t i o n a l c o n s t r u c t i o n b e i n g used i n Canada a t p r e s e n t . A f i e l d s u r v e y r e v e a l e d t h i s t o be a frame of 2 4 - f t span w i t h 2- by 6 - i n . j o i s t s , 2- by 4 - i n . c o l l a r t i e s , and 2- by 4 - i n . r a f t e r s a r r a n g e d a s shown i n Fig. 1 ( a l t h o u g h t h e s u r v e y showed 2 x 6 ' s t o be i n s l i g h t l y w i d e r u s e t h a n 2 x 4 ' s , t h e l a t t e r were used i n t h e t e s t f r a m e s
s i n c e t h e y a r e t h e minimum a l l o w e d ) . A l l lumber used i n t h e t e s t s was No. 1 ( C o n s t r u c t i o n ) E a s t e r n Spruce. I n t h e
c o n s t r u c t i o n of t h e frames a n a t t e n p - t was made t o o b t a i n a v e r a g e workmanship and t o m a i n t a i n it t h r o u g h o u t t h e t e s t program, The most common r o o f s l o p e r e v e a l e d by t h e s u r v e y was 5 i n 1 2 ; f r a m e s w i t h t h i s s l o p e were t h e r e f o r e used i n
Group A. I n Group B , frames w i t h s l o p e s of 3 i n 1 2 , 4 i n 1 2 , 6 i n 1 2 and
7
i n 1 2 , were used. To a l l o w a comparison of r e s u l t s t h e f r a m e s of Groups C and D had t h e same s l o p e s a s t h o s e of Group A .N a i l i n g a t t h e j o i n t s of a l l frames was determined by s e v e r a l g u i d e s and i s d e s c r i b e d i n d e t a i l i n T a b l e s I A , I B
and I C . With t h e e x c e p t i o n of t h e r a f t e r - r i d g e , a l l j o i n t s were f a s t e n e d w i t h 3$-in. common w i r e n a i l s . 1.s a s t a r t it was decided t h a t Group A f o l l o w t h e n a i l i n g s c h e d u l e g i v e n i n t h e NBC (1953) and i n t h e Housing S t a n d a r d s f o r frame con- s t r u c t i o n f o r a l l c o m ~ e c t i o n s e x c e p t t h e t h r o u g h n a i l i n g from j o i s t t o r a f t e r a t -the h e e l j o i n t . Por Group A t h i s t h r o u g h n a i l i n g v a r i e d from z e r o t o e i g h t n a i l s . A s t h e number of t h r o u g h n a i l s a t t h e h e e l I - n c r c a s e d , i t mas n e c e s s a r y t o i n c r e a s e a l s o t h e t h r o u g h n a i l i n g a t t h e o t h e r j o i n t s ( j o i s t s p l i c e and c o l l a r - r a f t e r j o i n t ) . A b a l a n c e d n a i l i n g was n o t s t r i v e d f o r , however, and t h e n a i l i n g a t t h e s e j o i n t s was made s t r o n g e r t h a n t h a t a t t h e h e e l t o have f a i l u r e o c c u r
f i r s t a t t h e h e e l j o i n t .
The n a i l i n g r e q u i r e d t o r e s i s t 100 psf was d e t e r - mined. f o r Group
B
( d i f f e r e n t roof s l o p e s ) by t h e r e s u l t s of t h e Group A t e s t s and by t r i a l and e r r o r . S i m i l a r l y f o r Group C (unbalanced l o a d i n g ) t h e j o i s t s p l i c e and c o l l a r - r a f t e r n a i l i n g was based. on t h e Group A r e s u l t s w h i l e t h e t h r o u g h n a i l i n g a t t h e h e e l was a r b i t r a r i l y made 2 , 4 and 6 n a i l s . The n a i l i n g of t h e Group D ( l o n g - t e r m l o a d i n g ) frames was based e n t i r e l y on t h e Group A r e s u l t s . I n a l l t e s t s t h et o e n a i l i n g r e q u i r e d by t h e NBC (1953) n a i l i n g schedule was used, i . e . two n a i l s from j o i s t t o p l a t e and t h r e e n a i l s from r a f t e r t o p l a t e a t t h e e x t e r i o r w a l l and two n a i l s f r o m j o i s t t o p l a t e a t t h e i n t e r i o r p a r t i t i o n .
TEST ARRANGEMENT ( a ) T e s t Method
Two methods were used f o r t e s t i n g t h e roof frames:
i n
p a i r s and s i n g l y .( i ) Tested i n P a i r s .
-
This i s t h e same method used i n previous t e s t s ( 3 , 4 , 5 ) . I n t h e p r e s e n t s t u d y it was used f o r t h e f i r s t t h r e e t e s t s of GroupA
only,i . e . f o r t h o s e frames w i t h t o e n a i l i n g b u t no
through n a i l i n g a t t h e h e e l j o i n t . The use of t h i s method was made n e c e s s a r y by t h e s e p a r a t i o n r e q u i r e d a t t h e h e e l j o i n t between t h e j o i s t s and r a f t e r s f o r proper t o e n a i l i n g . B r i e f l y it c o n s i s t e d of a p p l y i n g a l o a d e q u a l l y t o two roof frzmes, t h e r a f t e r s of which were c e n t r e d 16 i n . a p a r t and covered w i t h s h e a t h i n g . Loads were a p p l i e d by means of eigh% s h o r t beams spanning between t h e frames and loaded a t t h e i r mid-points by e i g h t h y d r a u l i c 'acks connected t o a common o i l l i n e
( F i g s 2,3a
3
.
( i i ) Tested Singly.
