Loading tests on conventional and trussed roof constructions

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Loading tests on conventional and trussed roof constructions Hansen, A. T.

NATIONAL FLESEARCH COUNCIL CANADA

D I V I S I O N O F B U I L D I N G RESEARCH

LOADING TESTS ON CONVENTIONAL AND TRUSSED ROOF CONSTRUCTIONS by A.T. H a n s e n R e p o r t N o ,

81

of t h e D i v i s i o n of Building R e s e a r c h O t t a w a M a y

1956

PREFACE

Development of economies i n house d e s i g n and

~ o r ~ s t r u c t i o n i s a continu-ing r e s p o n s i b i l i t y of t h e D i v i s i o n o f B u i l d i n g R e s z a r c h . I n s u c h s t u d i e s t h e D i v i s i o n h a s b e 2 2 ~ o n s i d e r i ~ ~ g c r i t i c a l l y e a c h of t h e p r i n c i p a l compo- n s t s of t h e s t a n d a r d house w i t h a view t o i m p r o v m e n t i n d e s i g n 2nd economy, T h i s has l e d t o a n i n v e s t i g a t i o n of r o o f d e a i g n i n which t h e Dj~rfs!-on bas f o l l o w e d t h e l e a d 0% American

seseascb, workers i n c o n s i d e r i n g t h e p o s s i b l e use of p r e -

f a b r i c a t e d t r u s s e s f o r houae r o o f s I n p l a c e of t h e c o n v e n t i o n a l b u i l t - i n r o o d e s i g n .

Report No. 77 of t h e D i v i s i o n , " S t r u c t u r a l T e s t Fne;

of Two W - t r u s s e s f ' ~ e c o s d s d r e s u l t s of l o a d t e s t s on t r u s s e s u s i n g a d e s i g n developed i n t h e United S t a t e s but s u b j e c t e d t o t h e more s e v e r e l o a d i n g s t y p l c s l of Canadian c o n d i t i o n s . T h i s p r e s e n t r e p o r t d e a l s w i t h l o a d i n g t e s t s on t r u s s e d r o o f s and s e v e ~ a l c o n v e n t i o n a l r o o f c o n s t r u c t i o n s . The t y p e s of t r u s s e d r o o f s t e s t e d were t y p i c a l of t h o s e s u g g e s t e d For use i n t h e U n i t e d S t a t e s . Ottawa, May

1956

N.B. A s s i s t a n t D i r e c t o r . Xutcheon,

TABLE OF CONTENTS

Page

A ,

Conventional Roof Framing

(1) Description of Test Structures

a, General Description

b.

Collar Ties and Rafters

c.

Joists

d.

Nailing

(2) Testing of Test Structures

a. Test Equipment

b.

Instrumentation

c ,

Application of Load and Testing

Procedure

d.

Recording of Results

(3) Results of Tests

a.

Deflection Characteristics

b,

Load-@am?;~ing Capacities

B.

Trussed Roof Construction

(1) Description of Test Structures

a, General Description

b.

Type

A Truss

c.

Type B Truss

d.

Type C Truss

e. Types D, E and F Trusses (experimental)

(2) Testing of Test Structures

a.

Test Equipment

b.

,Inst~umentation

c,

Application of Load and Testing

Procedure

d .

Recording of Results

(3) Results of Tests

a. Type

A

Trusses

b.

Type B Trusses

c.

Type

C

Trusses

d.

Types D,

E and F Trusses

C. Coet Study

D. General Discussion of Results

E,

Conclusions

LOADING

TESTS

ON CONVENTIONAL AND TRUSSED ROOF CONSTRUCTIONS

A.T. Hansen

Conventional wood-frame construction as used in house building has evolved to its present status largely through experience in use. The construction system as a whole is difficult to analyse by a standard engineering approach although parts of the system have, in recent times, been designed and specified on the basis of accepted loading requi~ements. Roof rafters and floor joists are typical of elements where allowable working stresses and design loads are used to select members.

When a change in the conventional system is contemplated it is not always simple to evaluate the effect or merit of the change since there has not been established any generally

accepted criteria upon which to base an over-all comparison. In spite of this, new types of construction have been tested and their performance and acceptance based on the knowledge gained through similar tests on conventional types of

construction.

In the course of a preliminary study of roof construc- tion, the author attempted to determine the nailing requirements for conventional roof framing under design loads as specified

in the National Building Code. From this study there was

evidence that conventional roof framing may not be capable of supporting design loads with the normal margin of safety. From theory it can be shown that the nailed joints which are commonly used

are

inadequate to develop the strength of the rafters. Loading tests on conventional roof structures might then provide a basis for more balanced and economical design in conventional roof framing.

Trussed roof construction is a relatively new develop- ment in house construction practice but the possibilities of this form of roof constsuction have not been explored widely in Canadian practice. One of the difficulties in the way of general acceptance is lack of knowledge of the performance

characteristics and cost of trussed constructions. A truss

designed according to recognized engineering practice for house construction cannot compete with the conventional roof construction on an economic basis.

In view of the apparent inconsistency of a trussed roof construction as compared with conventional roof construc- tion, both acceptable under similar conditions, it was

both s y s t e m s . S i n c e t r u s s e s d e s i g n e d f o r a l o c a t i o n r e p r e s e n - t a t i v e of Canadian c o n d i t i o n s had been t e s t e d p r e v i o u s l y * , a number of t r u s s d e s i g n s t y p i c a l of t h o s e s u g g e s t e d f o r u s e i n t h e United S t a t e s were i n v e s t i g a t e d t o g e t h e r w i t h s e v e r a l r e p r e s e n t a t i v e c o n v e n t i o n a l c o n s t r u c t i o n s .

A . CONVENTIONAL ROOF FRAMING

( 1 ) D e s c r i p t i o n of Test S t r u c t u r e s a . General D e s c r i p t i o n

I n o r d e r t o d e t e r m i n e r e p r e s e n t a t i v e t y p e s of r o o f

c o n s t r u c t i o n , v i s i t s were made t o s e v e r a l h o u s i n g developments i n t h e Ottawa a r e a . From t h e s e v i s i t s i t was d e c i d e d t o t e s t t h r e e b a s i c t y p e s of c o n v e n t i o n a i r o o f c o n s t r u c t i o n . A l l

t e s t s were c a r r i e d o u t on a 24-foot span u s i n g a roof s l o p e of 5/12. Yard-run e a s t e r n s p r u c e was used i n a l l a s s e m b l i e s .

I n o r d e r f o r t h e t e s t s t r u c t u r e s t o r e s i s t l a t e r a l b u c k l i n g due t o t h e a p p l i e d l o a d , t h e t e s t specimens were

t e s t e d i n p a i r s p l a c e d a t 16 i n c h e s on c e n t r e w i t h 1- by 6 - i n c h s h e a t h i n g a p p l i e d t o t h e r a f t e r s .

The r e s i s t a n c e t o l a t e r a l t h r u s t t h a t may be p r o v i d e d

by walls w i l l v a r y and may be q u i t e small. T e s t s were conducted, t h e r e f o r e , b o t h w i t h r o l l e r s under one end of t h e assembly t o s i m u l a t e t h e c o n d i t i o n where walls o f f e r e d l i t t l e l a t e r a l r e s i s t a n c e , and a l s o w i t h b o t h s u p p o r t s of t h e s t r u c t u r e

r e s t r a i n e d t o d u p l i c a t e t h e o t h e r extreme where t h e walls a r e assumed n o t t o move.

A t o t a l of t h r e e t e s t s were done on most a s s e m b l i e s and t h e r e s u l t s were a v e r a g e d . Three b a s i c t y p e s of r o o f f r a m i n g t e c h n i q u e s were t e s t e d ( F i g s .

36,

37,

and

381,

and w i l l be r e f e r r e d t o a s Types I, 11, and 111. b. C o l l a r T i e s and R a f t e r s The s i z e and p o s i t i o n of t h e c o l l a r t i e s f o r t h e t e s t s was d e c i d e d upon a f t e r a n e x a m i n a t i o n of t h e N a t i o n a l B u i l d i n g

Code and CMHC B u i l d i n g S t a n d a r d s . The N a t i o n a l B u i l d i n g Code r e q u i r e s t h a t a minimum of 2- by 8 - i n c h e a s t e r n spruce r a f t e r s

*

S t r u c t u r a l t e s t i n g of two W-trusses by D.B. Dorey. Report No.

