Proposed structural testing of wooden house frames

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Ser

TH1 B92 no. k

c . 2

NATIONAL E S ~ A R C H

COUNCIL OF

CANADA D I V I S I O N

OF

B U I L D I N G RFSEAHCH

\

PROPOSED

STRUCTURAL

TESTING

OF

a d

aj,,

WOODEN HOUSE FUMES y ~ E D

G.Vt,S. 1,10,49, Buildina Note No, 4

The Division of Buflding Research I s vitally interested

in the building o f houses, especially t h o s e of wooden frame con-

struction, This is due to the extensive u s e of wooden f'rames

in Canada, as Illustrated by the fact that 4 out a f 5 homes employ thts type oS s t r u c t u r ~ , Central Mor6gage and Housing C o r p o r a t i o n , Canadars f e d e r a l housing agency, estimates that 90% of t h e dwellings constructed under their juriadfction a r e o f wooden frame construction.

Existing Canadian Practice

Rouse design in Canada has been of a standard style for a great many years, The l o c a l building codes stipulate

s p e c i f i c sizes and spacing of joists and r a f t e r s f o r roof con- . .*

atruction and standard s i z e s and spacing of studs for w a l l sections to ensure structural s t r e n g t h , In a d d i t i o n , walls have an e x t e r i o r sheathing a p p l i e d beneath the e x t e s l o r clad- ding which probably originally served as

an

insulation material, as w e l l as adding strength to t h e structure. As more efficient

Insulating materials were d i s c o v e r e d , t h e y were i n c o r p o r a t e d

into the w a l l section and of recent years due to the p r o b l e m of condensation, vapour barpiers have n e w been added, Finally,

the i n s i d e finish I s added t o form the completed w a l l s e c t f o n , These constructPon designs have been evolved through experience and n o t as a r e s u l t of rfgourously c a l c u l a t e d design,

3 Requirements of the NatSonal Building Code

The design loads as s e t o u t in the National B u i l d i n g Code ( 7 ) were originally established a r t e r a study of the exist- ing codes of' a large number of Canadian cities, Until recently there has been no need to r e v i s e t h e Code as dwellings erected under it have safely w i t h s t o o d the most serious of l o a d i n g

c o n d i t % o n s , As a r e s u l t of this structural sufficiency, t h e m

has been l i t t l e correlation between design l o a d s and common b u i l d i n g practice,

4, T , o a d k n ~ and Desfan P r a c t i c e

With materials and labour at an a l l t h e high c o s t there is a great need f o r research into loading and design

pracl5ce, Whfle s a f e t y must s t i l l rernafn the governing factor, I t is necessary to gfve more concern to effecting economies wherever possible f n t h e c o n s t n c t l o n of houses.

It

i s felt

t h a t t h e d a t a to be secured f r o m the proposed structural t e s t s , outlined

in

a subseque~t e ~ c t f o l z in this Building Note,

will

enable a more efficient correlation between s e r v i c e l o a d i n g and t h e s t r u c t u r a l -suffic%ency o f t h e v a ~ i o u s members t o be established, Some members might be reduced in s i z e , their s p a c i n g increased, some claddtng materials eliminated and a more efficient form o f fastening found, r e s u l t i n g in an appre-

ciable saving in t h e cost of materials and labour, 5, Investfgatfon of' House D e s l g n in Other Counkries,

A thorough study of investigations c a r ~ i e d o u t

in

other countries has been made and e x t r a c t s f r o m subsequent reports are herewith included, In the Unlted S t a t e s , in an endeavour t o apply engineering pPinciples t o house d e s i g n , the

U,

5 , 3ureau or Standards, as o u t l i n e d in BMS2, 1938, (la),

established standard laboratory t e s t s t o be performed on unl- formly sized specimens of the elements of a house as shown in Table

I,

TABLE

I

These t e s t s a r e made on e l e m e n t s c o n s t m c t e d f r o m a g r e a t v a r i e t y of materials, All tests i n v o l v i n g wood conptruc- tion are conducted with the close co-operation of the U, S,

Forest Products Laboratory, (1), E l e m e n t - Walls NonLoad B e a r i n g

1.