-
T h i s method, which had t h e advantage of r e q u i r i n g only one frame p e r t e s t and t h u s was r e l a t i v e l y quick and economical, was used f o r a l l t e s t s i n which t h e r e was through n a i l i n g from t h e r a f t e r s t o t h e j o i s t s . I t r e q u i r e d a l o a d i n g r i g which c o n s i s t e d of e i g h t l e v e r arms pin-connected t o t h e l o a d i n g r i g a t one end, b e a r i n g on t h e roof frame a t t h e o t h e r , and loaded a t t h e i r mid-spansby e i g h t h y d r a u l i c jacks ( P i g s 2 , 3 b ) . With t h i s , arrangement h a l f of t h e downward p u l l of each jack was a p p l i e d through each l e v e r arm t o t h e r a f t e r s
of t h e roof frame. The l e v e r arms were l o c a t e d a t d i f f e r e n t h e i g h t s corresponding t o t h e s l o p e of t h e roof and were kept approximately h o r i z o n t a l d u r i n g each t e s t . Wooden wedges of t h e same a n g l e a s t h e roof s l o p e were placed between t h e l e v e r arms and t h e r a f t e r s t o provide a h o r i z o n ? a l b e a r i n g s u r f a c e , Besides l o a d i n g t h e frames t h e l e v e r arms were used t o provide l a t e r a l s u p p o r t t o t h e r a f t e r s and t h u s avoid l a t e r a l buckling (prevented i n r o o f s by t h e
s h e a t h i n g ) . T h i s was accomplished by extending,
f r o m t h e l e v e r arms, wooden b l o c k s which s t r a d d l e d t h e r a f t e r s .
Since t h e h e e l j o i n t and t h e j o i s t s p l i c e were " e c c e n t r i c " connections, t h e y introduced a couple through t h e t o e n a i l s i n t o t h e r e s p e c t i v e p l a t e s . These couples were counteracted i n t h e frames t e s t e d s i n g l y by extending t h e p l a t e s i n one d i r e c t i o n and by pin-connecting t h e long ends t o t h e loading r i g . By extending t h e p l a t e it was p o s s i b l e t o reduce t h e h o r i z o n t a l f o r c e r e s i s t i n g t h e couple.
For t h e frame t e s t e d s i n g l y i t was a l s o necessary t o prevent buckl.ing of t h e c o l l a r t i e when t h e frame s t r e n g t h increased beyond approximately 50 p s f .
This i s f u r t h e r evid.ence t h a t t h e c o l l a r t i e should i n f a c t be c a l l e d a c o l l a r s t r u t . A s t h e c o l l a r t i e always buckled i n t h e same d i r e c t i o n because o f
i t s e c c e n t r i c connection t o t h e r a f t e r s , it was necessary t o brace it i n one d i r e c t i o n only. This was done by i n s e r t i n g a s t r u t between t h e c o l l a r t i e and t h e loading r i g . Because t h e frames t e s t e d i n p a i r s f a i l e d a t l e s s t h a n 50 p s f , it was n o t necessary t o brace t h e i r c o l l a r t i e s .
Both methods had s e v e r a l f e a t u r e s i n common. Besides having t h e roof loads a p p l i e d by means of e i g h t h y d r a u l i c
jacks connected t o a common o i l l i n e , both methods had a c e i l i n g load of 10 psf simulated by l e a d - f i l l e d bags p i l e d along t h e j o i s t s .
Support f o r t h e frames was provided a t t h r e e p o i n t s i n both methods. F r i c t i o n between t h e p l a t e and t h e supporting concrete block held one h e e l j o i n t i n a f i x e d p o s i t i o n . The o t h e r h e e l j o i n t was mounted on a r o l l e r support and was f r e e t o move i n t h e h o r i z o n t a l d i r e c t i o n p a r a l l e l t o t h e frame. The t h i r d support, a l s o a r o l l e r support, was provid.ed a t t h e j o i s t s p l i c e a t t h e mid-span of t h e frame.
I n both methods, d e f l e c t i o n s were measured a t t h e same l o c a t i o n s and i n t h e same way. Dial gauges were used t o measure t h e h o r i z o n t a l movements of t h e p l a t e s a t t h e two h e e l j o i n t s . The v e r t i c a l d e f l e c t i o n of t h e r i d g e was
i n d i c a t e d on a s c a l e by a weight hanging on a wire from t h e r i d g e . The d e f l e c t i o n of t h e r a f t e r s was measured by means of a t a u t wire running f r o m r i d g e t o h e e l , t h e movement of t h e mid-span of t h e r a f t e r perpendicular t o t h e wi're being t h e d e f l e c t i o n of t h e r a f t e r r e l a t i v e t o i t s ends. During t h e l a t t e r t e s t s i n Group A , it was n o t i c e d t h a t t h e j o i s t s
were d e f l e c t i n g because of t h e end moments induced i n them by t h e h e e l j o i n t s . I n subsequent t e s t s , t h e r e f o r e , t h e d e f l e c t i o n of t h e mid-spans of t h e j o i s t s were a l s o measured
( w i t h d i a l gauges).
The frames t h a t were t e s t e d under unbalanced load and long-term load were t e s t e d s i n g l y i n t h e loading r i g .
Unbalanced l o a d i n g was accomplished by d i s c o n n e c t i n g t h e jacks on one s i d e of t h e frame and a l l o w i n g t h e jacks on t h e o t h e r s i d e t o a c t a s usual. A l o a d of 1 0 psf was a p p l i e d by means of l e a d - f i l l e d bags t o t h e s i d e n o t loaded by jacks
i n o r d e r t o s i m u l a t e more r e a l i s t i c a l l y a c t u a l snow l o a d i n g c o n d i t i o n s , c o n s i s t i n g of a l i g h t l o a d on one s i d e and a heavy load on t h e o t h e r . Long-term l o a d i n g was a p p l i e d by d i s c o n n e c t i n g a l l jacks and by a p p l y i n g t h e f u l l l o a d w i t h l e a d - f i l l e d bags t o t h e frame i n t h e l o a d i n g r i g .
( b ) T e s t Procedure
I n t h e t e s t s , each roof frame was loaded u n t i l it
f a i l e d completely, o r u n t i l t h e d e f l e c t i o n s were s o l a r g e a s
t o prevent f u r t h e r loading. Loads were a p p l i e d t o t h e frames i n increments of 1 0 p s i o i l p r e s s u r e which, by c o i n c i d e n c e ,
i s n e a r l y equal t o 1 0 l b / f t of span o r 6.7 psf w i t h frames 16 i n . O.C. A f t e r every t h r e e increments of 1 0 p s i , t h e o i l
p r e s s u r e was reduced t o z e r o then i n c r e a s e d t o t h e previous maximum and t h e 1 0 p s i increments resumed. (The l i v e l o s d on t h e frames was n o t reduced completely t o z e r o because of t h e f r i c t i o n i n t h e jacks and t h e weight of t h e l e v e r arms.) The l o a d i n g c y c l e , t h e r e f o r e , was a s f o l l o w s :
0 + , 10, 20, 30, O + , 30, 40, 50, 60, 0+, 60, 70
....
l b / f t . The c e i l i n g l o a d of 1 0 psf was a p p l i e d b e f o r e anyroof load and was l e f t c o n s t a n t throughout t h e t e s t . Deflec- t i o n s a t a l l p o i n t s were measured a t z e r o load and a f t e r every increment of load.
