Bracing for Wall Formwork

7/30/2019 Bracing for Wall Formwork

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Brac ing for form w o rk is like

an insura nce po licy; yo u

m ay think you dont ne ed

it , but when the storm s of

life h it, youre m ighty glad its th ere.

Co n c ret e Co n s t ruc tio n re c e n t l y

h e a rd from a con tractor who was

w o rking with gang form pa nels 20

feet h igh wh en a strong wind flat-

tened som e of the already b ra c e d

p a n e l s. In som e cases n ails pu lled

ou t of the stakes to wh ich th e b ra c e s

we re attach ed; in oth ers the sta kes

t h e m s e l ves p ulled ou t of th e

g rou n d. Tra g i c a l l y, on e wo rker was

killed as the gusts reached 50 m ph .

What went wrong? We dont have

enou gh det ails to m ake an accura t e

diagnosis of this case, but so me o f

the wa rning lessons are clear.

Avoid work in st ron g winds. A

l a rge form pan el ca n becom e a

kitewith or with ou t a stri n g a n d

w rea k h avoc at th e con st ru c t i o n

Br a c in g fo rwa ll fo r m wo r k Its needed for stability and a lignm ent

BY M. K. HURD

Figure 1. Double-channel braces share duty with adjustabletubular braces on this high w all form. To make t he braces,two lightweight channels are welded together 2 12 inchesapart, using end plates and spacers. Four-inch round holesin t he channels reduce weight and open up space betw eent he channels for ease in making bolt ed connections.Similar double-channel members serve as strongbacks fort he ganged form panels.

Figure 2. Adjustable t ubular metal braces may be part of amanufactured forming system or they may be add-ons for

job-built forms. Here brac es are anchored t o massiveportable cast concrete blocks. In addition to resisting windand maintaining alignment, braces for this t all form mustbalance unsymmetrical loads on the access scaf folds.


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s i t e. Cran e op era to rs cea se work i n gwh en wind s a re too high. Ma n u a l

o p e rations with large p ane ls shou ld

be stopped too.

Brace a dequ ately. In ad dition to

the fam iliar wood wedges and 2x4s

an d 2x6s, there are m an ufacture d

b rackets for atta ching an d ad justing

b ra c e s, adju stab le pip e bra c e s, an d

heavier manufactured members

( Fi g u re 1) design ed for effective

b racing. For high form s an

e n g i n e e rin g desig n o f

the bracing sys-tem is advis-

a b l e.

Pay at tent ion to conn ection de-tailswheth er they be na ils or m an-

u f a c t u red h ar d w a re . Fo rm w o rk is

often nailed sparingly to

sp eed th e str i p p i n g

p roces sa p ra c-

tice that can

b a c k-

f i re wh en un usu al loads occu r. An-

chor braces to som ethin g solid. A

wooden stake casu ally dri ven int o

n o n u n i f o rm so il m ay have in su ffi-

cien t re s i s t a n c e. Anch orage to con -

c rete slabs, po rtable con crete b locks

( Fi g u re 2), or dead m en be low gra d e

m ay be nece ssary for high form s

with h eavy loads an ticipat ed.

High form s re q u i re m ore car e f u lplann ing. Wind is more intense a t

h igher leve l s. Brace s car ry less loa d

as their unsupported length in-

c re a s e s.

Bracing requirements

With dou ble-side d form w o rk, ties

h old th e form s toge th er, re s i s t i n g

th e latera l p re s s u re of th e con cre t e,

but external supp orts are needed to

resist applied working loads and

wind pre s s u re. On th e occasion al

job th at p roh ib it s t ies t h rou gh th ewall, the b racin g m ay also ha ve to

resist the late ral pre s s u re. Su p p o rt is

Figure 3. Hardware items simplify the use of t imber bracing. Patent ed turnbuckle form aligners have a variety of endbrackets and clamps that make it easy to connect the aligner to the timber and to the formwork framing members.Turnbuckle adjustment helps set t he form plumb. Other devices such a swivel bracket s can be used to vary t he inclinationof the form, at the same t ime anchoring it t o a slab or other solid member.


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needed n ot only when the form-

w o rk is erected in p osition but also

when it is m erely standing on site

awa itin g t he n ext us e (a fre q u e n t l y

neglected condition).

The wall form is m ost vulnera b l e

d u ring erect ion o f the first side, b e-

f o re any of th e re i n f o rcin g s teel has

been placed. The larger the p an els

bein g ha nd led, th e great er th e ri s k .Bracing should b e pu t in place as

early as possible for su ch form s,

p a rticular ly whe n there is any pos si-

bility of strong wind .

Many form builders prefer to

b race wall form s on on e side o nly.

Bracing is usu ally installed to m ain-

tain the posi t ion and a lignmen t of

the first side of the form ; then th e

second side can be set an d aligned

f rom the first with spreaders or

s p read er ties t o m ain tain th e corre c t

wall thickn ess.This bracing m ust be stro n g

enough and well enough a nchore d

to resist wind and other loads such

as imp act from con crete as its

du m ped , equ ipm ent m ove m e n t ,

and access scaffolds that ma y be at

sub ject say tha t braces fastened to

one side of the form s and nailed to

stakes set into th e ground ab out 8

to 10 feet apar t p re ven t th e form s

f rom shifting when concrete is

p l a c e d . This ru le-of-th u m b ap -

p roach is best limited to 8-foot wallsat grade with no attached work plat-

f o rm s or scaffolding. Even th en ,

un anticipated loads m ay make i t

h a z a rd o u s. Bra ce s ar e fre q u e n t l y

made from 2x4 or 2x6 lumber,

which provides limited strength un -

less knee braces or trussing mem -

bers are add ed.

