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Full-scale loading test on a valley and hip roof
NATIONAL RESEARCH COUNCIL CANADA
DIVISION OF BUILDING RESEARCH
FULL-SCALE LOADING TEST ON A VALLEY AND HIP ROOF
by
W. A. Dalgliesh
Internal Report No. 267 of the
Division of Building Research
OTTAWA April 1963
P R E F A C E
T h e s t r e n g t h and adequacy of roof f r a m i n g s y s t e m s in c u r r e n t u s e in Canadian houses have been t h e subjects of r a t h e r intensive s t u d i e s by t h e Division of Building R e s e a r c h . T h e need for an evaluation of t r a d i t i o n a l r a f t e r and joist roof f r a m e s a r o s e p a r t l y in connection with the r e v i s i o n of the National Building Code. T h e Building S t r u c t u r e s Section t e s t e d s e v e r a l full-scale r o o f s a s well a s individual joist and r a f t e r f r a m e s under simulated snow loads up to failure.
Valley and hip r o o f s had not been included in t h i s s e r i e s of t e s t s . In specifying valley r a f t e r and hip r a f t e r s i z e s for the Housing S t a n d a r d s 1963, t h e s e r a f t e r s w e r e s i m p l y r e q u i r e d t o b e 2 in. deeper than t h e common r a f t e r s . Calculations, m a d e t o check the adequacy of t h e r e q u i r e m e n t , showed that u n l e s s the sheathing could b e counted on for a considerable p a r t of the l o a d - c a r r y i n g capacity of t h e valley section, t h e valley r a f t e r deflections would b e excessive. It s e e m e d , t h e r e f o r e , d e s i r a b l e to obtain verification of the p e r f o r m a n c e of the valley r a f t e r b y actual t e s t , provided t h i s could b e done in the limited t i m e that r e m a i n e d b e f o r e publication of the Housing Standards. A single f u l l - s c a l e loading t e s t of a valley and hip roof was accordingly c a r r i e d out t o evaluate t h i s unknown element in i t s s t r u c t u r a l action.
. T h e t e s t was planned jointly b y H. J. T h o r b u r n and
W. A. Dalgliesh, R e s e a r c h Officers in t h e Building S t r u c t u r e s Section. After t h e d e p a r t u r e of H. J. T h o r b u r n f o r post graduate s t u d i e s in England, W. A. Dalgliesh was r e s p o n s i b l e for the execution of the t e s t and t h e completion of this r e p o r t under t h e direction of W, R. S c h r i e v e r , Head of t h e Section.
Ottawa, April 1963
R o b e r t F. Legget, Director.
FULL-SCALE LOADING TEST ON A VALLEY AND H I P ROOF
by
W. A. Dalgliesh
T h e Housing Standards 1963, now Supplement No. 5 to the National Building Code of Canada, r e q u i r e that hip and valley r a f t e r s b e 2 in. deeper than t h e corresponding common r a f t e r s and that, in addition, r i d g e support b e provided in the hip o r valley
section where a t i e cannot b e m a d e between opposite r a f t e r ends. Calculations w e r e m a d e t o check the adequacy of t h i s r e q u i r e m e n t . In t h e s e p r e l i m i n a r y calculations, the s t r e n g t h of the roof f r a m i n g m e m b e r s only (i. e. without the sheathing) was c o n s i d e r e d ; the uniform load on the jack r a f t e r s was a s s u m e d t o b e divided equally between the valley r a f t e r and t h e r i d g e beam. T h i s approach indicated that the bending moment in t h e valley r a f t e r
might b e a s much a s 5 to 10 t i m e s g r e a t e r than in the common r a f t e r in which c a s e t h e 2-in. i n c r e a s e in depth for the valley r a f t e r would b e inadequate.
It was thought, however, that the valley portion of a roof s y s t e m might d e r i v e an additional. amount of s t r e n g t h and s t i f f n e s s f r o m the sheathing, an e f f e c t which is difficult to evaluate theoretically,
i n view of the m a n y complications caused by the yielding of nailed
connections in an actual roof. Consequently, i t was decided to d e t e r m i n e the t r u e s t r e n g t h of a hip and valley roof, due t o both "framing action" and "sheathing action,
"
by conducting a load t e s t on a f u l l - s c a l e roof. Since t h e r e was only a limited period of t i m e within which a change t o the valley r a f t e r r e q u i r e m e n t s could s t i l l be m a d e in the Housing Standards, only one s i m p l e t e s t was c a r r i e d out.DESCRIPTION O F ROOF
An L-shaped roof 28 by 28 ft in plan, with 24-ft s p a n s and 5/12 s l o p e s was constructed in s t r i c t a c c o r d a n c e with the Housing Standards 1963 ( F i g u r e s 1 and 2). R a f t e r s , joists and r i d g e b e a m s w e r e 2 by 6 in. except for the hip and valley r a f t e r s which w e r e
2 by 8 in. At 4-ft i n t e r v a l s , 2- by 4-in. r i d g e s u p p o r t s w e r e provided in the valley section, and knee ,walls w e r e used in place of c o l l a r t i e s w h e r e t h e jack r a f t e r s w e r e too s h o r t t o i n s t a l l a collar t i e a t the mid-point of the r a f t e r span. A l l m e m b e r s w e r e No. 1 (construction)
E a s t e r n Spruce, except f o r t h e sheathing which was No. 2 E a s t e r n S p r u c e and Pine.
