Formwork Design Tables for Temporary Structures by Professor Kamran M. Nemati, Study notes of Introduction to Business Management

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Formwork Design Tables

Professor Kamran M. Nemati

CM 420 Temporary Structures

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Formwork Design Tables

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„ Based on the principles outlined so far, safe

spans for many timber and plywood formwork

components have been calculated and

arranged in tables for use by formwork

designer.

„ The tables cover single span beams, two-span

beams, and beams continuous over three or

more spans carrying a uniform distributed load.

„ The tables can be used to develop a

preliminary design for cases where rigorous

structural analysis is required for formwork

design.

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„ Four sets of allowable (adjusted) stresses

are included in the tables.

„ Adjusted stresses for long term and short

term loading stresses for formwork made of

No. 2 grade Southern Pine and Douglas Fir-

Larch.

„ Adjusted stresses for both short term and

long term loading of No. 2 Spruce-Pine-Fir

and No. 2 Hem-Fir.

„ Table 7-1 shows the expressions which

are used to calculate the safe support

spacings (spans).

Formwork Design Tables

Formwork Design Tables

Professor Kamran M. Nemati

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„ Table 7-1: Expressions Used in Calculating the

Safe Support Spacings of Chapter 7 Design Tables

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„ The tables are in four groups:

„ Table 7-2 through 7-4 for plywood

sheathing

„ Tables 7-5 through 7-7 for joists, studs,

stringers or any other beam

components of the formwork where

framing members are used singly

„ Tables 7-8 through 7-10 for wales or

other formwork components where the

members are used double

„ Table 7-11 and 7-12 for shore loading

and bearing checks

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„ Nominal lumber

sizes are shown

in the tables.

„ All calculations

are based on

lumber finished

on all four

surfaces (S4S).

„ Actual thicknesses are shown for plywood.

„ In each table, it is shown whether the safe span is

controlled by bending, deflection of shear.

Formwork Design Tables

Formwork Design Tables

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„ Sheathing Design: Tables 7-2—7-

„ Tables 7-2, 7-3, and 7-4 were calculated for

both long term and short term loading using the

information from Table 4-2, for face grain

parallel and perpendicular to direction of the

span.

„ Tables 7-2 through 7-4 are applicable to

plywood sheathing for columns, slabs, and

walls.

„ They cover plywood supports as a single span

beam, two span beam, or a beam continuous

over three or more spans.

„ Theoretical deflection of spans in these tables

do not exceed 1/16 inch.

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„ Joists, Studs, Beams: Tables 7-5—7-

„ Tables 7-5, 7-6, and 7-7 are applicable to

joists, studs, or any other form members

loaded uniformly as a beam.

„ Tables 7-5.1 through 7-5.4 are beams

continuous over three or more spans with the

following adjusted (allowable) stresses:

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„ Joists, Studs, Beams: Tables 7-5—7-

„ Tables 7-6 and 7-7 are like Table 7-5, except that

span length are calculated for simply supported

and two-span rather than continuous beams.

„ Note: Beam sizes are given in conventional

fashion with b or the width of beam face to which

load is applied given first and the second number

indicating depth of beam or d.

„ 2 x 4 (nominal size)

„ 4 x 2 (nominal)

d = 4

d = 2

b = 2

b = 4

Formwork Design Tables

Formwork Design Tables

Professor Kamran M. Nemati

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„ Double Members: Tables 7-8—7-

„ Tables 7-8, 7-9 and 7-10 are similar to

Tables 7-5, 7-6, and 7-7 in terms of

allowable (adjusted) stresses and

general layout, but they cover double

members which are commonly used for

wales and frequently for stringers.

„ Spans are calculated on the basis of

these members side by side with their

longer dimension as the depth of the

beam.

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„ Wood Shores: Tables 7-11—7-

„ Table 7-11 shows allowable loads on

wood shores for some of the more

commonly used timber sizes, with base

value of compression parallel to the grain

F

c

ranging from 1100 to 1600 psi, with

modulus of elasticity values from

1,200,000 psi to 1,600,000 psi.

„ Table 7-11 shows no load when l /d

exceeds the recommended limit of 50.

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Formwork Design Tables- EXAMPLE

Flat Slab

„ Use the design table to make a

preliminary selection of a stringer,

joist, and sheathing combination

suitable for forming a flat slab with

dead plus live load of 200 psf

supported on shores spaced 4 ft on

centers in both directions. Assume

that No. 2 Douglas-Fir Larch is

selected for multiple-use forms.

Formwork Design Tables

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Formwork Design Tables- EXAMPLE

Flat Slab

DL + LL = 200 psf

4 ft.

4 ft.

4 ft.