77,

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

16 i n c h e s on c e n t r e be used t o span a 12-foot h o r i z o n t a l d i s t a n c e , assuming a snow l o a d of 40 pounds p e r square f o o t ,

if t h e r a f t e r s have no i n t e r m e d i a t e v e r t i c a l s u p p ~ r t ~ . There i s no i n d i c a t i o n i n t h e Code t h a t t h e c o l l a r t i e i s i n t e r p r e t e d t o a c t a s an i n t e r m e d i a t e r a f t e r s u p p o r t . The Code a l s o

s 2 s t e s t h a t c o l l a r t i e s must be used when t h e ends of t h e r a f t e r s a r e not t i e d t o g e t h e r by c e i l i n g j o i s t s . It does not s t a t ? , however, what c o n s t i t u t e s a s u i t a b l e t i e . It i s

q u e s t i o n a b l e i f a s u i t a b l e t i e e x i s t s when Types I1 and 111

c o n s t r u c t i o n s a r e used. It i s conceivable t h a t even i n Type I c o n s t ~ u c t i o n t h e r a f t e r s and j o i s t s may not l a p but may be f a s t e n e d independently t o the r a f t e r p l a t e by t o e - n a i l s .

I1 f a c t t h i s w a s observed o f t e n i n t h e course of t h e f i e l d i n v z s t i g a t i o n . There i s an i n d i r e c t t i e t o be s u r e but i t i s obviously not as good as i n t h e case where t h e j o i s t s and r a f t e r s a r e joined d i r e c t l y by n a i l s as w e l l a s being t o e - n a i l e d t o t h e t o p p l a t e .

The Code a l s o r e q u i r e s t h a t i f a minimum of 1 by

5

i n c h e s i s used as c o l l a r t i e s t h e n t h e s e c o l l a r t i e s must be l a t e r a l l y supported at t h e i r c e n t r e s by a 1- by 4-inch s t r i p a t r i g h t a n g l e s t o t h e c o l l a r t i e s .

CMHC Building Standards have p r a c t i c a l l y t h e same requirements w i t h one important e x c e p t i o n . Although CMHC Building Standards do not s t a t e s p e c i f i c a l l y t h a t c o l l a r t i e s may be a c c e p t a b l e a s i n t e r m e d i a t e r a f t e r s u p p o r t s t h i s i s t h e

i n t e r p r e t a t i o n which i s commonly acknowledged i n p r a c t i c e . Thus,since a n unsupported l e n g t h of 2 - by 4-inch e a s t e r n

spruce r a f t e r s , 16 lnches on centre, i s p e r m i t t e d t o span a

6-f oot -11-inch h o r i z o n t a l d i s t a n c e , i t i s consl dered a c c e p t - a b l e t o CMHC t o use 2 - by 4-inch r a f t e r s provi2ed t h a t t h e c o l l a r t i e d i v i d e s t h e r a f t e r s o t h a t t h e h o r i z o n t a l p r o j e c - t i o n s of each of t h e two r a f t e r segments do not exceed

6 f e e t 11 i n c h e s . POP a h o r i z o n a l span a 12-foot 2 - by 4- inch r a f t e r may be used i f t h e c o l l a r t i e i s placed n e a r t h e mid-span of t h e r a f t e r . The Standards a l s o s t a t e t h a t a ninimum of 2 - by 4-inch c o l l a r t i e s may be used when t h e

c o l l a r t i e s support a c e i l i n g J o r i n u n f i n i s h e d a t t i c s 1- by 5-Xnch c o l l a r f i e s may be used provided t h e c o l l a r t i e s a r e L a t e r a l l y supported a t t h e i r c e n t r e s oy a continuous 1- by 4-inch s t r i p . The Standards do not s t a t e , however, i n what c a s e s c o l l a r t i e s must be used.

I n both t h e National B u i l d i n g Code and CMHC Building Standards t h e p o s i t i o n of t h e c o l l a r t i e s i s l i m i t e d t o t h e middle t h i r d of t h e r a f t e r s .

I n o r d e r t h a t a s e r i e s of t e s t s should r e p r e s e n t a f a i r c r o s s - s e c t i o n of c u r r e n t b u i l d i n g p r a c t i c e s , i t w a s decided t h a t both i n t e r p r e t a t i o n s of t h e f u n c t i o n of t h e c o l l a r t i e be acknowledged. It was t h e r e f o r e decided t h a t both 2- by 4-inch and 2 - by 8-fnzh r a f t e r s be t e s t e d .

t h e r a f t e r s and j o i s t s o r t h e p i t c h of t h e r o o f , t h e t y p e of wood used and t h e a c c e p t e d snow l o a d , a l l of which s e r i o u s l y a f f e c t t h e naurber of n a i l s r e q u i r e d .

I n t h e t e s t s conducted h e r e a l l j o i s t s were lapped a t t h e s p l i c e over t h e b e a r i n g p a r t i t i o n and f a s t e n e d w i t h t h r e e 3*-inch n a i l s through t h e two j o i s t s as w e l l a s two 33-inch t o e - n a i l s from t h e j o i s t s t o t h e t o p p l a t e of t h e b e a r i n g p a r t it io n a s shown i n F i g .

39.

I n Type 1 c o n s t r u c t i o n t h e r a f t e r s and j o i s t s were t i e d t o g e t h e r with t h r e e 3*-inch n a i l s t h r o u g h b o t h members as w e l l as t h ~ e e 3*-inch, t o e - n a i l s from t h e r a f t e r t o t h e t o p p l a t e and two 33-inch t o e - n a i l s from t h e j o i s t s t o t h e t o p p l a t e . T h i s was observed t o be seldom done i n p r a c t i c e but s i n c e t h i s n a i l i n g schedule s a t i s f i e s a l l t h e r e q u i r e m e n t s of t h e codes, t h i s scheme of n a i l i n g w a s decided upon ( F i g . 3 6 ) . I n Types 11 and I I E c o n s t r u c t i o n s t h e n a i l i n g r e q u i r e - ments f o r c o n n e c t i n g t h e r a f t e r p l a t e t o t h e j o i s t s and h e a d e r s a s w e l l a s c o n n e c t i n g t h e h e a d e r s t o t h e j o i s t s were n o t covered by t3he n a i l i n g s c h e d u l e s . F i e l d o b s e r v a t i o n s of t h e s e t y p e s of c o n s t r u c t i o n showed t h a t p r a c t i c e s v a r i e d c o n s i d e r a b l y . It was d e c i d e d , t h e r e f o r e , t o a r r i v e a t

n a i l l n g schemes which would r e p r e s e n t t h e b e s t p r a c t i c e s i n t h e s e c o n s t ~ u c t i o n s . The n a i l i n g schemes a r e shown i n

F i g s . 37 and

38.

It w a s a l s o noted i n f i e l d o b s e r v a t i o n s t h a t c o l l a r t i e s were f a s t e n e d t o t h e r a f t e r s by e i t h e r two o r t h r e e 3$-inch n a i l s . I n t h e t e s t s t r u . c t u r e s t h r e e 33-inch n a i l s were used. ( 2 ) T e s t i n g of T e s t S t ~ u c t u r e s a . Teat Equipment

Loads were applLed t o t h a r a f t e r s by means of e i g h t h y d r a u l i c t e n s i o n Ja,cks a ~ c h a r e d t o t h e f l o o r and connected t o a h y d r a u l i c pump by a common o i l l i n e . The pump was f i t t e d w i t h

a

p r e s s u r e gauge t o measure t h e o i l p r e s s u r e i n t h e

system. The f o r c e e x e r t e d by t h e j a c k s was c a l i b r a t e d a g a i n s t t h e p r e s s u r e gauge on t h e pump by p l a c i n g t h e j a c k s i n a

h y d r a u l i c t e s t i n g machine and r e c o r d i n g t h e p r e s s u r e on t h e pump p r e s s u r e gauge w i t h t h e corresponding r e a d i n g s on t h e t e s t i n g machine. Corsesponding r e a d i n g s were t a k e n f o r a range of l o a d s s o t h a t t h e n e c e s s a r y c a l i b r a t i o n c u r v e s could be o b t a i n e d .

