Stress measurements in timbering for excavations

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Stress measurements in timbering for excavations

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NATIONAL RESEARCH COUNCIL

OF

CANADA Ser TH1 1392 no. 5 c. 2 D I V I S I O N

OF

BUILDING RESEARCH

m s J s . ~ . 5 0

-

Bullding Note No 5

T h e Pollowing notes were prepared as a r e s u l t o f d i s -

cussions and

exchange

of correspundenee w i t h some i n t e r e s t e d people i n G r e a t B r i t a i n , particularly Dr.

E D

J, Mwdack of t h e

Research Branch of h l e s a r s , G o Wimpey and Co, L t d , , Fubllc Works and Civil Engineering Contractors. Before coming to Canada In

the s m e r aS 1949 1 had

an

opportunity of two v i s i t s *D some

of t h e Hydro Electrie Schemes b e i n g canstsucted b y WirnpeyRsa

A t one of t h e s e s i t e s at Pitluchrg t h e constructfon of a cut-off

w a l l e n t a f l e d the excavation o f a t r e n c h 100 f e e t or so deep, Some pressure measurements were b e i n g made

In

the t i m b e r s in

t h f s trench, These measurements a r e d i s c u s s e d l a t e r in these n o t e s ,

A _{r e v i e w of}_{t h e}_{methods w h f c h}_have _{been used f o r}_{t h e}

measurement of s t r e s s e s in struts in t r e n c h e s shows t h a t the methods described below have been attempted w i t h v a r y i n g de-

grees of success, S t r e s s e s a r e measured e i t h e r by measuring

the s t r a i n in the timber or by the use of extraneous measuring

devices Ghrough which the load is t r a n s m i t t e d t o the timberingd

Method

I

D i r e c t measurement between two gauge p a i n t s serewed

' i n t o the timbsrp using a p o r t a b l e gauge i n c o r p o r a t i n g a micrometter

.

G o l d e r has d e s e r t b e d such an apparatus in a oontributlon to the 2nd International Conference on S o i l Plechanls s and Foundation Engineering, V o l . 11, page 76. A s k e t c h o f Goldergs arrangement is shown ir, Figure

I

and the stress measurements aa t h e y varied

~ 5 t h tine are shorn

In

Figure 2. Fuller d e t a i l s of this work

can b e o b t a i n e d in the paper r e f e r r e d to above,

A

s o m e w h a t

similar arrangement w a s u a e d b y WimpeyPs at. Pltlochrgn D e t a i l s

af this a r r a n g e m e n t a r e g i v e n in F i p e 3, G o l d e r u s e d a 5 f o o t gauge l e n g t h , A 2 f o o t gauge l e n g t h was used at Pitlcschry,

The advantage of t h i s method i s i t s simplicfty and

t h e absence of d e l i c a t e gauges which a r e easily damaged or upset over a p e r f o d of s e v e r a l months, Disadvantages lie in

t h e variability of t i m b e r , p a r t i c u l a r l y i t s modulua of

elasticity and the e f f e c t of moisture c o n t e n t

in

causing swelling

and shrinking, The l a t t e r has an i m p o r t a n t e f f e c t on stresses

(3)

From Lnf"omat1on o b t a i n e d f s o m the Fore st Products Research L a b o r a t o r y , I t appeaFs t h a t a change

in

1engt.h

of

the

o r d e r of 0,0C4 p e r cent m y be expected f o r a 1 p e r en*

change

in

moisture c o n t e n t , f o r Douglas

Ffr

99. s i m i l a r timber, Aaswnfng a modulus of elasticity of aboufi 2,000,000 lbJ1nz9 the stress produc.ed is about 80 1bJin2, that is a load of over

5 t o n s ir? a 12 i n c h by 1 2 i n c h strut f o r a

I

per c e n t change in mcr;srure c o n t e n t , , Obviously the rate of change of muisture

c o n t e n t in t h e h e a r t o f a 12 inch by 32 _{i n c h}_sfru%2.8 extremely sl,cw,, %he change fn the surface l a y e r s between w e t a n d d r y weather may b e s e v e r a l p e r c e n t , and t h e r e l a t h e p o ~ s i b i l l t y of a n

average c b n g e o f 1. per c e n t or m o r e ir_ a m A t t e 7 of t w o or t h r e e weeks, producing effects which canno% be ignoredm Moisture con-

tent ohangea caln also ocep In wallnga and ff t h e s e o e c w simul- taneously with m o i s k u ~ e conten* changes In the m e a s u ~ i n g a % . r u t ,

ocnsfderabLe f o r c e s can be s e t up in t h e a W u t depending on %he

d e g r e e of reatrain? 't.0 whfch the s t m " f a subjected,

Method 2 A metal gauge length is i n s e r t e d b e t w e e n t h e strut and

-

the walxngo To t h i s gauge e l e c t r f c s t r a i n gauges or o t h e r e ~ t e n s o r n e t ~ e r s are f i x e d , Elec$slc s t r a i n gauges a r e an i d e a l merhod o f m e a s w i n g strains if the condit ions a r e such as t o give