T h e p r i m a r y p u r p o s e of t h e t e s t was t o d e t e r m i n e the s t r u c t u r a l action and whether t h e valley r a f t e r would deflect unduly under load; consequently, only the valley, hip and four common r a f t e r s w e r e i n s t r u m e n t e d for deflection readings. In addition, pencil l i n e s w e r e m a r k e d a c r o s s v a r i o u s joints t o indicate whether movement would
t&ke p l a c e during loading.
T e n deflection gauges w e r e used: 5 on the valley r a f t e r , 1 each on t h e c e n t r e of t h e hip and t h e 4 common r a f t e r s ( F i g u r e 3). After each load application, deflection r e a d i n g s w e r e m a d e by a s u r v e y o r ' s l e v e l s e t up outside t h e roof.
T h e load consisted of 8-in. c o n c r e t e blocks applied over the e n t i r e roof s u r f a c e i n l a y e r s consisting of 216 blocks each. Spaces w e r e left between r o w s of blocks to prevent a r c h i n g of t h e load a s the roof deflected. Blocks in the f i r s t l a y e r averaged 47. 5 l b each, while a l l o t h e r s a v e r a g e d 40 lb. B e c a u s e each block contributed load t o an a r e a (on t h e horizontal) of 3. 55 s q ft, t h e f i r s t l a y e r gave a uniform loading of 13. 4 psf, while succeeding l a y e r s added 11. 3 psf each ( F i g u r e 4).
Supports w e r e s e t up under t h e roof s e p a r a t e d f r o m the s t r u c t u r e by gaps of a few inches w h e r e and when they w e r e considered n e c e s s a r y , a s a safeguard against complete collapse of t h e roof under t h e dead weight method of loading used.
OBSERVAT I ONS
T a b l e I l i s t s t h e loads and corresponding deflections of t h e
valley, hip and common r a f t e r s . Generally, the r o o f behaved " e l a ~ t i c a l l y ' ~ up t o 40 psf, but a s the downward deflection of the valley r a f t e r i n c r e a s e d , jack r a f t e r s began t o pull away f r o m the N-S r i d g e b e a m on t h e valley side. As t h e r i d g e beam was supported by kingposts i t could not move downwards with the valley r a f t e r and consequently e i t h e r t h e jack-to -valley r a f t e r connection o r the jack-to-ridge beam connection would have t o
s e p a r a t e ; the s e p a r a t i o n o c c u r r e d at 45 t o 50 psf a t the jack-to-ridge connection.
J a c k r a f t e r s Nos. 2 and 3 ( F i g u r e 3) in the hip section a l s o seemed to be rotating upwards with r e s p e c t to the hip r a f t e r at a load of 45 to 50 p s f .
At a load of 55 to 60 p s f the pulling away of the jack r a f t e r s on the west side of the N-S ridge had increased to a maximum of 3/4 in. At 70 psf the middle portion of the N-S ridge beam ( a 2 by 6) could
be seen bowing slightly to the east. The jack r a f t e r s on the west
side had slipped downwards, and the sheathing had a pronounced bow in the middle ( F i g u r e 7).
Two rows of blocks had been added around the bottom, two rows laid a t the top on the e a s t side of the N-S ridge, and one row completed on the west side of the N-S ridge at the top when failure occurred by slippage of the jack r a f t e r s down the ridge beam. T h e average load over the roof at this t i m e was 70 to 75 psf ( F i g u r e 5).
The r a f t e r s , in the c e n t r a l region especially, on the west side of the N-S ridge had pulled down and out f r o m the ridge. A support was built to catch t h e s e r a f t e r ends and loading was continued up to
80 to 85 psf ( F i g u r e 6). Because i t appeared that t h e r a f t e r ends were moving towards the ridge t o thrust against i t in bearing, the support was removed, readings taken, and then loading was continued.
Before long the north r a f t e r s on the E-W ridge began t o pull away and the roof s u r f a c e began t o settle. Loading was stopped
immediately and blocks w e r e removed t o leave a load of 70 p s f on the roof over the weekend.