The J a c k s were p l a c e d midway between t h e p a i r s of t e s t specimens and p o s i t i o n e d s o t h a t t h e a p p l i e d l o a d would simu- l a t e t h e same bending moments i n t h e r a f t e r s and t h e same end r e a c t i o n s as f o r a uqif orm l o a d .

The f o r c e s e x e r t e d by t h e j a c k s were t r a n s m i t t e d t o t h e r a f t e r s by means of $-inch s t e e l r o d s from t h e j a c k s t o s t e e l c h a r n e l s e c t i o n s l y i n g a c r o s s t h e r a f t e r s a s shown i n F i g s .

16

and 40.

The c e i l i n g load was a p p l i e d d i r e c t l y t o t h e c e i l i n g j o i s t s by means of l e a d - f ' l l l e d bags placed i n such a way a s t o s i m u l a t e a u n i f o r n c e i l i n g l o a d .

F i g u r e 41 shows t h e p o s i t i o n s and magnitudes of t h e a p p l i e d l o a d s .

The t e s t s t r u c t u r e s were supported on c o n c r e t e b l o c k s , as shown i n F i g .

16,

w i t h b r a c e s s u p p o r t i n g t h e end blocks a g a i n s t movement ( F i g .

1 3 ) .

S h o r t p e i e e s sf $-inch d i a m e t e r s t e e l b a r s sandwiched between two s t e e l p l a t e s were used a s s u p p o r t s under one end of t h e s t r u c t u r e i n t h o s e t e s t s s i m u l a t i n g t h e c o n d i t i o n of no l a t e r a l r e s i s t a n c e t o t h r u s t by t h e e x t e r i o r w a l l s

( F i g * 1 3 ) .

Fos t h o s e t e s t s i n which t h e w a l l s a r e assumed n o t t o move, t h e double 2- by 4-inch b e a r i n g p l a t e s a t each end of t h e s t r u c t u r e were s e c u r e l y b o l t e d t o t h e f l o o r beam t o r e s t r a i n movement ( F i g s .

6

and

8 ) .

b

.

I n s t r w n e n t a t i o n

D i a l gauges were placed under each end of t h e s u p p o r t i n g p l a t e s t o measure t h e s e t t l e m e n t of t h e s t r u c t u r e under t h e a p p l i e d l o a d s . D i a l gauges were a l s o placed a t t h e ends of t h e r a f t e r s t o measure any h o r i z o n t a l movement of t h e r a f t e r s under l o a d ( F i g .

13).

D i a l gauges were a l s o placed on t h e r a f t e r s

a t

t h e ends of t h e c o l l a r t i e s t o measure t h e r e l a t i v e movement between r a f t e r s and c o l l a r t i e s t o determine when t h e c o l l a r t i e a c t s i n t e n s i o n snd when i t a c t s i n compression. Only one c o l l a r t i e was instrumented i n each t e s t ( F i g . 1 4 ) .

V e r t i c a l d e f l e c t i o n s of t h e r a f t e r s were measured i n t h e f o l l o w i n g manner.

Fine

i;

i a n o w i r e w a s s t r u n g from t h e peak of t h e assembly and a t and

6

f e e t on e i t h e r s i d e of t h e peak (measured h o s l a o n t a l l y ) . The w i r e s were f a s t e n e d t o t h e s h e a t h i n g midway between t h e two s e t s of r a f t e r s and passed over p u l l e y s f a s t e n e d t o t h e f l o o r d i r e c t l y below.

From t h e p u l l e y s below t h e r a f t e r s t h e w i r e s were l e d by o t h e r p u l l e y s t o a r e c o r d i n g board. A t t h e ends of t h e w i r e s were f a s t e n e d 1-pound w e i g h t s , t h e v e r t i c a l p o s i t i o n of which could be noted on t h e r e c o r d i n g board ( F i g . 15). The average

v e r t i c a l movement of t h e two s e t s of r a f t e r s r e g i s t e r e d t h e same movement on t h e r e c o r d i n g board.

c

.

A p p l i c a t i o n of Load and T e s t i n g Procedure

A t t h e beginning of t h e t e s t a 10-pound-per-square- f o o t c e i l i n g l o a d was a p p l i e d t o t h e c e i l i n g j o i s t s and allowed t o remain f o r t h e d u r a t i o n of t h e t e s t .

The weight of t h e r a f t e r l o a d i n g equipment hanging from t h e r a f t e r s , p l u s t h e weight of t h e a p p l i e d s h e a t h i n g , was p r a c t i c a l l y e q u a l t o t h e weight of r o o f i n g and s h e a t h i n g which would be a p p l i e d i n p r a c t i c e s o t h a t t h e dead weight of t h e r o o f i n g d i d n o t have t o be allowed f o r i n t h e h y d r a u l i c l o a d a p p l i c a t i o n .

The h y d r a u l i c l o a d s were a p p l i e d i n increments c o r r e s - ponding t o 10-pound--per-square-foot uniform snow l o a d s .

A f t e r each incremenL of l o a d i n g was reached, t h e l o a d was h e l d f o r

5

minutes and a l l d i a l gauge r e a d i n g s and r a f t e r d e f l e c t i o n s recorded. The l o a d s were i n c r e a s e d t o a t o t a l of 40 pounds p e r square f o o t a t which p o i n t t h e l o a d s were reduced t o z e r o . Loading was a g a i n i n c r e a s e d in lo-pound- per-square -foot increments u n t i l f a i l u r e occurred.

Three t e s t s on both r o l l e r s u p p o r t s and f i x e d end s u p p o r t s were not c a r r i e d out on a l l t y p e s of conventional c o n s t r u c t i o n . I n Types I1 and I11 constructions some f a i l u r e s were such t h a t f a i l u r e was independent of t h e type of s u p p o r t s used. I n t h e s e c a s e s t h e r e s u l t s of t h e f a i l u r e s were

included i n both t h e average f a i l u r e w i t h r o l l e r s u p p o r t s and f i x e d end s u p p o r t s , even though t h e a c t u a l t e s t s i n some

c a s e s were on r o l l e r s u p p o r t s only (Table 11).

Photographs were t a k e n of t h e f a i l u r e s of most of t h e s t r u c t u r e s t e s t e d and t h e moisture c o n t e n t s of t h e s t r u c t u r e s necorded by means of a r e s i s t a n c e - t y p e moisture meter.

d . Recording 0% R e s u l t s

A l l d i a l gauge r e a d i n g s were taken t o t h e n e a r e s t

.001 inch and r a f t e r d e f l e c t i o n r e a d i n g s t o t h e n e a r e s t . O 1 i n c h . The r e a d i n g s of t h e d i a l gauges placed under t h e double

2- by 4-inch p l a t e s a t e i t h e r end of' t h e t e s t s t r u c t u r e s were deducted p r o p o r t i o n a t e l y from t h e r a f t e r d e f l e c t i o n r e a d i n g s t o determine t h e t r u e v e r t i c a l r a f t e r d e f l e c t i o n s . From t h e s e c o r r e c t e d r a f t e r d e f l e c t i o n s t h e d e f l e c t i o n s normal t o t h e r a f t e r s were c a l c u l a t e d f o r t h e 40-pound-per-square-foot l o a d s (Table 11).

( 3 )

R e s u l t s of T e s t s

s. 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

A s can be s e e c i n T a b l e V I j t h e r e i s a c o n s i d e r a b l e v a r i a t i o n i n t h e d e f l e c t i o n r a % i o s of s i m i l a r s i z e r a f t e r s i n both t h e 2- by 4-inch and 2 - by 8-inch r a f t e r c o n s t r u c - t i o n s , and t h e d e f l e c t i o n r a t i o v a r i e d with t h e type of c o n s t r u c t i o n and w i t h t h e type of end support.