Many of t h e frames t e s t e d were loaded t o f a i l u r e more than once. A f t e r t h e frames were loaded f o r t h e f i r s t time and t h e weakest j o i n t had f a i l e d , t h e f a i l e d j o i n t was r e p a i r e d and strengthened w i t h e x t r a n a i l s and clamps. The frames were t h e n reloaded and t h e f a i l u r e of t h e second weakest j o i n t observed. I n a few c a s e s t h i s procedure was c a r r i e d t o t h e t h i r d weakest j o i n t . Sometimes second o r t h i r d l o a d i n g s were prevented by f a i l u r e of a member. TEST RESULTS
( a ) S p e c i f i c R e s u l t s
Grou A
-
T e s t s on t h e frames of Group A i n d i c a t e d t h a t t h e u tlma e s h o r t - t e r m l o a d - c a r r y i n g c a p a c i t y of t h e h e e l j o i n t s-I+€
i n c r e a s e d from 19.5 psf f o r no through n a i l i n g t o 1 2 1 psf f o r e i g h t through n a i l s . The i n c r e a s e was l i n e a r and a t a r a t e of approximately 12.8 psf p e r n a i l . The i n d i v i d u a l t e s t r e s u l t s i n d i c a t i n g t h i s a r e given i n Tabl-e I A and a r e g r a p h i - c a l l y summarized i n F i g . 4.
The h e e l j o i n t n a i l i n g r e q u i r e d t o r e s i s t a roof load of 100 psf w i t h frames a t 16 i n . 0.c. was found t o be
s i x t o seven through n a i l s . E x t r a p o l a t i o n o f t h e r e l a t i o n - s h i p shown i n Fig. 4 t o t h e r e s i s t a n c e r e q u i r e d a t 24-in.
s p a c i n g f o r t h e same roof s t r e n g t h , i n d i c a t e s t h a t t e n through n a i l s would be needed.
-
R e s u l t s of t h e t e s t s on frames of Group B i n d i c a t e d t h a t o r e s i s t a given roof l o a d , t h e number of through n a i l s=v
a t t h e h e e l j o i n t must i n c r e a s e a s t h e s l o p e of t h e roof
d e c r e a s e s (Table I B ) , The r e l a t i o n s h i p i s n o t l i n e a r , however, a s shown by Fig.
5.
Although t h e extremes o f t h e r e l a t i o n s h i p were n o t i n v e s t i g a t e d , it i s l o g i c a l t o assume t h a t t h e y a r e l i m i t e d by p r a c t i c a l c o n s i d e r a t i o n s . On t h e one hand t h e r e d u c t i o n i n t h e number of n a i l s r e q u i r e d a s t h e s l o p e i n c r e a s e s i s l i m i t e d because o t h e r l o a d s such a s wind may govern; on t h e o t h e r hand t h e maximum number of n a i l s i sl i m i t e d by t h e a v a i l a b l e n a i l i n g a r e a , With t h e s e assumptions, t h e r e f o r e , it seems probable t h a t t h e n a i l i n g of frames w i t h s l o p e s g r e a t e r t h a n 8 o r 9 i n 12 i s governed by o t h e r l o a d s and t h a t frames w i t h s l o p e s of
3
i n 12 o r l e s s r e q u i r e a p r o h i b i t i v e number of n a i l s ,Group C
-
The r e s u l t s of t h e t e s t s on t h e frames of Group Cwere somewhat unexpected, b u t l o g i c a l . I t was found t h a t t h e u n i t load which caused c o l l a p s e o f a frame loaded on one
s i d e o n l y , was approximately 1.5 times g r e a t e r t h a n t h e u n i t l o a d which c o l l a p s e d a n i d e n t i c a l frame loaded on b o t h s i d e s
(Table 11). The f i g u r e s i n d i c a t i n g t h i s a r e given i n Table I C and by t h e accompanying comparison w i t h t h e v a l u e s from Table I A , For example, a frame with two through n a i l s a t t h e h e e l j o i n t f a i l e d a t a n average of approximately 45 psf of balanced load compared w i t h approximately 60 psf of
unbalanced l o a d . I t w i l l be a p p a r e n t t h a t t h i s r e s i s t a n c e of t h e frames t o unbalanced l o a d i n g i s due almost e n t i r e l y t o t h e c o l l a r t i e a c t i n g a s a s t r u t and t r a n s f e r r i n g p a r t of t h e load from t h e d i r e c t l y loaded r a f t e r t o t h e unloaded
r a f t e r . This r e s i s t a n c e t o unbalanced l o a d i n g i s a d e s i r a b l e f e a t u r e when viewed i n t h e l i g h t of how snow t e n d s t o
accumulate on p i t c h e d r o o f s .
Grou D
-
Two frames were t e s t e d i n t h i s group and o n l y one e s e y i e l d e d v a l i d r e s u l t s . This frame, which had n a i l i n g+
(determined from Group A ) t o r e s i s t 100 p s f , was loaded t o h a l f t h a t v a l u e f o r almost f i v e months, and d u r i n g t h a t time showed no v i s i b l e signs of f a i l u r e .
A t t h e end of t h e five-month l o a d i n g p e r i o d , t h e frame was unloaded and a s h o r t - t e r m l o a d i n g t e s t t o f a i l u r e was
c a r r i e d out. The frame c o l l a p s e d a t a l o a d of 100 psf which was e x a c t l y t h e l o a d f o r which it was n a i l e d . This would i n d i c a t e t h a t t h e long-term l o a d i n g had no n o t i c e a b l e e f f e c t on t h e s t r e n g t h of t h e frame.