Partitions loors

lo.s

_-

The IT, S , Bureau of Standards adopted the above method

of tests in preference to testing a complete house for the

f o l l o w i n g r e a s o n s :

"Subjiecting c o m p l e t e houses to known l o a d s is v e r y expensive and r e q u i r e s much time; t h e r e f o r e , this method of

carrying o u t investigations of houses i s n o t likely t o be u s e d Specimen S i z e 41 x 8P . 4 f x 8p 4q x 1 2 ' 6 f t 4c x 14v 6" Type of ~ o a d Applied compressive, t r a n s v e r s e , con- c e n t r a t e d impact, rackfng, impact, concentrated, transverse, concentrated, impact , transverse, concentrated,

to any g r e a t e x t e n t , Such tests have t h e f u r t h e r disadvantage

that only the s t r e n g t h o f t h e weakest element o f a p a r t i c u l a r

house c o u l d be measured, As one i l l u s t r a t f o n , t o determine t h e

strength of t h e second-storey floor in a house, loads c o u l d be

applied

to

the f l o o r .

If'

the walls crushed b e f o r e the

maximum

load f o r t h e f l o o r was appkled, F t would be necessary

t o

t e s t

another house having st rongar walls,

'F

(13)

It

should be n o t e d , however, that the testing of specf- mens only, neglects t h e important e f f e c t of adjacent elements on

the element to be tested, and also the quality of workmanship would tend to b e superior to t h a t found in the f i e l d , The s p e c i -

mens t a s t e d a r e usually of a uniform size of 4 r e e t by 8 feet, t h u s allow'lng the use of standard laboratory testing apparatus

and recording fnstmments, . .

In

Great Britain the B u r t Committee was appointed in 1942 to consfder the housing problem, due fn

no

small r e s p e c t

to the ravages incurred during World Nar 11,

It

w a s d e c i d e d to l o a d complete prototype houses at a constmiction site or an t e s t beds sftuated on the grounds of ths Building Rssearch S t a t i o n (2,

3 , 4, 6 , 9 , 10, 111, This method was adopted as I t was imperative

to approve new designs and have construction s t a r t e d immediately,

These loadlng t e s t s determined the s t r e n g t h and elasticity of the s t r u c t u r e for a prescpibed design l o a d and gave no indica-

tion of the m o u n t of' overstrength, These approvals d e a l t

essentially with new types of construction using materials which w e r e cheap and readfly accessible, The method of Loading t h e

complete house i s illustrated in Table

If,

and

in

P l a t s 1,

I V e r t i c a l l Flat r o o f

(

t t !

IAs

me Of Load V e r t i c a l Element F l o o r Vertical P i t c h e d r o o f

1

H Load a p p l i e d at p u r l i n

1

i points, either by weights

I

I

l

or by p u l l - l i f t s ,

i

i

Vertical

I

Impact r e s i s - (~ifferent s i z e weights 1 tance dropped f r o m prescribed

!

heights , i

Structural

Roof sheath-

1

Local s t ~ e n g t h A small p l a t f o r m

is

loaded I

i n g j(ma1ntenance

lowds)

1

with weights,

I

I

V e r t f cal

Method of Load ing

F l o o r ! ! S t r e n g t h and ! stiffness

1

1 I A pull at f i r s t floor l e v e l and eaves l e v e l by a pull-lift using a g r i d Property S t r e n g t h and stiffness

I

k t

d i s t r i b u t e the Load,

I

1

Weights a r e stacked so as

t o secure equal distribu- I

, t i o n oS t h e l o a d ,

I

E

A great many c o u n t r ~ i e s , Including the United S t a t e s ,

Great Brktain, Australia and Sweden, have been examining exist- ing f i g u r e s f o r design l o a d s w i t h a view to reducing them. The establishing of a r b i t r a r y d e s l g n l o a d s or f o m l a e f o r a r r f v f n g

at them I s very difficult due to the g r e a t variance I n the actual

l o a d i n g of house structures.

In

Brltain ( 4 ) t o conduct t h e t e s t s outlined prev9oualy, the design l o a d s were revfsed and the f o l l o w - fng summary of strength and stiffeness requirements prepared,

In Canada t h e r e I s a g r e a t variance

En

the d e s i g n l o a d s as contained in

the

municipal building codes and

in the

N a t i o q a l Building Code (7') as illustrated by Table IV,

TAB=

IV

LIVE LOADS FOR

DWELLINGS

l ~ o d 8

I

Floors

I

R o o f

I

Wind B, C, I

1

50 i b s .

/

10 i b s , st 50' slope boronto , f 20 I b s . a t 50' ! 40 l b s , at 5" s l o p e

/

slope

I

i

straight Ifne calcula- I

I

!

i

' tlon

o

to W 0 slope, Montreal. 4 0 Z b s , - - -20 lbs,-40 I b s . accord- 20 l b s , vertical I

i n g to snow f a l l In area projection

1

4 0 I b s ,

I

i

!