An e x p l a n a t i o n of t h e v a r i o u s r a f t e r d e f l e c t i o n r a t i o s i s as follows: If t h e ends of t h e r a f t e r s (A) a r e assumed not t o move under l o a d , t h e d e f l e c t i o n of t h e r a f t e r s a t B w i l l cause t h e c o l l a r t i e BB t o a c t i n compression. On t h e

o t h e r hand, i f we cor.sider t h a t t h e r a f t e r does n o t d e f l e c t under load and t h e r a f t e r ends can move outward a t A , t h e n t h e c o l l a r t i e must a c t i n t e n s i o n . I n p r a c t i c e , both a c t i o n s occur simultaneously. The s t i f f e r t h e r a f t e r , t h e l e s s w i l l be t h e tendency t o compress t h e c o l l a r t i e , and t h e more r i g i d t h e j o i n t s a r e a t A and C t h e l e s s w i l l be t h e tendency t o s t r e t c h t h e c o l l a r t i e . The n e t e f f e c t of t h e two a c t i o n a may cause e i t h e r a t e n s i o n o r a compression of t h e c o l l a r t i e depending on t h e r e l a t i v e movements produced by t h e loading.

The g r e a t e r t h e compressive f o r c e i n t h e c o l l a r t i e , t h e g r e a t e r w i l l be t h e a c t i o n t o r e s i s t t h e d e f l e c t i o n of t h e r a f t e r s under a p p l i e d load. Conversely, t h e g r e a t e r t h e t e n s i o n i n t h e c o l l a r t i e t h e g r e a t e r w i l l be t h e tendency

of t h e c o l l a r t i e t o i n c r e a s e t h e r a f t e r d e f l e c t i o n . T h e r e f o r e , it f o l l o w s , t h a t t h e d e f l s c t i o n s a t i ~ of t h e r a f t e r s w i l l

depend upon t h e degree of compression o r t e n s i o n i n t h e c o l l a r t i e f o r any given s i z e and span of r a f t e r ,

T h i s e x p l a n a t i o n w i l l show why t h e r a f t e r d e f l e c t i o n r a t i o s were g r e a t e r f o r t e s t s on r o l l e r s u p p o r t s t h a n f o r t e s t s on f i x e d end s u p p o r t s , and would e x p l a i n why t h e r a f t e r d e f l e c t i o n r a t i o s of t h e s t r u c t u r e s w i t h t h e s t r o n g e s t h e e l j o i n t d e t a i l (Type I ) wepe l e s s t h a n t h o s e for t h e weakest type of h e e l j a i n t (Type 111).

A s a g e n e r a l i z a t i o n based on o b s e r v a t i o n s of d i a l gauges a t t h e ends of t h e c o l l a r t i e , i t m y be s a i d t h a t

when 2- by 8-inch r a f t e r s were used t h e c o l l a r t i e s a c t e d i n compression a t t h e lower l s a d s , and a t t h e h i g h e r l o a d s t h e s t r e s s u s u a l l y r e v e r s e d and t h e c o l l a r t i e s a c t e d i n t e n s i o n . With t h e 2- by 4-inch r a f t e r s , t h e c o l l a r t i e s u s u a l l y a c t i n

compression throughout t h e t e s t due t o t h e f l e x i b i l i t y of t h e s m a l l e r r a f t e r s . The l o a d s causing a r e v e r s a l of s t r e s s i n t h e c o l l a r t i e may be seen i n Table 11.

b . Load-carrying C a p a c i t i e s

The u l t i m a t 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 f o r Type I c o n s t r u c t i o n b u i l t w i t h 2- by 4-inch r a f t e r s was found t o be

56

pounds p e r square f o o t f o r r o l l e r s u p p o r t s and 72 pounds p e r square f o o t f o r f i x e d s u p p o r t s . For Type I1

c o n s t r u c t i o n t h e average f a i l u r e l o a d s were between

40

and

43

pounds p e r square f o o t and f o r Type I11 c o n s t r u c t i o n only

18

pounds p e r square f o o t

a able

V I ) .

The t e s t r e s u l t s f o r t h e 2- by 8-inch r a f t e r c o n s t r u c - t i o n s showed t h e s e s t r u c t u r e s t o possess c o n s i d e r a b l y g r e a t e r s t r e n g t h t h a n 2- by 4-inch r a f t e r c o n a t r u c t i o n s (Table V I ) . The f a i l u r e load f o r Type I c o n s t r u c t i o n was 89 pounds p e r

square f o o t when t h e s t r u c t u r e s were t e s t e d on r o l l e r s u p p o r t s and 125 pounds p e r square f o o t i f t h e ends of t h e s t r u c t u r e s were f i x e d . For Type I1 c o n s t r u c t i o n t h e average u l t i m a t e

l o a d was from 82 t o 84-pounds p e r square f o o t and only 46 pounds p e r square f o o t f o r Type 111 c o n s t r u c t i o n .

It i s important t o keep i n mind t h a t c o l l a r t i e s were a l s o used w i t h a l l 2- by 8-inch r a f t e r c o n s t r u c t i o n and were placed a t mid-span of t h e r a f t e r s . If no c o l l a r t i e s were used, which i s allowed under t h e National Building Code f o r Type I c o n s t r u c t i o n and p o s s i b l y f o r Types I1 and I11

c o n s t r u c t i o n s (depending on how t h e Building Code i s i n t e r - p r e t e d ) , then t h e s t r u c t u r e s would probably be weaker t h a n those 2- by 8-inch s t r u c t u r e s t e s t e d . Again i f t h e c o l l a r t i e s were placed a t t h e upper t h i r d of t h e r a f t e r s which i s t h e l i m i t i n g p o s i t i o n , t h e i r e f f e c t on p r e v e n t i n g t h e r a f t e r ends from spreading would be only about two-thirds as g r e a t t h a n i f t h e c o l l a r t i e s were placed a t mid-span of t h e

r a f t e r s , and t h e o v e r - a l l s t r e n g t h of t h e s t r u c t u r e s would i r , all p r o b a b i l i t y be l e s s .

A comparison of t h e r e s u l t s of 2 - by 4-inch and 2 - by 8-inch r a f t e r c o n s t r u c t i o n s r e v e a l s some i n t e r e s t i n g f a c t s .

As can be seen from t h e average r e s u l t s i n Table V I , t h e f a i l u r e l o a d s of t h e 2- by 8-inch r a f t e r c o n s t r u c t i o n s a r e s u b s t a n t i a l l y h i g h e r than t h e f a i l u r e l o a d s f o r t h e 2- by 4-inch r a f t e r c o n s t r u c t i o n s . I n many c a s e s t h e t y p e s of f a i l u r e s were i d e n t i c a l f o r both 2 - by 4-inch and 2 - by 8- inch r a f t e r c o n s t r u c t i o n s and occurred a t t h e n a i l e d j o i n t s l

a t t h e j o i s t l a p o r h;el j01r:t. The n a i l i n g f o r b o t h 2- by 4 - i n c h and 2 - by & i n c h r a f t e r s , however, was i d e n t i c a l .

It must be a s s m s d t h e n t h a t t h e outward t h r u s t e x e r t e d by t h e 2 - by 4 - i n c h r a f t e r s wax c o n s i d e r a b l y more t h a n t h e t h r u s t f o r t h e 2 - by $ - i n c h r a f t e r s w i t h similar l o a d s . One p o s s i b l e e x p l a n a t i o n i s as f o l l o w s : The outward t h r i ~ s t of a r a f t e r

sv-pporting W - pounds p e r l i n e a l f 00% and apabrlnirkg L f e e t on a

5/12 s l o p e i s

4

x

5

WL if t h e a g t i o n of t h e c o l l a r t i e i s i g n o r e d . The outwa d t h r u s t i s t x l2 WL if t h e c o l l a r t i e i s assumed t o be a n a d e q u a t e l y d e s i g n e d T i n connected member.

The 2- by 8 - i n c h r a f t e r s d i d n o t d e f l e c t s u f f i c i e n t l y f o r t h e c o l l a r t i e s t o c o n t r i b u t e v e r y much as a compression member, and i n many c a s e s t h e c o l l a r t i e s a c t u a l l y a c t e d i n t e n s i o n .

