Charts Aid in Design of Horizontal form work

7/28/2019 Charts Aid in Design of Horizontal form work

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Char ts aidin design of

horizontal

formworkSelect plywood decking, joists,

and stringers; find shore loadings;all without lengthy calculations

BY PAUL H. SOMMERSCHIEF ENGINEERALGERNON BLAIR INCORPORATEDM ONTGOM ERY, ALABAMA

Fo rm w o rk design can be simple and stra i g h t f o rw a rdwhen the designer understands the fundamentals t ru c t u ral re q u i rements of each member making up thef o rming system. Each piece of wood or plywood in adeck forming system must meet standard limits forbending, deflect ion, and shear. Once these re q u i re-ments are met, a stru c t u rally sound forming system canbe built.

By doing design calculations for commonly used size sand grades of materials and combining the results in a

s e ries of charts like those presented here, we can avo i dtedious recalculations and get some answers in a fewm i n u t e s. These charts are much like the ones publishedby CONCRETE CO N S T RUCTION in Ap ril (page 392), ex-cept that they cover form w o rk for hori zontal instead ofve rtical stru c t u ral members.

The charts provide a quick re f e rence and we re deve l-oped especially with the smaller contractors in mind.They will also be valuable to the estimator who is pre p a r-ing bids for form w o rk, and they will help the contra c t o ror engineer in pre p a ring or checking form w o rk shopd ra w i n g s.

The charts for supported hori zontal form w o rk prov i d eenough information to choose the following for the av-e rage forming system, using materials of the quality in-dicated later in the text.

Plywood thickness

Joist spacing

Joist size

St ringer size

St ringer spacing

HOW TO USE THE DESIGN CHARTSFOR HORIZ ONTAL FORMS

St a rt in the upper left-hand corn e r. The dotted lines h ows a sample problem which has already beenw o rked out.

1. DETERMINE LOAD ON THE FORMS. Enter theupper left section of the chart at the top with desire dslab thickness (PointA) or at the bottom with a pre-

d e t e rmined design load (Point B). NOTE: If you usethe slab thickness, this chart automatically prov i d e sfor the construction live load and weight of form s.

2. CHOOSE PLYWOOD THICKNESS. T h ree choicesof plywood thickness are ava i l a b l e

12,

58, and

34

inch. Draw a ve rtical line from PointAor Point B t ointersect the curve for the desired plywood thicknessat Point C.

3. FIND JOIST SPACING. Draw a line straight acro s sf rom Point C to intersect the curve for the chosen ply-wood in the upper right chart at Point E. This linec rosses the first chart boundary at Point D, where weread the plywood span. THIS IS ALSO THE MAX I-

MUM JOIST SPAC I N G .4. SELECT A JOIST AND FIND ITS SPA N . From Po i n tE d raw a line down to intersect the curve for the de-s i red size of joist. Six choices here range from 2x4 to4x6. The intersection is Point F. Now extend the linea c ross to the left until it intersects a curve re p re s e n t-ing the combination of joist and plywood already se-lected. In the example shown by the dotted line,

34 -

inch plywood and 4x4 joists we re chosen. The newintersection is Point H . Go back along this hori zo n t a lline to Point Gw h e re it crosses the chart border andread the maximum joist span. THIS IS ALSO THESTRINGER SPAC I N G .

5. SELECT THE STRINGER AND FIND ITS SPAN. Inthe lower left-hand chart, find the curve re p re s e n t-ing the desired stri n g e r. Choices range from a single4x4 to a double 2x10. Draw a line straight down fro mPoint H to intersect the desired stringer curve at Po i n tK. From this intersection extend the line across intothe lower right chart until it intersects the curve forthe same stringer size at Point N. ( In the samples h own, a double 2x8 stringer is being used.) At Po i n tMw h e re the line crosses the chart boundary, read themaximum span of the stri n g e r s. THIS IS ALSO THEM AXIMUM SPACING OF THE SHORES.

6. FIND THE SHORE LOA D.From Point N d raw a lines t raight down to the bottom of the chart and find the

s h o re load at P. Numbers must be multiplied by 100pounds to get the actual load. For the usual 4x4d ressed post shore of So u t h e rn ye l l ow pine or Dou-glas fir, it is desirable to limit the load to 4000 pounds.If it goes above 4700 pounds, crushing of the stri n g e r scan be expected. The chart gives LOAD APPLIED TOTHE SHORE BY ONE FLOOR OF CONCRETE ANDFORMS. For shore design in multistory work, a larg e rload must be considered because of loading fro ms e ve ral stories above. Consult Fo rm w o rk for Co n c re t efor more details plus bracing re c o m m e n d a t i o n s.


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Sh o re spacing

Load imposed on shores (for multistory work, loadf rom the chart is multiplied by 1.5 to comply with AC I3 4 7 - 7 8 )

The drawing shows how these members are arra n g e din a simple slab forming system.