The o t h e r frame which was t e s t e d i n Group D c o l l a p s e d prematurely a s t h e r e s u l t of a n e r r o r i n judgment. It had been thought, on t h e b a s i s of previous t e s t s , t h a t t h e 50-psf l o a d could be a p p l i e d without buckling t h e unbraced c o l l a r t i e . This was n o t t h e c a s e , however, f o r t h e t i e buckled while t h e load was being a p p l i e d . The l o a d i n g was t h e n stopped, t h e c o l l a r t i e was braced, and t h e l o a d i n g was completed. Some damage had been done, however, f o r t h e frame c o l l a p s e d w i t h i n t h r e e days, a p p a r e n t l y t h e r e s u l t of t h e f a i l u r e of t h e c o l l a r - r a f t e r connection where t h e n a i l s had been p u l l e d s l i g h t l y
when t h e c o l l a r buckled. Perhaps t h e l e s s o n t o be l e a r n e d from t h i s i s t h e confirmation t h a t even a s l i g h t s e p a r a t i o n between t h e members a t a e n a i l e d j o i n t e f f e c t i v e l y d e c r e a s e s t h e s t r e n g t h of t h a t j o i n t . ( b ) General R e s u l t s S t r e n g t h of J o i s t S p l i c e Many of t h e t e s t s y i e l d e d u s e f u l i n f o r m a t i o n on j o i n t s o t h e r t h a n t h e h e e l j o i n t . Frequent f a i l u r e s of t h e j o i s t s p l i c e s , f o r example, give a n i n d i c a t i o n of t h e v a r i a t i o n of t h e f a i l u r e load of t h e frame w i t h t h e number of through n a i l s a t t h e s p l i c e . When compared w i t h t h e r e l a t i o n s h i p between frame s t r e n g t h and t h e number of through n a i l s a t t h e h e e l j o i n t , a s i s done i n Fig. 6 , t h i s v a r i a t i o n i n d i c a t e s t h a t t h e number of through n a i l s r e q u i r e d a t t h e j o i s t s p l i c e exceeds by one t o two n a i l s t h e number r e q u i r e d a t t h e h e e l
j o i n t f o r t h e same frame s t r e n g t h . T h i s i n d i c a t i o n was i n v e s t i g a t e d i n t h e l a s t seven t e s t s of Group A . I t was found t h a t i n f i v e c a s e s one a d d i t i o n a l n a i l was s u f f i c i e n t t o balance t h e s t r e n g t h of t h e s p l i c e w i t h t h a t of t h e h e e l , and t o b r i n g it w i t h i n
5
p e r c e n t i n t h e o t h e r two cases. One n a i l r a t h e r t h a n two would seem t o be s u f f i c i e n t when it i s remembered t h a t i n a completed house it i s p o s s i b l e t h a t t h e s h e a t h i n g a c t i o n of t h e c e i l i n g w i l l t e n d t o reduce t h e f o r c e on t h e j o i s t s p l i c e s .There a r e two a p p a r e n t r e a s o n s why more through n a i l s a r e r e q u i r e d a t t h e j o i s t s p l i c e t h a n a t t h e h e e l j o i n t i n o r d e r t o have balanced s t r e n g t h . The f i r s t i s t h e d i f f e r e n c e i n t o e n a i l i n g . A t t h e h e e l j o i n t two t o e n a i l s h e l p t o t r a n - s f e r t h e t h r u s t , whereas a t t h e j o i s t s p l i c e o n l y one t o e
n a i l a s s i s t s . Prom t h e t e s t s of t h e f i r s t t h r e e frames of Group A , i t can be shown t h a t t h i s d i f f e r e n c e of one t o e n a i l accounts f o r approximately 10 psf o r roughly h a l f t h e d i f f e r e n c e
i n t h e s t r e n g t h of a h e e l and s p l i c e w i t h e q u a l through n a i l i n g . The o t h e r f a c t o r c o n t r i b u t i n g t o t h e d i f f e r e n c e I n s t r e n g t h i s t h e l a r g e d i f f e r e n c e between t h e f r i c t i o n f o r c e developed between t h e j o i s t and p l a t e a t t h e h e e l , and t h a t developed a t t h e s p l i c e . A t t h e h e e l , t h e normal f o r c e c a u s i n g f r i c t i o n
which i s t r a n s f e r r e d t o t h e j o i s t by t h e t h r o u g h n a i l s , whereas a t t h e s p l i c e t h e normal f o r c e i s t h a t r e s u l t i n g from t h e weight of t h e c e i l i n g . I n a completed house o t h e r f a c t o r s may a l s o i n f l u e n c e t h e d i f f e r e n c e i n s t r e n g t h , b u t t h e i r c o n t r i b u t i o n w i l l u s u a l l y be s m a l l . S t r e n g t h of C o l l a r - R a f t e r J o i n t Some i n f o r m a t i o n was a l s o g a i n e d on t h e c o l l a r " t i e " and i t s c o n n e c t i o n t o t h e r a f t e r s . A s i n t h e t e s t s c a r r i e d o u t p r e v i o u s l y on s i m i l a r f r a m e s , it was found t h a t e x c e p t a t l o a d s n e a r f a i l u r e when l a r g e d e f o r m a t i o n s had o c c u r r e d , t h e c o l l a r d i d ' n o t a c t a s a t i e b u t a s a s t - r u t , t h u s making it l i a b l e t o b u c k l i n g . T h i s was t o be expected w i t h 2- by 4 - i n . r a f t e r s and it was n o t s u r p r i s i n g t h a t