I n t h i s i n s t a n c e t h e r a f t e r s p r o b a b l y a c t much l i k e s i m p l e beams and t h e outward t h r u s t a p p r o a c h e s a v a l u e l e s s t h a n

4

x

4

WL pounds. There i s consLderably more f l e x i b i l i t y i n t h e 2=' by 4 - i n c h r a f t e r s and t h e compressive f o r c e e x e r t e d by f h e c o l l a r t i e s i s s u b s t a n t i a l l y g r e a t e r t h a n i f 2- by 8 - i n c h r a f t e r s were I n thPs c a s e t h e h o r i z o n t a l r a f t e r t h r u s t w i l l a g p r o a c h u ~ ~ . ~ WL pounds. I f , on t h e b a s i s of the assumption t h a t t h e h o r i z o n t a l t h r u s t of 2- by 8 - i n c h r a f t e r s e q u a l s x 5/12 WL and 2 - by 4 - i n c h r a f t e r s e q u a l s

%

x 5/12 WL, t h e f a i l u r e l o a d s of t h e n a i l e d j o i n t s a r e c a l c u l a t e d f o r e a c h t y p e of c o n s t s u c t i o n , t h e r e s u l t s show t h a t t h e u l t i m a t e s t r e n g t h s of t h e n a i l e d j o i n t s are similar r e g a r d l e s s of r a f t e r s i z e . The dead w e i g h t of t h e r o o f s t r u c t u r e must n a t u r a l l y be t a k e n i n t o a c c o u n t i n t h e s e c a l c u l a t i o n s , and i n Types I1 and 111 c o n s t r u c t i o n s t h e f r i c t i o n a l r e s i s t a n c e i n t h e h e e l J o i n t must be a l l o w e d F o r . 'This would seem t o i n d i c a t e t h a t t h e e x p l a n a t i o n f o r t h e d i f f e r e n c e i n t h e h o r i z o n t a l t h r u s t w i t h t h e d i f f e r e n t s i z e of r a f t e r s i s s u b s t a n t i a l l y c o r r e c t .

B

.

TRUSSED ROOF CONSTRUCT I O N ( 1 ) D e s c r i p t i o n of T e s t S t r u c t u r e s

a , General D e s c r i p t i o n

A l l t e s t s were c a r r i e d o u t on 2 4 - f o o t s p a n t r u s s e s u s i n g a r o o f s l o p e of' 5/12. Yard-run e a s t e r n s p m c e w a s lased cn a l l a s s e m b l i e s f o r t h e s t r u c t u r a l members and $-inch

Douglas f i r plywood used f o r g u s s e t p l a t e s .

The t r u s s e s were t e s t e d i n p a i r s p l a c e d

16

i n c h e s on c e n t r e and s h e a t h e d w i t h 1- by 6 - i n c h s h e a t h i n g . A s i n t h e t e s t s on c o n v e n t i o n a l c o n s t r u c t i o n , t h 2 t e s t s were conducted u s i n g b c t h s o l l e ~ s u p p o r t s and f f x e d end s u p p o r t s . Three t e s t s were conducted on most a s s e m b l i e s t o o b t a i n an a v e r a g e v a l u e f o r r e s u l t s .

The connections used i n f a s t e n i n g t h e t e s t t r u s s e s t o t h e s u p p o r t i n g 2- by 4-inch p l a t e s were p u r e l y a r b i t r a r y and c o n s i s t e d of' f a s t e n i n g by t o e - n a i l i n g r a t h e r t h a n by u s i n g p a t e n t e d c o n n e c t o r s . T h 2 number and s i z e of n a i l s were a r b i t - r a r i l y a d j u s t e d t o s u i t t h e t y p e of h e e l j o i n t encountered. These n a i l i n g schemes a r e shown i n F i g s . 42, 43, 44, 45 and

46.

The t y p e s of t r u s s e s t e s t e d a r e shown i n F i g s . 42, 43, 44, 45 and 46 and w i l l be r e f e r r e d t o a s Types A , B y C, D, E, and F.

b . Type A T- a ias s

Type A t r u s s i s s i m i l a r t o t h e U n i v e r s i t y of I l l i n o i s Small Homes Council s p l i t r i n g W-truss w i t h some m o d i f i c a t i o n s . Two-b - f o u r ' s were used r a t h e r t h a n

_Y

1

x

4 ' s f o r t h e d i a g o n a l s and 32-inch n a i l s were used throughout. Whereas 1100 p . s . i . s t r e s s grade Douglas fir was suggested by t h e Small Homes Council f o r t h e s t r u c t u r a l members, yard run 2- by & i n c h e a s t e r n spruce was used i n t h e s e t e s t s ( ~ i g . 4 2 ) .

c . Type B Truss

Type B t r u s s i s s i m i l a r t o t h e n a i l e d t r u s s d e s i g n developed by E.G. S t e r n of t h e V i r g i n i a P o l y t e c h n i c I n s t i t u t e w i t h some m o d i f i c a t i o n s a l s o . Common n a i l s were used through-

out r a t h e r t h a n h e l i c a l l y t h r e a d e d n a i l s as suggested i n M r . S t e r n ' s d e s i g n . It w a s a l s o decided t o use $-inch p l y - wood g u s s e t p l a t e s and s p l i c e p l a t e s i n t h e t e s t s t r u c t u r e s r a t h e r t h a n 1- by 6-inch d r e s s e d boards. Although M r . S t e r n ' s d e s i g n c a l l e d f o r t h e use of 1450 p . s . i . graded lumber, 2- by 4-inch e a s t e r n spruce was used. Some r e v i s i o n s were a l s o made i n t h e n a i l e d j o i n t s ( F i g . 4 3 ) .

d . Type C T r u s s

Type C t r u s s was modelled a f t e r t h e U n i v e r s i t y of I l l i n o i s glued t r u s s d e s i g n f o r a 4/12 s l o p e . The s l o p e was r e v i s e d t o 5/12 and t h e g u s s e t p l a t e s i z e s a t t h e h e e l j o i n t s changed a c c o r d i n g l y . The p o s i t i o n s of t h e d i a g o n a l s were changed t o correspond t o t h e p o s i t i o n s of t h e d i a g o n a l s i n Types A and B t r u s s e s , t h a t i s , t h e s h o r t d i a g o n a l s connected t o t h e c e n t r e s ~f t h e t o p chord t o t h e t h i r d p o i n t s of t h e lower chord ( F i g . 4 4 ) . Here, as i n t h e o t h e r t r u s s e s ,

e a s t e r n spruce was used i n s t e a d of 1450 p . s . i . graded Douglas f i r as c a l l e d f o r i n t h e p l a n s .

e . Types D, E and F T r u s s e s

Types D, E and F t r u s s e s were b a s i c a l l y similar t o each o t h e r w i t h minor d i f f e r e n c e s i n t h e h e e l j o i n t s . These t r u s s e s were developed a s t h e r e s u l t of t e s t s on o t h e r roof frames w i t h t h e hope of o b t a i n i n g a t r u s s which would be both adequate and ecanomical. More w i l l be s a i d about t h e s e t r u s s e s l a t e r ( F i g s . 45 and 4 6 ) .

( 2 ) T e s t i n g of T e s t S t r u c t u r e s a . T e s t Equipment

Loads were a p p l i e d i n much t h e same way as f o r t e s t s on c o n v e n t i o n a l c o n s t r u c t i o n s . The p o s i t i o n s and magnitude of t h e t o p chord l o a d s were t h e same as shown i n F i g . 41. The 10-pound-per-square-foot c e i l i n g l o a d was a p p l i e d by means of l e a d - f i l l e d bags l a i d d i r e c t l y on t h e lower chords of t h e t r u s s a t t h e end q u a r t e r p o i n t s of t h e t h r e e bottom p a n e l s of each t r u s s .

The t e s t s t s u c t u r e s were supported on c o n c r e t e blocks under each end of t h e t r u s s e s and t h e blocks were braced a g a i n s t movement i n t h e same manner a s f o r t e s t s on conven- t i o n a l c o n s t r u c t i o n .

b. I n s t r u m e n t a t i o n

D i a l gauges were placed a t each end of t h e double 2 -

by 4-inch p l a t e s a t t h e ends of t h e t r u s s e s t o measure t h e s e t t l e m e n t of t h e t r u s s e s under t h e a p p l i e d l o a d s . D i a l gauges were a l s o placed a g a i n s t t h e o u t s i d e ends of each of t h e double p l a t e s t o measure any h o r i z o n t a l movement of t h e s e p l a t e s (Figs.

33

and 34).