/Sudbury 40 i b s . up to 20' sbope

S t e p s are now being taken to evolve a g r e a t e r degree of standardization between t h e varfous codes, and 2 t 5 s felt t h a t the proposed t e s t s should afd materially In thfs t a s k ,

30 l b s . on r o o f

1

1

[Vancouver

t

i

Ne.

D. V,

Isaacs of A u s t r a l i a has made a thorough study of design l o a d s for domestic dwellings and his ffndings are pub- l i s h e d In Bulletfn No, lg '9?he Structural Sufficiency of Domestic Build9ngsv, ( 5 ) Extracts f r o m the Recommended Code for* S t r u c t u r a l Sufficiency contaened In the above bulletin a r e shown in Table V,

50 l b s .

-

20' s l o p e or less 4001bs.

-

30 l b s .

i

50 lbs0

i

i 20 i b s . f o r over 20

.

1

surface if slope

o v e r 20'.

I

f

1

I

i

:

4 0 i b s ,

!

40 i b s , f o r 20' s l o p e 110 i b s . p e r sq. ft. 1 less 25 lbs. f o r 2Q00 of v e r t i c a l p r o j e c -

1

to 45'

o

f o r o v e r 45 ,

1

tion.

!

May b e fgnored over 20 and 112 I b a , I I i reductton per d e g m e in 1 i excess of 20°, i 1 I

TABLE

Type of 1 I

Element _{Loading Allowable Load} I

I 4 F l o o r

I

Floor 1 Roof a ( n u t open ! to r e s o r t ; ) I ! t

1

Roof

1

Floor ! 1 Localized l i v e load Impact load Superimposed Localf zed ( f o r main- tenance ) D e f lee t l o n I

W

,

450-5s2 Wesuperimposed l o a d i n g

L s p a n of joist beams, etc, i

Ssspaclng of joists, beams, .

e t c . i I

400 i b s . on 5 sq. in. " c i ~ c l e l o r 1/4 hr.;

!

Static l o a d of 3000 I b s , on 5 sq, in. c i r c l e f o r 1/4 hr.

W

=

-

200 but not less than 10

L*

4

W=superimposed load

L-span of roof truss or beam

200 l b s . 1 ft. dia. c i r c l e Por 114 hr.

\

Deflection n o t to exceed 1 to 600 under

(

a 80 lbs,/sq, ft, l o a d ,

_j

i Roof Deflection

1

Span

/

300 (combined dead and lave l o a d ) /

I I

It

should be realfzed that the above summary shows only some of t h e b a s i c l o a d s as proposed by

MY,

I s a a c s O and i t s c h i e f function is to show the Intense study t h a t is befng m d e in t h i ~

f i e l d ,

I

Wall Deflection V e r t i c a l span of wall

/

450 under I

The TJn5ted S t a t e s Bureau of Standards has conducted

exhaustive s t u d i e s into the a p p l i c a t i o n of engineerfng prtnciples to s t r u c t u r a l d e s i g n and have p u b l i s h e d

B,M.

S. I l e p o r t 109 (123

on

this subject,

\

V e r t i c a l

:

Pressure and

, s u c t i o n

This is a comprehensive r e p o r t d e a l i n g w i t h the d e t a i l e d

analysis of house frame.s and does not lend itself to condensing,

15 shows t h e p e a t number of variables ta be encountered In the de- sign of the house and the e f f e c t of var2ous cl4matic eond%tfons,

specff ied wLnd pressure, i

I

T o t a l p r e s s u ~ e and s u c t i o n of f r o m 8 t o 14 l b s , p e r sq, ft, in b a s i c table,

i

In

Performance Standards, s truc t u s a l and insulation require- ments for houses, as issued by the Housing and Home Finance Agency, Washington 25, D O C , , ( 8 ) design l o a d s a r e recommended a s f o l l o w s :

1

The loads shown in tabular form fn Table

VI

show the

t r e n d towards lowering design l o a d s on structures

In

the

U,

5,

It

fs realized t h a t in a great m a n y areas having d i f f e r e n t c l i -

matic conditfons l o c a l codes wfll

of

necessity be

In

variance,

It

can be seen f r o m the foregoing ffgures on destgn

l o a d s that g r e a t variations exist, and consequently one of the

chfef objectives of the proposed tests w i l l be

to

study and re- vise, if possible, the present Canadfan design loads, A study will then b e made i n t o their c o r r e l a t i o n with She actual l o a d s imposed during t h e proposed t e s t a ,