unbraced 2- by 4 - i n . c o l l a r s buckled a t a p p r o x i m a t e l y 50 p s f . On t h e o t h e r hand 2- by 4-in. c o l l a r s which were b r a c e d a t t h e i r mid-points d i d n o t b u c k l e u n d e r l o a d s of up t o 1 2 0 p s f . When f a i l u r e d i d o c c u r i n t h e c o l l a r , it was a t t h e c o l l a r - r a f t e r c o n n e c t i o n . Only a few f a i l u r e s o c c u r r e d a t t h i s j o i n t , however, and a s a r e s u l t no g e n e r a l c o n c l u s i o n s c a n be drawn; it c a n be s a i d , however, t h a t t h r e e t h r o u g h n a i l s p r o v i d e a n a d e q u a t e j o i n t i n f r a m e s w i t h f o u r o r f e w e r t h r o u g h n a i l s a t t h e h e e l j o i n t . S i u i l a r l y it a p p e a r s t h a t f i v e n a i l s a r e adequate' when seven a r e used a t t h e h e e l . D e f l e c t i o n s Although t h e prime o b j e c t i v e i n t e s t i n g t h e f r a m e s was t o determine t h e i r s t r e n g t h c h a r a c t e r i s t i c s , c o n s i d e r a b l e c a r e was a l s o t a k e n t o measure t h e i r d e f l e c t i o n s . S i n c e t h e d e f l e c t i o n c h a r a c t e r i s t i c s of t h e frames a r e l a r g e l y dependent on t h e j o i n t s ( 3 ) , and s i n c e t h e n a i l i n g i n some of t h e j o i n t s was v a r i e d f o r d i f f e r e n t r e a s o n s , few c o n c l u s i o n s c a n be drawn from t h e p r e s e n t work. General i n d i c a t i o n s were o b s e r v e d , however, such a s t h o s e shown i n Fig. 7 where t h e d e f l e c t i o n
of t h e mid-span of t h e r a f t e r s i s p l o t t e d a g a i n s t l o a d f o r t h e frames of Group A. F i g u r e 7 shows t h a t t h e d e f l e c t i o n . a t a g i v e n l o a d d e c r e a s e s a s t h e number of n a i l s a t t h e h e e l and o t h e r j o i n t s i n c r e a s e s . T h i s f i g u r e f u r t h e r shows t h a t t h e d e c r e a s e p e r n a i l i s n o t e q u a l , and t h a t t h e a d d i t i o n of n a i l s i s a p p a r e n t l y s u b j e c t t o a "law of d i m i n i s h i n g r e t u r n s , e q which c a n be p a r t l y a t t r i b u t e d t o t h e i n c r e a s e d c o n t r i b u t i o n of t h e members t o t h e d e f l e c t i o n a s t h e j o i n t s become s t r o n g e r . A more p r a c t i c a l comparison of t h e d e f l e c t i o n s c a n be made on t h e b a s i s of d e f l e c t i o n s a t d e s i g n o r working l o a d s . I f , f o r t h e sake of comparison, it i s assumed t h a t a l o a d f a c t o r of 2 i s a d e q u a t e f o r r o o f f r a m e s , t h e n t h e d e s i g n l o a d s f o r t h e frames t e s t e d a r e e q u a l t o t h e f a i l u r e l o a d s d i v i d e d by 2. Thus t h e d e s i g n l o a d f o r a frame w i t h z e r o t h r o u g h n a i l s a t t h e h e e l which f a i l e d a t 20 p s f i s 1 0 psf
( p u r e l y a n academic v a l u e ) . A t t h i s l o a d t h e d e f l e c t i o n of t h e mid-spans of t h e r a f t e r s r e l a t i v e t o t h e i r ends i s l e s s t h a n 1/3000 of t h e span. Such a low v a l u e r e s u l t s because t h e members a r e o n l y s l i g h t l y s t r e s s e d owing t o weak j o i n t s . S i m i l a r l y f o r one, two, t h r e e , and up t o e i g h t t h r o u g h n a i l s d e f l e c t i o n a t working l o a d s can be determined. A comparison of t h e s e d e f l e c t i o n s shows t h a t t h e y i n c r e a s e from l e s s t h a n 1/3000 of t h e span f o r z e r o t h r o u g h n a i l s t o a p p r o x i m a t e l y 1/400 of t h e span f o r e i g h t n a i l s . Even a t t h i s , however, t h e d e f l e c t i o n i s n o t e x c e s s i v e , b e i n g below t h e u s u a l l y a c c e p t e d
l i m i t of 1/360 of t h e span.
D e f l e c t i o n s of' t h e r a y t e r s i n Groups
b
and C a r e a l s o worth c o n s i d e r i n g a l t h o u g h few r e s u l t s a r e a v a i l a b l e on which t o base any c o n c l u s i o n s . B r i e f l y , it can be s a i d t h a tf o r frames w i t h d i f f e r e n t s l o p e s , a g a i n assuming a l o a d f a c t o r of 2 , t h e d e f l e c t i o n s a t working l o a d s i n c r e a s e d a s t h e s l o p e d e c r e a s e d . F o r example, r a f t e r s of frames of 7
i n
1 2 it was 1/750, a t 4 i n 1 2 it was 1/52T and a t 3 i n 1 2 it
was 1/370, none of which i s e x c e s s i v e . Under unbalanced l o a d e q u a l t o %he d e s i g n l o a d of t h e c o r r e s p o n d i n g frames of Group A , t h e r a f t e r s of t h e Group C frames d e f l e c t e d a t l e a s t t w i c e a s much a s t h e r a f t e r s of t h e Group A f r a m e s , i . e . t h e i r d e f l e c t i o n s were a s l a r g e a s 1/200 of t h e span and e x c e s s i v e on t h e b a s i s of t h e 1/360 c r i t e r i o n . The outward d e f l e c t i o n of t h e unloaded r a f t e r was n o t e x c e s s i v e , however, because of t h e s l i p p a g e of t h e c o l l a r - r a f t e r c o n n e c t i o n .