D e f l e c t i o n measurements of t h e t r u s s e s were t a k e n a t

a l l p a n e l p o i n t s and a t t h e mid-spans of t h e lower chords of t h e t r u s s e s by means of' w i r e s and p u l l e y s i n much t h e same way as t h e r a f t e r d e f l e c t i o n s were t a k e n i n t e s t s on conven- t i o n a l c o n s t r u c t i o n s . These measurements were t a k e n midway between t h e p a i r s of t r u s s e s s o t h a t t h e average d e f l e c t i o n s of t h e two t r u s s e s being t e s t e d were measured.

c . A p p l i c a t i o n of Load and T e s t i n g Procedure

A l l d i a l gauge r e a d i n g s and t r u s s d e f l e c t i o n s were noted before t h e c e i l i n g l o a d s and h y d r a u l i c l o a d s were a p p l i e d . The c e i l i n g l o a d was t h e n a p p l i e d and allowed t o remain f o r t h e d u r a t i o n of t h e t e s t . F i v e minutes a f t e r t h e c e i l i n g l o a d was a p p l i e d a l l t h e r e a d i n g s were a g a i n n o t e d .

The h y d r a u l i c l o a d s were t h e n a p p l i e d i n increments s i m u l a t i n g 10-pound-per-square-foot snow l o a d i n g s . F i v e minutes a f t e r each l o a d i n g increment t h e r e a d i n g s were a g a i n taken. The l o a d i n g was continued a n t i 1 a t o t a l of 40-pound-per-square- f o o t load was a p p l i e d , a t which p o i n t t h e l o a d s were

reduced t o z e r o . Loadang w s s a g a i n i n c r e a s e d i n increments of 10 pounds p e r square f o o t u n t i l f a i l u r e occurred.

A s f o r c o n v e n t i o ~ r l c o n s t r u c t i o n t e s t s , photographs were t a k e n of t h e f a i l u r e a and t h e moisture c o n t e n t s of t h e

t r u s s e s were recorded.

d . Recording of R e s u l t s

A l l d i a l gauge r e a d i n g s were recorded t o t h e n e a r e s t .001 i n c h and t r u s s d e f l e c t i o n s t o t h e n e a r e s t . O 1 i n c h . The d i a l gauge r e a d i n g s under t h e end b e a r i n g p l a t e s were deducted p r o p o r t i o n a t e l y from t h e t r u s s d e f l e c t i o n s t o o b t a i n t h e t r u e v e r t i c a l d e f l e c t i o n s .

Although t h e t r u s s e s were t e s t e d a t 16 inches on c e n t r e t h e r e s u l t s were a d j u s t e d $9 apply t o t r u s s e s spaced a t 24

inches on c e n t r e a s w e l l ,

( 3 )

R e s u l t s of T e s t s a . Type A Trusses

It may be seen i n Table V t h a t t h e average f a i l u r e l o a d s f o r t h i s type of t r u s s spaced 24 i n c h e s on c e n t r e were 6 5 pounds persquape f o o t f o r t r u s s e s t e s t e d on r o l l e r s u p p o r t s and 68 pounds p e r square frsot f o r t r u s s e s on f i x e d end

s u p p o r t s . The correspondfng average maximum d e f l e c t i o n s of t h e lower chords were .86 inch and

.77

inch r e s p e c t i v e l y f o r a 40-pound-per-square-foot snow l o a d . The maximum d e f l e c t i o n s d i v i d e d by t h e t o t a l span i s

1/335

f o r t r u s s e s on r o l l e r

s u p p o r t s and 1/370 f o r t r u s s e s on f i x e d end s u p p o r t s . The commonly a c c e p t e d l i m i t i n g r a t i o d e f l e c t i o n t o prevent p l a s t e r c r a c k i n g i s 1/360. The r e s u l t s show t h a t t h i s type of t r u s s i s very c l o s e t o t h i s v a l u e . However, f o r long-term l o a d i n g of 40 pounds p e r square f o o t t h e d e f l e c t i o n s would i n a l l p r o b a b i l i t y be g r e a t e r and t h e d e f l e c t i o n r a t i o might s u b s t a n - t i a l l y exceed t h e 1/3&0 l i m i t .

The most eommon cause of f a i l u r e i n t h e s e t r u s s e s was i n t h e lower chord n e a s o r a t t h e s p l i t r i n g s , w i t h o t h e r f a i l u r e s being due t o t h e t o p chord breaking i n bending o r f a i l u r e i n t h e n a i l e d j o i n t s of t h e long d i a g o n a l ( F i g s . 1 7 ,

Due t o t h e n a t u r e of t h e t o p chores of t h e s e t r u s s e s i n which t h e lower h a l v e s of' t h e t o p chords were o f f s e t from t h e upper h a l v e s p t h e t o p chords a l l had a very d e f i n i t e tendency t o bend l a t ? s a ! l y undek l c a d i n g , The d e s i g n of t h e s e t r u s s e s was such t h a t t h t s berdlng was i n t h e same d i r e c t i o n r e g a r d l e s s of kow t h e t r u s s e s

were

o r i ~ ~ + t a t e d . That i s , it was not p o s s i b l e t o zou:2te~act t h i s bending

e f f e c t by twrri.ng one t r u s s a t 180" t o the. o t h e r . The over- a l l e f f e c t i n a r o o f , t . h e s e f o ~ e , would be t h a t t h e e n t i r e roof would tend t o warp under l o a d i n g . J u s t how much warping any roof system could w i t h s t a n d without showing any v i s i b l e

s i g n s i s not known. Thl3 tendencyp howevers might be over- come i f t h e design of t h *

trusses

was a l t e r e d f o r h a l f of t h e t r u s s e s s o t h a t t h e uppep chords of each t r u s s tended t o bend i n opposite d i r e c t i o n s t o each a d j a c e n t t r u s s .

b. Type B T r u s s e s

T h i s type was t h e second s t r o n g e s t and t h e second most r i g i d type t e s t e d . F a i l u r e s i n most c a s e s were due t o n a i l j o i n t f a i l u r e s a t e i t h e r 2nd of t h e long d i a g o n a l s while o t h e r f a i l u r e s were due t o j o i n t f a f l u r e i n t h e s h o r t diagonal o r t o t e n s i o n f a i l w e i n t h e bottom chord ( F i g s . 2 2 ,

2 3 , 24, 25, 26 and

2 7 ) .

I n two c a s e s ( T e s t s Nos.

3&

and 4 0 ) , t h e t r u s s e s f a i l e d when t h e t o p chord bowed l a t e r a l l y under t h e h i g h e r l o a d s .

The f a i l u r e load f o r t h i s type of t r u s s a t 24 i n c h e s on c e n t r e averaged

99

pounds p e r square f o o t w i t h r o l l e r end s u p p o r t s and 110 pounds p e r square f o o t w i t h f i x e d end

s u p p o r t s

.

The d e f l e c t i o n r a t i o f o r t h e bottom chord of t h e s e t r u s s e s a t 40-pound-per-square-foot snow l o a d and spaced 24 inches on c e n t r e i s 1,600 for r o l l e ~ a ~ l g p o r t e d t r u s s e s and 1/650 f o r t r u s s e s o n f i x e d end s u p p o r t s . T h i s i s considesably l e s s than t h e 1/360 l i m i t i n g r a t i o . The f a c t t h a t t h i s

was

under short-term loading,, however, must be t a k e n i n t o con- s i d e r a t i o n .

c

.

Type C T r u s s e s

T h i s type of t r u s s was t h e s t r o n g e s t and most p i g i d of a l l t y p e s t e s t e d . The average f a i l u r e load f o r t r u s s e s a t 24 Inches on c e 3 t r e was 106 pounds p e r square f o o t on r o l l e r s u p p o r t s and 119 pounds per* square f o o t on f i x e d end s u p p o r t s

.

The average d e f l e c t i o n ~ a t i o of t h e bottom chord f o r t h i s type of t r u s s a t 4 0 - p ~ u d - p e r - s q u w a - f o o t snow l o a d i n g spaced 2 f e e t on c e n t r e is h/:440 kos t r u s s e s on r o l l e r

s u p p o r t s and 1/1310 f o r t r u s s e s on f i x e d end s u p p o r t s . Both t h e s e v a l u e s a r e c o n s i d e r a b l y l e s s t h a n t h e 1/360 l i m i t even when c o n s i d e r i n g t h e f a c t t h a t t h e t r u s s e s were loaded by

s h o r t - t e r m l o a d i n g .