BRACING AND LACING MUST BE ADDED

Bracing and lacing needed for lateral stability are not

included in the chart s. THEY MUST BE PROV I D E D, tomeet OSHA and local code re q u i re m e n t s. We re c o m-

mend that they also comply with Section 2.3.2 of theA m e rican Co n c rete Institu te form w o rk standard, AC I347-78. The lateral bracing must be able to resist windloads and other lateral loads. ACI recommends a mini-mum of 2 percent of the dead load on the form, distri b-uted evenly along the slab edge, but not less than 100pounds per lineal foot of slab edge. Lacing (hori zo n t a lties) is needed to maintain alignment and reduce theu n s u p p o rted length of tall shore s.

PRECAUTIONS NEEDED IN M ULTISTORY WORK

If these charts are used for slabs in a multistory build-ing, the user is cautioned that the shore loadings show n

a re for only one floor. In multistory b uildings, shore smay carry load from seve ral floor levelssometimes asmuch as three times the weight of a single floorde-pending on the particular building cyc l e. Refer to Fo r m -w o rk for Concrete, ACI Special Public ation No. 4, form o re details.

BASIS FOR THE CHARTS

Some compromises we re made in setting up the de-sign charts to take care of as many situations and op-

tions as possible without making them complex and un-re a s o n a b l e. They are in compliance with the ACI form-

w o rk standard (ACI 347-78). Most of the data for thec h a rts we re taken from Fo r m w o rk for Concre t e .

DESIGN STANDARDS AND STRESSES

Listed below are the working stresses and some otherassumptions used in pre p a ring the chart s.

Plywood sheathing

Plywood is assumed to be continuous over four orm o re support s, and it is used the strong waythat iswith face grain parallel to the span. Deflection is limitedto 13 6 0 of the plywood span, but never is allowed to ex-ceed 11 6 i n c h .

Bending stressf= 1930 psi

Rolling shear vr= 80 psi

Modulus of elasticityE= 1,500,000 psi

These are the stresses recommended by the Ameri-can Plywood Association for Class I concrete form gra d eof plywood, based on short - t e rm loading.

Lumber or t imbers

The wood members are assumed continuous over twoor more spans. Allowable stresses used in pre p a ring thec h a rts are suitable for use with No. 2 Douglas firlarc hsubject to repeated usage or for No. 2 So u t h e rn pinewhich will have only limited re u s e.

Bending stressf= 1500 psi

Ho ri zontal shearH= 140 psi

Modulus of elasticityE= 1,700,000 psi

Six joist and six stringer choices are provided for hori-zontal form s. Deflection is limited to 13 6 0 of the span, but

n e ver is allowed to exceed 14 i n c h .

CONCLUSIONS

Fo r m w o rk for Concrete,AC I s Special Publication No.4, provides many design tables which reduce work i n gtime in the pre p a ration of form w o rk shop dra w i n g s.Howe ve r, design charts like those presented here can cutd own further on the math needed for design of form sand reduce most calculations to simple ari t h m e t i c. If them a t e rials being used are significantly different in quali-ty from those whose pro p e rties are shown here, anotherset of charts will be re q u i re d .

It is hoped that this chart approach will help thec raftsman or small contractor to build forms with somee n g i n e e ring logic, and not just by guess and rule oft h u m b.

Editors note:

Information in this article was condensed from a talk, Sim-plified Design of Formwork, presented by Paul H. Sommersat a seminar on quality conc rete co nstruction sponso red b ythe ACI Kansa s and Miss ouri Cha pters at the ACI Conven-tion in Kans a s City, S eptemb er 27, 1983. A dra ft of the o rigi-

Parts of typical wood formwork for slabs, oftendescribed as a stick -built syst em. The span of t hesheathing sets the spacing for the joists, whichpick up load and transmit it to the stringers.Support requirements of the joists determine theallowable spacing of t he stringers. In turn, thespan of the stringers between supports determineswhere the shores go. Shore and bracing design isnot covered in the charts of this article.


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nal talk, including equations used todevelop the des ign c harts, is a vailab lefrom C oncrete C onstruction P ublica -tions , 426 S outh Westg a te, Add iso n,Illinois 60101. C os t o f the co mpletetalk is $3.00, payable in advance.

Formwork for Concrete, by M. K.Hurd, reference d b y Mr. S ommers , isalso a vailab le from C oncrete Co n-struction P ublica tions. Co st o f the464-pag e illustrate d bo ok is $43.45,

and it co ntains the c omplete text ofthe ACI S tand ard 347-78.

PUBLICATION #C840648Co py right 1984, The Ab e rdeen Gro u p

All rights re s e rve d

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Charts Aid in Design of Horizontal form work