Measurements taken on the following Monday showed that r a f t e r s on the west side of the N-S ridge had dropped in some places a s much a s 4 1/4 in. vertically down the ridge at the connection but s t i l l were t h r u s t -bearing on the ridge for a length of 1 1/4 in. ( F i g u r e s 7, 8). This was t r u e of r a f t e r s Nos. 5 and 6 ( F i g u r e 3). R a f t e r s Nos. 1 t o 4 were not bearing but were separated from the ridge by amounts varying from 3/4 in. at r a f t e r No. 1 t o 1/8 in. at r a f t e r No. 4. R a f t e r s Nos. 7 t o 12 ( F i g u r e 3) had shifted downwards by p r o g r e s s i v e l y l e s s e r amounts, and w e r e bearing on the ridge except for r a f t e r s Nos. 10 and 11 which were 1/4 in. f r o m the ridge.
A s i m i l a r situation was found on the north side of the E-W r a f t e r , where r a f t e r s Nos. 2 t o 8 ( F i g u r e 3) had slipped about 4 in. leaving 1 t o 2 in. of bearing on the ridge ( F i g u r e
9).
RESULTS
J a c k r a f t e r s began t o s e p a r a t e f r o m the N-S r i d g e b e a m along t h e w e s t s i d e a t 45 t o 50 psf. At 70 t o 75 psf s o m e of t h e r a f t e r s pulled completely f r e e of the r i d g e b e a m , causing a sudden s e t t l e m e n t of the sheathing; f a i l u r e can b e s a i d to have been r e a c h e d , although complete c o l l a p s e did not occur a t t h i s point. As loading continued, t h o s e r a f t e r s which had pulled f r e e dropped back onto t h e N-S r i d g e b e a m , 2 o r 3 in. below t h e i r o r i g i n a l positions. Loading was stopped a t 80 t o 8 5 p s f a f t e r r a f t e r s on the n o r t h s i d e of the E-W r i d g e had begun t o p u l l away f r o m t h e r i d g e b e a m and t o t a l c o l l a p s e of the roof s e e m e d imminent.
T h e m o s t i m p o r t a n t r e s u l t was that t h e deflections e x p e r i e n c e d by t h e v a l l e y and hip r a f t e r s w e r e not r e a l l y e x c e s s i v e when c o m p a r e d t o t h o s e of t h e four i n s t r u m e n t e d common r a f t e r s ( F i g u r e 9 ) . T h e upper end of t h e v a l l e y r a f t e r was pushed downwards a t o t a l of 0. 9 in. a t t h e r i d g e ; only n e t deflections ( f r o m a s t r a i g h t l i n e joining the ends of the r a f t e r s ) which r e p r e s e n t the v e r t i c a l component of the a c t u a l m o v e m e n t s of t h e gauge points a r e given in F i g u r e
9.
DI SCUSSI ON O F RESULTS
In t h e f i r s t a n a l y s i s of t h e f r a m i n g s y s t e m in t h e valley a r e a , the jack r a f t e r s w e r e c o n s i d e r e d t o span a s s i m p l e b e a m s between the r i d g e and t h e valley r a f t e r . Half t h e load c a r r i e d by e a c h jack r a f t e r was a s s u m e d t o b e t r a n s f e r r e d t o t h e valley r a f t e r which would have caused e x c e s s i v e deflections at working loads.
T h e f u l l - s c a l e t e s t showed that no such e x c e s s i v e deflections o c c u r r e d a t working loads. T h e unexpected s t i f f n e s s of the valley s e c t i o n a p p e a r e d t o be provided by t h e combined action of the sheathing,
jack r a f t e r s and t h e v a l l e y r a f t e r in spanning between t h e h e e l and the peak in the valley s e c t i o n , and can p r o b a b l y b e explained a s follows:
T h e v a l l e y section c o n s i s t e d e s s e n t i a l l y of two right-angled t r i a n g l e s joined along t h e i r hypotenuses by t h e valley r a f t e r . T h e two p l a n e s m e t a t t h e f a i r l y f l a t angle of 148 deg. Sections at r i g h t angles t o t h e valley r a f t e r w e r e V-shaped and varied in depth f r o m 0 ft a t e i t h e r end t o a m a x i m u m of 2 1/3 ft.
As load w a s applied t o t h e p l a n e s of t h i s V-shaped "beam, " p a r t of i t was t r a n s m i t t e d through t h e jack r a f t e r s t o the v a l l e y r a f t e r which deflected v e r t i c a l l y downwards. T h i s caused s t r e s s t o b e applied
in the planes of t h e roof and t h e r e l a t i v e l y high stiffness of t h e s e planes apparently prevented e x c e s s i v e deflection of the valley
r a f t e r . T h e valley section thus acted somewhat like a single-folded plate, the action of which can be visualized a s t h a t of two inclined
deep b e a m s joined together along a bottom "seam. T h e s e two b e a m s b y their interaction w e r e loaded in-plane and gave mutual l a t e r a1 support to each other t o p r e v e n t out -of -plane bending.