The most common t y p e of f a i l u r e i n t h i s t r u s s was a t t h e glue j o i n t of t h e g u s s e t p l a t e a t t h e peak w i t h t h e only o t h e r t r u e f a i l u r e s o c c u r r i n g at t h e glue j o i n t of t h e g u s s e t p l a t e connecting t h e bottom chord t o t h e d i a g o n a l s . The

p r i n c i p a l cause of f a i l u r e seemed t o be due t o t h e f a c t t h a t i t was d i f f i c u l t t o g e t a good glued j o i n t a t t h e peak where t h e membera were connected by a g l u e bond t o a r i g i d g u s s e t p l a t e on one s i d e and a 1- by 4-inch s t r i p of wood on t h e o t h e r . Unless a l l of t h e members were of t h e same t h i c k n e s s t h e n a good g l u e bond was not p o s s i b l e . Also i f t h e members were cupped due t o d r y i n g s h r i n k a g e , which was o f t e n observed d u r i n g t h e course of t h e t e s t s , t h e glued bond was e f f e c t i v e only over p a r t of t h e r e q u i r e d bonding a r e a . When t h e long d i a g o n a l s were s l i g h t l y warped i t was v e r y d i f f i c u l t t o g e t a good bond a t both peak g u s s e t p l a t e and t h e p l a t e connecting t h e long d i a g o n a l t o t h e bottom chord. It should be added h e r e f o r t h e sake of i n t e r e s t t h a t c a s e i n g l u e was used and was

a p p l i e d by a s t i f f brush t o one s u r f a c e of each glued j o i n t &

d . Types D y E and F T r u s s e s

A l l of t h e s e t y p e s were q u i t e similar i n d e s i g n . The only d i f f e r e n c e i n each w a s a t t h e h e e l j o i n t . The d i f f e r e n c e between Type D and E t r u s s e s w a s t h a t i n Type E t r u s s $-inch common washers i n c h i n d i a m e t e r were used a t t h e h e e l j o i n t and i n Type D t r u s s 2 - by 2 - by 1/8-inch washers w i t h *-inch d i a m e t e r h o l e s were used, ( F i g . 4 5 ) . Type F d i f f e r e d from Type D only i n t h e s m a l l notch c u t i n t h e j o i s t of Type F which was added w i t h t h e hope t h a t i t s a d d i t i o n would s t i f f e n t h e t r u s s ( ~ 1 ~ . 4 6 ) . It was n o t e f f e c t i v e , however, and i t was decided t h a t f u r t h e r t e s t s would be done on Type D o n l y . T h e r e f o r e , t h e d i s c u s s i o n w i l l be l i m i t e d t o Type D . Only f o u r t e s t s were conducted i n Type D

-

two on r o l l e r s u p p o r t s and two on f i x e d end s u p p o r t s .

T h i s type of t r u s s was t h e second weakest of t h o s e t y p e s t e s t e d having a n average f a i l u r e l o a d , w i t h t h e t r u s s e s

2 f e e t on c e n t r e , of

88

pounds p e r square f o o t on r o l l e r

s u p p o r t s and

75

pounds p e r square f o o t on f i x e d end s u p p o r t s . The d e f l e c t i o n r a t i o of t h e bottom chord was 1/480 f o r r o l l e r s u p p o r t s and 1/370 f o r f i x e d end s u p p o r t s w i t h a 40-pound-per-square-foot snow l o a d . Although t h e s e v a l u e s a r e l e s s t h a G t h e l i m i t i n g 1/360 r a t i o i t should be s t a t e d t h a t t h i s type of t r u s s d e s i g n showed a c o n s i d e r a b l e tendency t o c r e e p under t h e h i g h e r l o a d s . T h i s c h a r a c t e r i s t i c was q u i t e

marked w i t h t h i s type of t r u s s as compared w i t h t h e o t h e r t y p e s t e s t e d . T h i s would l e a d t o t h e p o s s i b i l i t y t h a t t h i s type of t r u s s might d e f l e c t c o n s i d e r a b l y more under a

s u s t a i n e d 4O-pou,1?d

-

per-square-f oot load and have a n u l t i m a t e d e f l e c t i o n conslderlably g r e a t e r t h a n 1/480 and 1/370. T h i s p o s s i b i l i t y should be i n v e s t i g a t e d by long-term l o a d i n g t e s t s .

Another f e a t u r e of t h i s t r u s s which should be examined c l o s e l y i s t h e connection a t t h e h e e l j o i n t . If t h e members a r e b o l t e d t o g e t h e r when t h e wood i s a t a h i g h moisture

c o n t e n t and t h e n allowed t o d r y , i t i s probable t h a t t h e j o i n t may loosen. Just how much e f f e c t t h i s would have on t h e s t r e n g t h and r i g i d i t y of t h e t r u s s e s i s not known and should a l s o be i n v e s t i g a t e d .

STUDY

It was decided t h a t a comparative c o s t s t u d y of t h e v a r i o u s t y p e s of t r u s s c o n s t r u c t i o n s would be very u s e f u l i n e v a l u a t i n g t h e d i f f e r e n t t y p e s of t r u s s e s a g a i n s t each o t h e r and w i t h conventional c o n s t r : ~ c t i o n . A rough time s t u d y was t h e r e f o r e c a r r i e d out t o o b t a i n s u f f i c i e n t d a t a t o make such comparisons.

In

t h e c o s t s t u d y t h e wage r a t e s and m a t e r i a l s c o a t used were t h e p r e v a i l i n g r a t e s i n t h e Ottawa d i s t r i c t . From t h e information s u p p l i e d , however, it should be p o s s i b l e t o convert t h e v a r i o u s c o s t s t o apply t o any d i s t r i c t . I n t h e time s t u d y , t h e time was observed t o t h e n e a r e s t minute and i s a rough s t u d y only. In connection w i t h t h e time s t u d y i t should be mentioned t h a t two men assembled and c u t t h e lumber and p l a t e s and f u l l use was made of a power hand saw and a bench saw. Only owe

man

d i d t h e n a i l i n g i n a l l c a s e s and t h e h e l p e r , who was not a c a r p e n t e r , merely h e l p e d ' t o put t h e p i e c e s of t h e t r u s s e s 1.n p l a c e and g e n e r a l l y a s s i s t e d t h e c a r p e n t e r . The time s t u d y , however, doea n o t i n c l u d e t h e time o r m a t e r i a l used i n making t h e assembly j i g o r i n

c u t t i n g t h e p a t t e r n p i e c e s . A summary of t h e r e s u l t s of t h e time s t u d y i s shown i n Table 111,

I n o r d e r t o compare t h e c o s t of t r u s s e s t o t h e c o s t of conventicnal c o n s t r u c t i o n on a n approximate b a s i s , t h e c o s t of m a t e r i a l t h a t was used i n conventional roof framing was c a l c u l a t e d . The summary i s shown i n Table

IV.

Labour

c o s t s were not included s i n c e no time s t u d y was made on conventional c o n s t r u c t i o n .

The c o s t does n o t i n c l u d e o t h e r p o s s i b l e economical advantages which may be obtained by t h e use of t r u s s e s .

These i n c l u d e more freedom of d e s i g n , u n o b s t r u c t e d work a r e a , e l i m i n a t i o n of b e a r i n g p a r t i t i o n s , b e t t e r o p p o r t u n i t y t o

p l a s t e r t h e ceL'.l-L~~,g and o u t s i d e w a l l s , as w e l l as l a y i n g t h e f i n i s h e d f loop witb~out p s s t i t i o n s o b s t r u c t i n g t h e work, and t h e speed w i t h wkick a Su.;-lding may be e n c l o s e d . It should a l s o be added t h s t widsr. mass p r o d u c t i o n methods, t r u s s e s could be made f a s t e r and cheaper t h a n t h e y were made i n t h e s e t e s t s s i n c e t h e r e was 1Xttl.e o p p o r t u n i t y t o p e r f e c t f a b r i c a - t i o n t e c h n i q u e s and t o m k e t h e most e f f i c i e n t use of both men employed ir b u i l d i n g t h e t r u s s e s .

D.

GENERAL DISCUSSION OF RESULTS

The f a i l u r e l o a d s f o r t h e t r u s s e s were e q u a l t o o r g r e a t e r t h a n t h e s t r o n g e s t type 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 w i t h e q u a l s p a c i n g s for each.