Bracing design

Wind can com e from any dire c t i o n

and the bracing system must be

read y for it. If braces are p ositioned

on only one side of the wall, they

mu st be ab le to take either tension or

c o m p ression and the ir con nection s

h a ve to do the sam e. Lateral loads

ma y come not on ly from th e wind ,

but also from cable tensions, inclined

s u p p o rt s, dum pin g of con cre t e, or

impact from p lacing equipm ent.

Wall forms sh ould be braced for

the wind loads p re s c ribed by the lo-

cal co de for p erm an en t st ru c t u re s.

For use in de sign , th e wind ve l o c i t y

is co mm on ly co nve rted to re s u l t a n t

p re s s u res in pou nd s p er sq ua re foot

( p s f), for a given he ight zo n e. Wi n d

maps of the United States show

p re s s u res from 20 to 50 psf for va ri-

ous regions of the country, withsom e of the highest pre s s u res in the

coa stal South an d East. Howe ve r,

th e int en sity of p re s s u re in cre a s e s

with height a bove grou nd. For ex-

ample:

In the m ost m oderate win d zo n e,

15 psf m ay be u sed for design of

walls up to 30 feet high. In the

sam e zo n e, at elevation s a bove

100 feet, th e design pre s s u re is 30

p s f .

W h e re t here is n o local co de re-

q u i rem en t, th e Am erica n Co n c re t eInstitute (ACI) Com mittee 347, Fo rm -

w o rk, advises d esign ing wall form

b races for at least 100 poun ds p er lin -

eal foot (plf) of wall app lied at t he top,

or 15-psf wind load, whichever is

g re a t e r.

How effective

can 2x4s be in re-

sisting this load or

other wind loads?

Notice the axial

load capacities

for different typesof bra cing sh ow n

in the table. At

len gth s gre a t e r

than 6 feet, the

2x4 needs some

lacing or latera l

s u p p o rt to m ee t

desig n re q u i re-

ments for allow-

able loads.

A simple

exampleNea r De t roit , a

design value of 25

p sf is pe rm i t t e d

for wind load on

s t ru c t u re s les s

than 30 feet high.

What does this

mean for wall

f o rm s 10 feet high ? Fo l l ow ing th e

ACI p ro c e d u re for design (Ref. 1 ),

this equates to a force app lied at the

top of the wall of wall height/ 2 x

wind pre s s u re= 10/ 2 x 25 or 125

pou nd s per lineal foot of wall

If this calculation would com e out

less than th e ACI 100-plf m inimu m ,

use th e m inimu m 100 plf. Assum ing

the b race is attached 3 feet below thetop of th e form, a ho ri zont al re s i s t-

ing force of 10/7(125) plf or 179 p lf is

re q u i red to balan ce th e ove rt u rn i n g

effect of the 125-p lf wind forc e.

With the b race attach ed 3 feet be -

l ow the top of the form and the end

of the brace an chored 4 feet away

f rom the wall, you can use th e re l a-

tionship be tween sides of a right tri-

angle to find the length o f the b ra c e

and the load it mu st carry.

If bra ces we re sp aced at 8 feet,

each one would have to carry8 x 360 or 2,885 pou nd s

A 4x4 or some of the manufac-

t u red braces shown in the table

would b e suitab le. The 2x4 would

no t b e stron g en ough. Many b ra c e s

s t rong e no ugh t o carry this load in


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c o m p ression wou ld also be ad e-

qu ate in ten sion. Howe ve r, th e

s t rength o f the con ne ctions m ust b e

adequ ate for the ten sion load . An al-

t e rn a t i ve m eth od of b racin g u sing

guy wires that carry tension load on-

ly re q u i res b racin g on bo th sides of

the wall.

Walls below grade are not sub ject

to wind and bracing adequate tom aintain alignme nt is generally suf-

ficient for walls 8 feet ta ll or less.

Taller wall form s be low gra d e

shou ld be designed for at least 100-

plf lateral load ap plied at the t op

(the ACI m inimu m ).

This simplified example shows the

significance of wind forces on form -

w o rk. Bracing d esigns should always

be m ade with ap pro p riate data for

the site being considered.

M . K. Hu rd is an en gine er-w r i t e rbased in Farmington Hil ls, MI. Sheis also a forme r edito r of Concre t e

C o n s t ruc tio n an d was form erly as t a f f engi nee r for the American Con-crete Institute.

References

1. M. K. Hurd, Formwork for Concrete,American Concrete Institute, P.O. Box19150, Detroit, MI 48219, Fifth Ed.,1989.

2. ACI Committee 347, Guide toFormwork for Concrete (ACI 347R-89), ACI, 1989.

3. Formwork: A Guide to Good Prac-tice, prepared by a joint committee ofThe Concrete Society and The Institu-tion of Structural Engineers, The Con-crete Society, London, 1986.

Pub lication # C910545

Co py righ t 1991, The Ab e rd e e n

Gro u p. All rights re s e rve d

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Bracing for Wall Formwork