r e 2 i a b i l Z t 7 y t o t h e gauge z e r o r e a d i n g s by p r e v e n t i n g moisture p e n e t r a k i o n .to t h e gauges or o t h e r f a c t o r s whlch make than un-

certain<, An attempt whtch is b e f n g made t o o b t a i n r e l t a h l e s e a d i n g a using e l e c t r f c . s t r a i n gauges w i l l . S e described L a t e r . & % h o d 3 _In_{t h f s}_method_jacks _{a r e}_{x s e d to}_{r e l i e v e}_the_{f o r c e on}

'-rn5tal.-spacing p i e c e , The b a d taken by t h e jacks i s m e a s w e d

by pressure gauges a n d a s s u m e d to be e q u i - ~ z l e n t to t h e load

i n

the Y'F-rut, The method has been ueed bg T e ~ z a g h L and o t h e ~ s .

I

_how T i ! ? , W i l l i a m Schriever h a s f u l l I . f l o r m a t i c n

zn.

the u s e made

by El, Peck -3f t h i s m e t h o d B

In addition to s t r e s s e s b e i n g produced by mo-bstu-a changes

Pn

timber, the d r f v l n g of wedges can a l s o produce s'resses which

can compake in magnitude w i t h t h o s e r e s u l t i n g F ~ o m earth p r e s s w a s , An extr5me example cln a 12 inch by T-2 Inch s f ~ u t , I ~ d i o a t e d %a-t; wedge d r i v i n g ran produce a l o a d of 35 kens

fn t h e

s t ~ u f

A

case

c o i l l d arfse a r green t i m b e r b e j n g used for skruks an3 walings

In

9. t r e n c h e ~ n s t . r u e % e d in wintap, T h e m may b e s b i n k a g e a n d

t h e y e f o r e some ~ e d u @ $ i o n ef s t r e s s in t h e s p r i n g , T h l s a p p a r e n t

slackness

an

the tfmber w o u l d be taken up by the fuzthel: B r l v i n g

of w e d g e s , With the e n s u i n g w i n t e r , s w e l l f n g m y r e s u l t in high p r e s s u r r ~ s which my mobilltse the passive r e s i s t a n c e of the e a ~ t h ,

This passive resistance may therefore provide the most r e l f a b l e

(4)

In

o r d e r t o o b t a i n r e l i a b l e rneasu~ements, s e p a r a t i o n of the v a r i a b l e s

E s

essential, Keeplng in mind the r;bjee?ions o u t l i n e d above t o timber atruts as a measuring medium, s t e e l

s t ~ u t s and jacking methods appear to b e the m o s t advantageouso

E x p e r i m e n t a l W o ~ k Under Wag

-

- --'

-

Menklon haa been made o f t h e fnaccuracfes in m e a s w e -

merits due tc ma%stxwe changes and wedging of the st~uts, Tern-

p a y a t w e ~ a r l a t i o n s can a l s o cause e r r o r s *

In

one case on re- card a s t ~ u t was measured and f o u n d to b e

In

tension, Thfs vms

attributed r o a mistake by the e n g i n e e r r e s p o n s i . b l e p but f u r t h e r thought has brought the @ o n e l u s f o n t h a t it m a y have been 3 u e to

s w e l l f n g of the t i m b e r , T ~ p e r i r n e r a t a l work is being unde~taken by ?Vlmpeys to d e t e m f n e the e f f e c t s of the various f a c t o r s

This wo-k i s b e i n g conducted under c o n t r o l l e d conditions in the

l a b o r a t o r y yard* A n experimental trcensh is b e i n g eug8. strutted

,

and a c c u r a t e r e c o r d s kept of strut measurements. At- the same time a 3 t e m p t ; s are being m d e to d e v e l o p a r e l i a b l e elect-sf c serain gauge measping d e v i c e and fair progress has been made

a,Lcng these ISaes, As y e t

I

have not a c q u i r e d t h e f u l l . details of tk%s work a s

I

was a o t s u r e if they would be required by

the Division o f Buildfng Research"

A oonsfderable amount of interest 1s b e i n g a h a m in

.this work in B r f t a f n , A-t; a r e c e n t meeting of the B r ~ t l s h

N a ~ l o n a l C o r n i t t e e

on

Soil Mechanics in London a discussion

was h e l d on the measurement af st.resses in t h b e r % n g

in

trenches a

The m f n o o n t r i b g t i o r a s to the discussion were made b y I k 3

Lo

J,

Murdoek and R, GoLder a l o n g t h e l f n e s d i s c u s s e d in t h e s e

Stress measurements in timbering for excavations

NRC Publications Archive

Archives des publications du CNRC

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