With t h e f a i l u r e l o a d s of t r u s s e s c a l c u l a t e d f o r 24- inch

-

on-centre s p s c i n g , t h e r e s u l t s compare q u i t e f a v o u r - a b l y w i t h c o n v e n t f c n z l c c n s t r u c t i o n spaced a t

16

i n c h e s on c e n t r e . Types B and C t m s ~ e s at 24 inches on c e n t r e have f a i l u r e l o a d s of the sd~sie o r d e r as t h e s t r o n g e s t type of

c o n v e n t i o n a l c o n s t r u c t i o ~ a t

16

i n c h e s on c e n t r e . Although Type A t r u s s had f a i l u r e l o a d s c o n s i d e r a b l y g r e a t e r t h a n Type 111 c o n s t r u c t i o n w i t h 2- by 4 - i n c h and 2 - by 8-inch

r a f t e r s , and Type I1 c o n s t r u c t i o n w i t h 2- by 4-inch r a f t e r s , i t was not a s stsang a s Types1 o r I1 w i t h 2- by 8 - i n c h

r a f t e r s and had approximately t h e same s t r e n g t h a s Type I w i t h 2- by 4-inch r a f t e r s . Type D t r u s s was a l s o s t r o n g e r

t h a n Type I11 c o n s t r u c t i o n w l t h 2- by 8 - i n c h and 2- by 4-inch r a f t e r s and Types I and 11 c o r ~ . s t r u c t i o n s w i t h 2- by 4-inch r a f t e r s . Although i t was n o t as s t r o n g as Type I w i t h 2- by 8 - i n c h r a f t e r s St had a p p ~ o x i r n a t e l y t h e same s t r e n g t h a s Type I1 w i t h 2- by & i n c h r a f t e r s .

The c ~ s t s f t r u n s e s a l s o compares quite f a v o u r a b l y w i t h t h e c o s t of c c ~ , v s ~ ? t i o ~ a l c o n s t r u c t i o n even when t h e l a b o u r c o s t f o r convent-f~r!a!. c o n s t r u c t i o n i s not allowed f o r . With t h i s c o s t o f l a b o u r f o r c o n v e n t i o n a l c o n s t r u c t i o n included t h e t r u s s e s would appear i n an even more f a v o u r a b l e l i g h t . The c o s t of t r u s s e s . a t 24 i n c h e s on c e n t r e i s impres- s i v e when compared w i t h c o n v e n t i a n a l c o n s t r u c t i o n u s i n g 2 - by 8 - i n c h r a f t e r s a t 1-6 i n c h e s on c e n t r e w i t h t h e average s a v i n g being i n t h e neighSourkood of 30 p e r c e n t .

CONCLUSIONS

The most p r o m i s i n g t r u s s e s a p p e a r e d t o be Types B and C from t h e sd:;andpoint of 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 and

u l t i m a t e s t r e n g t h s . Type A t r u s s was t h e l e a s t r i g i d and weakest of a l l t h e t r u s s e s t e s t e d as w e l l as b e i n g t h e most e x p e n s i v e t o b u i l d . Type D t r u s s , a l t h o u g h somewhat s t r o n g e r and more r i g i d t h a n Type A , was n o t as r i g i d o r s t r o n g as Types B and C . Type D was f a r c h e a p e r t h a n any o t h e r by a

c o n s i d e r a b l e margin ( a p p r o x i m a t e l y 15 p e r c e n t ) and because of t h i s it would a p p e a r t o J u s t i f y f u r t h e r development t o a t t e m p t t o improve t h e s t r e n g t h and d e f l e c t i o r - c h a r a c t e r i s t i c s . It i s d i f f i c u l t t o s a y j u s t how s t r o n g a t r u s s s h o u l d be i n o r d e r t o be a d e q u a t e . One of t h e p r i n c i p a l r e a s o n s why t e s t s on c o n v e n t i o n a l c o n s t r u c t i o n s were conducted w a s t o a t t e m p t t o e s t a b l i s h a c r i t e r i o n by w k i c h t o e v a l u a t e t r u s s e s . The wide v a r i a t i o n i n t h e s t r e n g t h of v a r i o u s c o n v e n t i o n a l c o n s t r u c t i o n s made such a comparison f o r t r u s s c o n s t r u c t i o n s d i f f i c u l t u n l e s s a minimum f a i l u r e l o a d i s a r b i t r a r i l y chosen from t h e r a n g e of f a i l u r e l o a d s shown f o r t h e v a r i o u s 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 s . F o r t h e time b e i n g t h i s would p r o b a b l y be t h e most l o g i c a l a p p r o a c h .

The second a p p r o a c h , which would be t h e use of t h e N a t i o n a l B u i l d i n g Code snow l o a d s w i t h a c c e p t e d d e s i g n

p r i n c i p l e s , would r u l e o u t t h e use of most i f n o t a l l t y p e s of c o n v e n t i o n a l r o o f f r a m i n g as w e l l as t h e use of a l l l i g h t - w e i g h t t r u s s e s .

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COST MTA FOR TrARIOUS TRUSSES

a The following unit prices were used in computing material costs in each t,russ:

...

Lumber

.

(u)

.I15 per fbm (1x6).

...

.12C per fSm

(2x&).

...

.I ?3 per ft.m

(3

plyW0Od). .1?6 Fa.' sqefi.

TCTAL CCST PER TRUSS $10.63 $ 9.78 $10.00 TYPE OF T;9USS A B C

...

Glue.. $1.00 p e r lb. Ftails

-

12.

...

.11R per lb. 24".

...

.I13 per lh. 3".

...

.I10 per lb. 33..

...

.1CV per lb. D y A T E R ~ L ?C $8.28 %7.7h $8.18 MATERIAIS LP. Born $2.35 $2.01r $l.A2 $6.68 Bblts 2 1 (*8)

-

-

LABOUR IiOURLY Carpenter (?ar hr. $1. @2 $1.82 $1. ~2 S p l i t Rings 3 ( 2 9 )

-

-

2

(ha)

' Cutting Plyvood (horns)

-

0.069 0.081 U4GE gelper (per hr.! $1.00 $1.00 Q.00

-

l n x l i " (fbm)

-

-

2 0.012 $lee2 b1.a QUAWITIES _- 1flx6fl (fi)

-

11

-

2 La&CUE TINE Cutting Cthers (hours) 0.L16 9.140 0.085 S 8.32 1 f1.00 7 OF MATERIAL 2 W W (fhm) 57 39 b7 Assembly (hours) @.Id6 0.46 o.lr78 0.180 1/2 in. plym3d (sq.fi)

-

7.73 8.60 39 Slue ( l b )

-

-

o.h@ 0.390 1.33 Nails (lb) 2.0 ( 3 3 ) 1.h7 (3") 0.15 (24") 0.l.461 ( 2 3 ) 0.370 (lgll)

-

0.39 (3") 0.635

(2P)

CONDEKSXD S W R Y OF TRUSSED-RAFTERS TESTS TABLE V COST PER TRUSS $13.63 u

a

9.78 n $IO.~O n $ 8.32 n - TYPE OF TRUSS A A B B

c

C D D

TYPE DEFLFCTIONS AT 40 psf SNOW LOAD ULTIMP.TF SXCW LOAD

OF

Em

Trusses a t 16" O.C. Trusses a t 24" O.C. Trussee _{a t} Ceflection Ridge

SUPWRTS Lower Chord Span ~ o w e r Chord

span

16" 3.c. (psf T n ~ s s e s a% 2bn O.C. (pnf) till 68 99 I l G 106 119 88 75 Rollers Fixed Rollers Fixed Rollers Fixed Rollers Mxed C.49" 0.41" 0.31" 0.27" 0.14~ 0.15" 0.33' 0.42" 1/590 1/700 1/930 1b070 11'2360 v.920 1/870 1/690 (3.37" 0.32" 9.22" 0.19"

O.oen

S.10" 0.23" 3.2511 0.86" 0.77" 9.481f @.U" 0.29" 0.22* 0.40" 0.78" 1/335 1/3 70

1/m

1/650 1hU0 1/1310 i/h80 1/370 0.67" C. 59" 0.34" 0.2811 0.1Ln 0.15' 0.40" 0.4811 193 109 157 173 167 1 8 5 lll0 120