A s would be expected f o r t h e frames of Group D, d e f l e c t i o n s were s i g n f i c a n t l y l a r g e r t h a n t h o s e o c c u r r i n g under s h o r t - t e r m l o a d i n g . From Table I D it can be s e e n t h a t d u r i n g t h e first- 24 h o u r s uncler l o a d , t h e d e f l e c t i o n of t h e r i d g e and s u p p o r t s r o u g h l y doubled, t h a t of t h e r a f t e r s t r i p l e d , and t h a t of t h e j o i s t s changed o n l y s l i g h t l y . A f t e r 24 h o u r s , however, t h e r a t e of d e f o r m a t i o n was much l e s s , and a f t e r one week t h e d e f l e c t i o n s of t h e r i d g e and s u p p o r t s were l e s s t h a n t h r e e t i m e s t h a t f o r t h e s h o r t - t c m l o a d i n g , t h a t of t h e r a f t e r s l e s s t h a n f o u r t i m e s , and t h a t of t h e j o i s t o n l y s l i g h t l y o v e r 1 . 5 t i m e s . S i m i l a r l y a t f o u r weeks and a t e i g h t weeks a d e c r e a s i n g r a t e of d e f o r m a t i o n was i n d i c a t e d by t h e d e f l e c t i o n f i g u r e s . On t h e whole t h e d e f l e c t i o n f i g u r e s a r e i n g e n e r a l agreement w i t h t h o s e determined by A. T. Hansen
( 5 ) ,
t h u s i n d i c a t i n g t h e t e s t t o be r e l a t i v e l y f r e e from e x p e r i m e n t a l e r r o r . I t c a n be s a i d , t h e r e f o r e , t h a t t h e frame had t o l e r a b l e d e f l e c t i o n c h a r a c t e r i s t i c s u n d e r long-term l o a d i n g . The d e f l e c t i o n o c c u r r i n g d u r i n g t h es h o r t - t e r m t e s t of t h e Group D frame were somewhat d i f f e r e n t from t h o s e of t h e same frames i n Group A . A t low l o a d s
( 0 t o
45
p s f ) , t h e d e f l e c t i o n s were l a r g e r , bu-t a t t h e h i g h e r l o a d s t h e y tended t o be s u b s t a n t i a l l y t h e same. T h i s would i n d i c a t e t h a t t h e long-term l o a d i n g d e c r e a s e d t h e r i g i d i t y of t h e frame i n t h e l o a d range below t h e long-term l o a d .C e r t a i n t r e n d s were observed i n t h e d e f l e c t i o n s of r i d g e s of frames i n each group. I n Group A frames t h e r e was
a tendency f o r t h e frames w i t h t h e h e a v i e s t n a i l i n g t o have l a r g e r d e f l e c t i o n s a t a d e s i g n l o a d e q u a l t o h a l f t h e f a i l u r e l o a d . On t h e o t h e r hand frames of Group C ( s u b j e c t e d t o
unbalanced l o a d ) showed l i t t l e v a r i a t i o n w i t h n a i l i n g . Frames w i t h d i f f e r e n t s l o p e s showed i n c r e a s e d d e f l e c t i o n s a s t h e
s l o p e was reduced. D e f l e c t i o n s under long-term l o a d i n g were t h r e e t o f o u r t i m e s t h a t f o r s h o r t - t e r m loading.
D e f l e c t i o n s of t h e j o i s t s which were measured when t h e j o i s t s were d e f l e c t i n g a p p r e c i a b l y d i d n o t prove t o be e x c e s s i v e . They approached t h e allowable l i m i t of 1/360 of t h e span o n l y n e a r t h e f a i l u r e l o a d s of t h e frames w i t h low s l o p e s .
CONCLUSIONS
Prom t h e s e r i e s of s h o r t - t e r m t e s t s d.escribed i n t h i s r e p o r t s e v e r a l d e f i n i t e c o n c l u s i o n s can be drawn:
1. The failure load of c o n v e n t i o n a l roof frames of t h e t y p e t e s t e d w i t h 5 i n 12. s l o p e
( a ) i s approximately 20 psf if n a i l e d a t t h e h e e l j o i n t w i t h o n l y t h e t o e n a i l i n g r e q u i r e d by t h e NBC (1953) o r t h e Housing Standards
( b ) i n c r e a s e s by a n average of 1 3 psf f o r each through n a i l a t t h e h e e l j o i n t .
2. The number of through n a i l s r e q u i r e d a t t h e h e e l j o i n t t o provide a given s t r e n g t h d e c r e a s e s a s t h e s l o p e of
t h e frames i n c r e a s e s . For a l o a d of 100 psf t h e r e l a t i o n - s h i p i s approximately a s shown i n Pig. 5.
3. Conventional frames of t h e type t e s t e d can s a f e l y s u p p o r t an unbalanced l o a d which i s roughly 1 . 5 times l a r g e r p e r square f o o t t h a n t h e load which can be c a r r i e d uniformly by t h e roof.
S e v e r a l o t h e r c o n c l u s i o n s were i n d i c a t e d by t h e r e s u l t s b u t t h e s e w i l l r e q u i r e c o n f i r m a t i o n by f u r t h e r t e s t s :
1. Conventional roof frames of t h e type t e s t e d w i t h
5
i n 12 s l o p e , r e q u i r e one more through n a i l a t t h e j o i s t s p l i c e t h a n a t t h e h e e l j o i n t i n o r d e r t o have balanced s t r e n g t h .2. When used i n t h e type of. frames t e s t e d , unbraced c o l l a r t i e s w i l l buckle a t a roof l o a d of approximately 50 p s f . 3 . I n a few c a s e s t h e d e f l e c t i o n of t h e mid-span of t h e
r a f t e r s becomes e x c e s s i v e o r c r i t i c a l a t t h e d e s i g n o r working load based on a load f a c t o r of 2.
I n c o n s i d e r i n g t h e s e c o n c l u s i o n s it should be remembered t h a t t h e t e s t s were c a r r i e d o u t under s h o r t - t e r m l o a d i n g w i t h a n a r b i t r a r y r a t e of l o a d a p p l i c a t i o n .
IMPLICATIONS
Considered w i t h t h e r e s u l t s of t h e p r e v i o u s l y
mentioned b r i e f f i e l d survey of roof frames, t h e r e s u l t s of
.
t h e t e s t s d e s c r i b e d i n t h i s r e p o r t r a i s e a s i g n i f i c a n t q u e s t i o n . This s u r v e y , which was c a r r i e d out t o determine t h e mostcommon type of c o n v e n t i o n a l roof frame b e i n g used a t p r e s e n t , a l s o i n v e s t i g a t e d t h e n a i l i n g used i n t h e s e frames. I n a sample o f 33 r o o f s it was found t h a t 1 0 p e r c e n t had no through n a i l i n g a t t h e h e e l j o i n t , 40 p e r c e n t had one
through n a i l , and 50 p e r c e n t had two through n a i l s . Assuming t h a t t h i s i s r e p r e s e n t a t i v e of most r o o f s and on t h e b a s i s of t h e p r e s e n t t e s t r e s u l t s , t h i s means t h a t 10 p e r c e n t of t h e r o o f s would a p p a r e n t l y f a i l a t 20 psf w i t h frames 16 i n . O . C . o r 1 3 psf a t 24 i n . o . c . , 40 p e r c e n t would f a i l a t 34 p s f , a t 16 i n . O . C . or.22.5 psf. a t 24 i n . o . c . , and 50 p e r c e n t would f a i l a t 44 psf a t 16 i n . O . C . o r 29.5 psf a t 24 i n . O . C . Despite t h e p r e s e n t d e s i g n l o a d s of 30 t o 50 p s f ,
few c o l l a p s e s have occurred except f o r t h e c o t t a g e f a i l u r e s a l r e a d y mentioned.