The fact that t h e sheathing was continuous back into the gable s e c t i o n s m a y have helped t o c a r r y s o m e of the load into the common r a f t e r s a s well, but i t s e e m s probable that a m a j o r portion of the load was borne d i r e c t l y by the valley section a s d e s c r i b e d above.
CONCLUSIONS
T h e f u l l - s c a l e loading t e s t , c a r r i e d out on a n L-shaped valley-and-hip r o o f , indicated that t h e valley r a f t e r used in the t e s t was able to m a t c h sufficiently the s t r e n g t h of the common r a f t e r s although neither r e a c h e d f a i l u r e . T h i s showed that the original assumption about the strength and stiffness of the valley section of the r o o f , based on the f r a m i n g action only, had been too p e s s i m i s t i c .
T h e deflections of the hip r a f t e r w e r e s m a l l a s expected. T h e deflections m e a s u r e d a t the mid-point of the hip r a f t e r w e r e slightly l e s s than the deflections of the common r a f t e r s .
In conclusion, i t can b e stated that the L-shaped valley-and- hip r o o f tested with a v e r t i c a l l y supported ridge, showed a load
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c a r r y i n g capacity c o m p a r a b l e t o that of a s t r a i g h t roof of the s a m e type of r a f t e r and joist construction. No m e m b e r s failed in the t e s t ; the weak link in the s t r u c t u r e proved t o b e t h e toe-nailed connection of the r a f t e r s t o the r i d g e beam.T A B L E I
LOAD - D E F L E CTION DATA
Net V e r t i c a l M i d - P o i n t D e f l e c t i o n s , in. Net D e f l e c t i o n s of V a l l e y R a f t e r , in. Load p s f 1 3 25 36 47 59 70 7 5 8 1 8 5 Hip e 0. 22 0. 37 0 . 5 7 0. 7 5 0 . 9 0 1. 12 1. 16 1. 43 1. 67 Load p s f 1 3 25 3 6 47 5 9 70 7 5 8 1 8 5 R e m a r k s B e h a v i o u r m a i n l y e l a s t i c J a c k - t o - r i d g e joint opening N e a r - f a i l u r e of r o o f r e a c h e d V a l l e y c 0. 16 0. 36 0 . 5 6 0 . 8 7 1. 23 1. 8 4 2. 1 5 2. 49 2. 7 4 C o m m o n R a f t e r s G a u g e P o i n t f 0. 17 0. 33 0 . 5 1 0 . 7 3 0 . 9 3 1. 30 1. 42 1. 58 1. 70 a 0. 07 0. 17 0. 27 0. 41 0. 56 0 . 8 5 0. 92 1. 07 1. 20 g 0. 22 0. 35 0 . 5 2 0 . 7 4 0 . 9 5 1. 33 1. 4 3 1. 6 4 1. 7 5 b 0. 16 0. 3 4 0. 5 3 0. 8 3 1. 15 1. 79 1. 99 2. 29 2. 58 h 0 . 0 5 0. 30 0 . 4 5 0 . 6 5 0 . 8 5 1. 26 1. 46 1. 8 4 2.06 c 0. 16 0. 36 0. 56 0. 87 1. 23 1. 8 4 2. 1 5 2. 49 2. 7 4 k 0. 1 4 0. 25 0 . 3 9 0. 56 0. 71 1. 00 1. 0 4 1. 42 2. 44 d 0. 13 0. 29 0. 47 0. 7 3 1. 00 1. 61 1 . 8 3 2. 05 2. 31
LEVEL 0
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F i g u r e 4 Loading p r o c e d u r e f o r v a l l e y a n d h i p r o o f t e s t
Figure 5 70-to 75-psf load caused failure of rafter along the N-S ridge (top of picture)
F i g u r e
6
F u r t h e r loading t o 80 t o 85 psf i n c r e a s e d r a f t e r movement along t h e N S r i d g e shown and a l s o caused a s i m i l a r f a i l u r e of t h e n o r t h r a f t e r joints along the E-W
r i d g eF i g u r e 7 Sheathing r e m a i n e d bowed even a f t e r unloading b e c a u s e the r a f t e r s had t h r u s t against the r i d g e beam and wedged t h e r e a t 8 0 - to 85-psf load
F i g u r e 8 Two s h e a t h i n g b o a r d s r e m o v e d ; n o t e t h e p o s i t i o n of t h e r a f t e r e n d s a f t e r a downward d i s p l a c e m e n t of up t o 4 1/4 in.
0 - 5 1.0 1-5 2.0
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IN I N C H E S I N I N C M E S