7, Cast Analgsin of a Frame Bungalow

In

order to ascertain the r e l a t i v e proportions of the

c o s t s of conatructing a t y p i c a l small frame dwelling, a cost

analysis-was s o l i c i t e d f r o m C e n t r a l Mortgage and Housing Corpora- t i o n , p o r n this detailed analysis submitted f o r f o u r frame

bungalows built

In

v a r i o u s sections

o r

Canada, Table VIf w a s

drawn up. This Table shows in percentages the c o s t s of materials and l a b o u r in relation to thefr total cost, neglecting t h e land c o s t s , contractorRs prafits and overhead expenses,

XEAKDOWIT 0-GALOW COSTS

Description 1. Excavation (Tiles, e t c . )

2 . Footings, Foundation Walls

3. Outside Walls

1

a, Wood s i d i n g

/

1. sheathing (paper) c, F r a m i n g

i

d. Insulation (;eather s t r i p ) e , Inside f i n f g h I

1

4, Partition Walle

I

a, Framing I b, FSnEsk each s i d ~ ' 5. Basement Floor

I

a. 3111

!

I b. S l ~ b c , Aux, wbpk 6, FSrst Floor a, Posts, beams, j ~ i ~ t s , brf b, Sul-fEabrPng c - Finish flaorfng 7 . Attic H.OOP

a, Finish f i r s t floor ceileng 1. Joists and bridging

8, Roof

R. rnfters, c o l l a r ties, ridge

b. I n s u l a t i o n

c , Roof sheathing (paper)

d. Ffnish r o o f i n g (shingles)

9, Mlllwork ( i n t e r f o r trim, doors, windows, Bftchen, bathroom, cabinets.

10, Stefrs (including ~ s r c h less 11. Beating 12. Plumbing 13, Electrical 14, Faintfng, glazing 15. Hardware 16, Grading, landscaping 17, Flaahing 1 0 , Chimney (Bsnonry, e t c . ) 19. Bathroam Tile, accesso~ies

From a study of Table

VII

it I s seen t h a t the c o s t s of

the structural elements

farm

approximately

ZL$

of the house c o s t , as shown in Table VIII,

It

is f e l t

in

additzon t h a t the finishes add t o t h e strength provided by the framing members, T h e r a f a r e , approximately 33% of t h e t o t a l c o s t i s d e v o t e d e i t h e r wholly or

partially to t h e strength of the house, TABLE

VIII

I

~esorlption

1

Average

$

1. Excavatfon ( T i l e s )

i

2,40

2 , Concrete work _I 1 2 , 5 2

1 3. S t r u c t u r a l Slements (above base p l a t e ) 1 2 o 0 9 8 ) 3 4 s

.1

I i

1 4, Fjlnlshes ( l a t h , p l a s t e r , shingles)

!

13,11) F a i n t , glazing I 5,99

I

17.30 I 1 7, M i l l w o r k 15,17 I 8 , ~ ~ i s c e ~ ~ n n e o u s

i

12.53 !

I

i

l00.00$

I

8, D e t a i l s of T e s t

Against the background of t h e foregoing information, a s t u d y of t h e most u r g e n t problems makes it _{c l e a r t h a t the f i r s t}

approach is t o experiment w P t h an actual f'rarne of a standard

Canadian house, ThLs frame should be subjected to s e r v i c e 1 oading conditions as closely as possfble sSnce this a p p a r e n t l y has n e v e r been done for this t y p e of structure, It i s proposed to e r e c t the framework of a one storey house fn a building at the N a t i o n a l llesearch Council so that it can be loaded,

~t is proposed in this test to d e t e r m i n e if overstrength exists in conventional wood frame structures and t o what degree,

If such a c o n d i t i o n e x l s t s s study will be made i n t o the possible reduction of material in the various elements and will also i n c l u d e an fnvestlgation i n t o the method of fastening members w i t h a vtew to superseding t h e present method of nalling,

This t e s t wfll be c a r r i e d o u t in one of the bulldings l o c a t e d on t h e grounds of the N a t i o n a l Research Council, Montreal R o a d , The house frame to be t e s t e d w i l l be a small f o u r room bungalow which will have its sill p l a t e s e c u r e d by anchor bolts

to s tee1 beams l o c a t e d In the l a b o r a t o r y f l o o r , simulating normal anchorage to a foundation, Tenders for the construction o f t h i s f r a m e will be c a l l e d f o r f r o m outslde contractors sa as to sim- l a t e ac%ual building p ~ a c t f c e as closely as possible,