T h i s . t h e r e f o r e . Doses t h e a u e s t i o n : "Why have t h e r e n o t been more ~ o l l a p s e s if t h e f a i l u r e l o a d s a r e a s Y l o w a s t h e
-
t e s t r e s u l t s i n d . i c a t e ? "There a r e two p o s s i b l e answers. F i r s t , t h e t e s t r e s u l t s p o s s i b l y do n o t i n d i c a t e t h e a c t u a l s t r e n g t h s of t h e complete r o o f s . Perhaps t h e s h e a t h i n g which c o v e r s t h e
frames provides a d d i t i o n a l s t r e n g t h by a c t i n g a s a deep beam on each s l o p e o r by "folded p l a t e " a c t i o n . Perhaps, t o o , t h e g a b l e s c o n t r i b u t e s i g n i f i c a n t l y t o t h e s t r e n g t h of t h e roof a s may a l s o s o f f i t s and o t h e r t r i m .
The second p o s s i b l e answer i s t h a t snow l o a d s which a c t u a l l y occur on t h e r o o f s may be s u b s t a n t i a l l y l e s s t h a n t h e common 30 t o 50 psf s p e c i f i e d by t h e N a t i o n a l B u i l d i n g Code.
REC OIMENDATI ONS
I n o r d e r t o f i n d o u t whether r o o f s a r e s t r o n g e r t h a n i n d i c a t e d by t h e single-frame t e s t s , o r whether t h e snow l o a d s do n o t i n f a c t r e a c h t h e c u r r e n t design v a l u e s , load t e s t i n g
of f u l l - s i z e house r o o f s , complete w i t h s h e a t h i n g and g a b l e s , would be v e r y u s e f u l and i s t h e r e f o r e recommended.*
Continued t e s t i n g of s i n g l e frames i s a l s o recom- mended i n o r d e r t o answer s e v e r a l o u t s t a n d i n g q u e s t i o n s . For example, f u r t h e r t e s t i n g i s r e q u i r e d t o c l a r i f y t h e n a i l i n g r e q u i r e d a t t h e j o i s t s p l i c e and t h e c o l l a r - r a f t e r connection t o o b t a i n balanced s t r e n g t h . The e f f e c t of moving t h e c o l l a r t i e should a l s o be i n v e s t i g a t e d a l o n g w i t h t h e e f f e c t of
providing d i f f e r e n t i n t e r m e d i a t e s u p p o r t s . The e f f e c t of using d i f f e r e n t member s i z e s i s a l s o u n b o r n , except f o r t h e i n f o r m a t i o n determined by A. T. Hansen.
F i n a l l y , a d d i t i o n a l t e s t s on frames of v a r i o u s s l o p e s i n c o r p o r a t i n g t h e above v a r i a b l e s would be u s e f u l . The aim of t h i s a d d i t i o n a l t e s t i n g would be t h e development of n a i l i n g s c h e d u l e s , which would give t h e n a i l i n g r e q u i r e d a t e v e r y
j o i n t of frames of any s l o p e w i t h any member s i z e s f o r use i n any snow l o a d a r e a .
REFERENC ES
(1) Dorey, D. B. S t r u c t u r a l t e s t i n g of a n experimental house. N a t i o n a l Research Council, D i v i s i o n of B u i l d i n g
Research, I n t e r n a l Report No. 62, Ottawa, January 1956. 6 1 p.
( 2 ) Dorey, D. B. S t r u c t u r a l t e s t i n g of two 1V-trusses. National Research Council, D i v i s i o n of B u i l d i n g Research,
I n t e r n a l Report No. 77, Ottawa, December 1955. 25 p .
( 3 )
Hansen, A. T. Loading t e s t s on conventional and t r u s s e d roof c o n s t r u c t i o n s . N a t i o n a l Research Council,D i v i s i o n of B u i l d i n g Research, I n t e r n a l Report No. 8 1 , Ottawa, May 1958. 50 p.
( 4 ) Hansen, A. T. Loading t e s t s on conventional r o o f . c o n - s t r u c t i o n s (second p r o g r e s s r e p o r t ) . National Research Council, D i v i s i o n of B u i l d i n g Research,
I n t e r n a l Report No. 113, Ottawa, December 1956. 9 p. 65) Hansen, A . T. Loading t e s t s on conventional and t r u s s e d
roof c o n s t r u c t i o n s ( t h i r d p r o g r e s s r e p o r t )
.
N a t i o n a l Research Council, D i v i s i o n of B u i l d i n g Research,I n t e r n a l Report No, 119, Ottawa, June 1957. 47 p.