' h e general t e s t procedure w o u l d be as f o l l o w s ~

( a ) Load the bare frame composed of the r a f t e r s , j o i s t s , and studding to a c r i t i c z l d e f l e c t f o n and r e c o r d the l o a d at this deflect5on0

( b )

I n s e r t

braclng between these members and apply t h e l o a d found above and record t h e d e f l e c t i o n ,

( c ) Apply claddfng and r e p e a t the above procedure,

( d ) A t t h e conclusi~n of the above tests the complete structures

will b e l o a d e d until aa obvfous f a i l u r e r e s u l t s ,

The filast twse tests s h o u l d show

on

a comparative b a s i s the effect of the bracing and cladding en the s t m c t u r a l s u f f l c - iency of the house frame, The f o u r t h test will determine to w h a t

degree t h e structurps i s overdesigned,

A i r c r a f t hydraulic jacks w i l l be used f o r applying the l o a d s to the v a r f a u s elements, The l o a d s w i l l be transmitted

f r o m the jacks t h o u g h a more e l a b o r a t e grSd system than t h a t

shown in Plate 1, ThSs g r i d wfll c o n s i s t of a sufffcfent number of members to insure a uniform l o a d i n g over the whole l o a d f n g surface, thus a l l o w i n g f o r compa~ison with the design Loads as set o u t

in

building codes,

In loading the structure the following design l o a d s as l a i d dawn by the Natfonal Bufldfng Code wP11 be used:

1, F l o o ~ l o a d

-

40 ~bs,/s~,ft,

2, Roof load

(a) Snow l o a d

-

30 lba,/sq,ft, ( b ) Wind load

( 1 ) s u c t i o n

-

9 ~bs,/sq,ft, n o m a l to slope of r o o f (if) pressure

-

(,ax

-

9 ) where x = pitch of. roof

therefore, with slope o f 30' pressure

=

z e r o , 3, Wfnd on v e r t i c a l w a l l surface

-

20: l b s , , / s q , f t , -

(a) 2/3 o f the assumed wind f o r c e s h a l l be consfdered to be a normal pressure a p p l i e d to windward wall, and (b] 1/3 shall be c o n s i d e r e d t o be a nomnal s u c t i o n a p p l l e d

to the leeward,

Actual combfnatfons for c r f $ l e a l l o a d i n g condltfons a r e s t i l l m d e s consfderation, It is proposed in this test to employ those combinations wMch most c l o s e l y approximate service condftions, A s an f l l u s t r a t f o n , when testing a v e r t i c a l w a l l surface f o r resfstance to wfnd pressure, suction l o a d s would be a p p l f e d to t h e leeward wall and to the leeward r o o f slope neg-

lecting any vertical live Loads, DurLng the test v a r i o u s combina- t f o n s of l o a d i n g will be employed t o determine lf a m a r e c p i t f e a l condftf on p r e v a i l s when t h e live l o a d s a r e Imposed, I

9 , Conclusion

The p ~ o p o s e d t e s t s should ahow the

stiffening

effect

on t h e f r a m e by bracing and cladding,

4

study w i l l then be made into the possible reduction o f materials in wooden house

frames a s w e l l as more e f f e c t i v e methods of fastening m e m b e r s ,

This testing of a conventfonal s t m c t u r e ' s b e h a v i o r under s h - r a t e d service c o n d i t i o n s should h e l p

fn

evaluating the m e r i t s o f new canstructfon methods and materials and s e t t l n g up stand- ards f o r them, A subsequent project might w e l l be the designing and testing of a completely engineered house, capable of with-

s t a n d i n g s e r v i c e l o a d i n g w i t h a peasonable factor of safety,

This proposal. has been prepared for submission to t h o s e groups inAt;erested in house c o n s t r u c t i o n and is submitted w i t h

a view to receiv-lng comments f r o m m e m b e r s of the foregoing groups,

?repared by:

G o W, S h o r t e r , O c t o b e ~ 1, 1949,

R e f e r e n c e s

Heck, G,E,, S t r u c t u r a l P r o p e r t i e s of Conventional

Wood-Frame

C ~ n s t m c t f o n s f o r Walls, P a r t f t f o n s , F l o o ~ s and Roofs,

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