*
These t e s t s have now been c a r r i e d o u t and t h e i r r e s u l t s w i l lTABLE I A
N A I L I N G AND FAILURE LOADS ( p s f ) OF FRAMES (16 i n . o . c . )
TESTED I N GROUP A
+ I n d i c a t e s t h a t one h e e l j o i n t f a i l e d a t 1 3 p s f , was r e p a i r e d , t h e load r e a p p l i e d and t h e o t h e r h e e l j o i n t f a i l e d a t 1 9 p s f . iw I n d i c a t e s t h a t one h e e l f a i l e d a t 30 p s f , was r e p a i r e d , t h e l o a d r e a p p l i e d and a r a f t e r f a i l e d a t 57 p s f . ( H I 3(H) I n d i c a t e s 3 "Ardox".Helicloid N a i l s . Remarks P l a t e s p l i t a t 28 p s f "Spongy" wood a t s p l i c e I n c i p i e n t f a i l u r e a t s p l i c e a t 93 psf Equal s t r a i n a t a l l j o i n t s up t o fail- Number Heel 0 0 0 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 7 7 7 7 8 8 8 of 34-in. S p l i c e 3 3 3 3 4 3 3 4 4 3 5 5 3 5 6 7 8 8 7 9 1 0 8 8 8 8 9 9 9 N a i l s C o l l a r Heel 3 1 3 ( 1 9 ) + 3 16 3 30 w ? 3 3 4 3 36(44) 3 22 3 4 6 ( 5 5 ) 3 3 4 3 42 3 6 6 3 60 3 67 3
-
3-
3(H) 70 4 73 5(H) 8 3 4(H) 75 4 8 7 5 9 4 5+2(H) 90 5 122 5 118 5 122 8 118 7 126 6 ( H ) 116 8 120 F a i l u r e S p l i c e-
-
-
-
-
-
3 6-
-
44 --
50 6 8 78 64 38-
87-
-
-
-
118-
1 2 1-
-
Loads ( p s f ) C o l l a r-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
79 7 5-
-
-
-
-
-
110-
R a f t e r-
-
57 w-
-
-
-
-
-
-
-
-
6 3 6 8-
-
114 100 9 0-
-
-
-
-
-
-
-
-
TABLE I B
N A I L I N G AND FAILURE LOADS ( p a f ) OF FRAMES (16 i n . o.c.)
I N GROUP B r r Number of 3 3 - i n N a i l a Remarks 7 S p l i c e
7
i n 1 2 Slope C o l l a r F a i l u r e Load ( p a f ) 44
4
-
R a f t e r Heel S p l i c e 6 6 6 6 i n 12 Slope C o l l a r 5 5 5-
-
-
,9 9 91 100-
-
-
6 5 5-
-
-
130 100 95 4 i n 12 Slope 94-
-
7 6 6 128-
-
6 6 6-
-
89 1 1 4-
-
9 9 9 103 90 85 3 i n 12 Slope 1 0 1 0 1 0-
-
-
8 8 8 No j o i n t f a i l u r e s ; maximum load=
72 p s f . 103-
-
1 2 1 3 1 4 1 4 1 3 14-
8 3 79-
-
-
11 11 11-
8 3-
-
-
-
TABLE I C
NAILING AND FAILURF LOADS* ( psf ) OF FRAMES (16 i n . o. c. ) TESTED I N GROUP C
*
Unbalanced l o a d c o n s i s t i n g of a jack l o a d on one s i d e o n l y anda
c o n s t a n t ( l e a d b a g s ) 1 0 p s f l o a d on t h e o t h e r s i d e .
TABLE I D
DEFLECTIONS OF A "100 p s f l ' FRAME UNDER A LOAD OF 50 p s f , t
Number of 33-in. N a i l s F a i l u r e Load ( p s f ) - L H e e l 2 2 2 4 4 4 6 6 6 9 D i a l gauges d i s t u r b e d making f u r t h e r r e a d i n g s i m p o s s i b l e . Remarks I n c i p i e n t h e e l f a i l u r e 56 P b b Both r a f t e r s f a i l e d Both r a f t e r s f a i l e d S p l i c e
3
3
3
5
5
5
7
7
7
D e f l e c t i o n a t { Ridge Support R a f t e r s J o i s t s S h o r t - t e r m Loading 0.27 0 . I 6 0.29 0.19 Long-term Loading 1 0 min 0.38 0.23 0.57 0.155 C o l l a r Heel 24 hr 0.55 0.36 0.865 0.22 R a f t e r-
-
56 95 92 116-
118 139 S p l i c e-
-
56-
-
-
134
-
-
3
3
3
5
5
5
C o l l a r-
-
-
-
-
-
-
-
-
78 77 I-
-
-
-
5
months1 - 1 5
46 1 . 6 3 9 1 wk 0.75 0.425 1 . 1 4 0,315 i 67
7
4 wk 0.92 0.54 1.36 0.39-
-
-
8 wk 1.01 0.561 . 4 5
0 - 4 3TABLE I1
COMPARISON
--
BALANCED VS. UNBALANCED LOADOn t h e b a s i s of t h e s e f i g u r e s it would seem t h a t a roof loaded on one s i d e o n l y can s u p p o r t load i n t e n s i t i e s of approximately 1.5 times t h a t which can be c a r r i e d over t h e whole roof.
No. of N a i l s
2 4 6
Heel F a i l u r e ( p s f ) Avg. Unbalanced Balanced 46,34,42,55 7 0 9 6 8 , 7 5 0 87,94,90 Avg. Balanced Unbalanced 77,78,>56 > 9 2 , > 9 5 , ) 1 1 6 > 1 3 4 , ) 1 1 8 , > 1 3 9
7 7 0 * 3
V4.2
= 7 1 - 5 9 >lol,-
= 1 - 6 12
>*5
=> 1 . 4 5
y~
4. COLLAR T I Ei x
4
RAFTERS I H E E L JOINT JOIST SPLICE 2 4 ' - 0 "FIXED SUPPORT ROLLER SUPPORT ROLLER SUPPORT
F I G U R E I
Figure 3a T e s t arrangement f o r l o a d i n g frames i n p a i r s .
/
LOADING BOOMS L! ! m FRAME BEING PIN CONNECTION LOADED7 -
ii
BAGS BOARD HYDRAULICJACKS ( PULLING DOWN )
FIGURE 3 b
140 0 OBSERVED VALUE 120 AVERAGE V A L U E 100 STRAIGHT- L I N E LL V) APPROXIMATION a
d
8 0a
0 1k
6 0 3 1-
2
4 0 2 0 0 I 2 3 4 5 6 7 8 9 NUMBER OF THROUGH N A I L S A T H E E L J O I N TFIGURE
4
VARIATION OF FAILURE LOAD W I T H T H E NUMBER OF THROUGH NAILS
14
12 OBSERVED VALUES ( P S F 1
0 FOR COMPARISON : REQUIREMENTS OF U.S. FEDERAL HOUSING
ADMINISTRATION 10 8 6 TESTS SHOW 5 IN 12 GOOD 4 FOR 7 0 P S F 2 0 2 4 6 8 10 12 14 16
NUMBER OF THROUGH NAILS AT THE HEEL JOINT