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„ From Table 4-2, the base stress values are:

„ F

b

= 875 psi

„ F

v

= 95 psi

„ E = 1,600,000 psi

„ As explained above, Tables 7-5.1, 7-6.1, and

7-7.1 are developed with adjusted stresses

that can be applied for No. 2 Douglas Fir-

Larch or Southern Pine, under long term

loads, with conditions as stated.

Formwork Design Tables- EXAMPLE

Flat Slab

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„ STRINGERS

„ With shores placed 4 ft on centers both ways, the

stringers will be 4 ft apart and have a span of 4 ft

between supports.

„ They will be designed as continuous beams with

an equivalent uniform load equal to the distance

between stringers times the uniform load on the

formwork (psf):

4 ft x 200 psf = 800 lb/lf

„ Use Table 7-5.1, since the stringers will be

continuous over three or more spans. Enter the

table at the left on the 800 lb/lf load line.

Formwork Design Tables- EXAMPLE

Flat Slab

Formwork Design Tables

Professor Kamran M. Nemati

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„ STRINGERS (Cont’d)

„ Note which members can be used for stringers

having a 48-in. span. Among the smaller

members that are suitable are:

„ 2x10 ⇒ Allowable span 55”

„ 3x8 ⇒ Allowable span 59”

„ 4x6 ⇒ Allowable span 55”

„ The 2x10 provides the necessary span with the

least lumber.

(But check with local suppliers for availability)

Formwork Design Tables- EXAMPLE

Flat Slab

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„ SHEATHING

„ Shore spacing places the stringers 4 ft apart, and

this 4 ft then is the span of the joists. How joists

are spaced depends on requirements of the

sheathing.

„ Assume 3/4-in. Plywood Class I or equal quality

plywood is used with its face running in the

direction of the span.

„ Since sheathing is continuous over several spans,

refer to Table 7-2.

„ The right side of the table, with F’

V

= 1545 psi,

applies since this is a multiple-use form.

Formwork Design Tables- EXAMPLE

Flat Slab

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„ SHEATHING (Cont’d)

„ From the column for 3/4-in. thickness

with face grain parallel to the span, for

load of 200 psf, read the allowable span

of 19 in.

„ In order to use 4x8 sheets of plywood

efficiently, a span of 96 ÷ 5 or 19.2 in.

probably will be used, dividing each 8-ft.

piece of plywood into five equal spans,

while permitting edge support for the

plywood panels.

Formwork Design Tables- EXAMPLE

Flat Slab

Formwork Design Tables

Professor Kamran M. Nemati

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„ JOISTS

„ This 19.2 in. becomes the required joist spacing,

and joist span has already been fixed at 4-ft.

„ What is the required joist size?

„ Joist loading = Joist spacing (ft.) x Load on forms

(19.2 ÷ 12) x 200 =320 lb/lf

„ Again using Table 7-5.1 since joists are

continuous over several spans, note that a 2 x 6

loaded at 300 lb/lf has an allowable span of 59 in.

and at 400 lb/lf has an allowable span of 51 in.

Formwork Design Tables- EXAMPLE

Flat Slab

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„ JOISTS (Cont’d)

„ By inspection, the 2 x 6 appears to be the lightest

joist that would be satisfactory on a 48-in. span.

„ But also consider the 4 x 4 which has an allowable

span of 53 in. at 400 lb/lf. The 4x4 is often

selected for this type of form, because its shape

provides inherent lateral stability.

„ Bearing

„ A check of bearing stresses where joists rest on

stringers and where stringers rest on shores

would be advisable.

Formwork Design Tables- EXAMPLE

Flat Slab

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„ Use the tables to determine spacing

of wall form members, assuming

continuous reuse of the forms and

No. 2 grade Douglas Fir-Larch or

equal lumber, with sheathing of

plywood. Design a 10-ft high wall

form for a maximum lateral pressure

of 600 psf, assuming no reduction of

pressure near the top of the form.

Formwork Design Tables- EXAMPLE

Wall Forms

Formwork Design Tables

Professor Kamran M. Nemati

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Formwork Design Tables- EXAMPLE

Wall Forms

1 in.

thick

Plywood

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„ SHEATHING

„ Assuming that 1-in. plywood is used with face grain

vertical, the grain will be perpendicular to the span

between the studs, and plywood will be continuous

across several spans.

„ The right side of Table 7-2 applies because the lower

stress levels are recommended when forms are

designed for continuing reuse, and the far right column

applies because the face grain is perpendicular to the

span.

„ Entering the table at 600 psf level, we find span of 13 in.

„ It is decided to set the studs 12 in. O.C. so that they can

be uniformly spaced and also support plywood at the

panel edges.

Formwork Design Tables- EXAMPLE

Wall Forms

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„ STUDS

„ With the studs 12 in. apart, the load per lineal ft is

12 / 12x(600) or 600 lb per ft.

„ Assuming that the studs are continuous over three or

more spans, refer to Table7-5.1 for choice of span

and member.

„ Entering table at left on the 600 lb/lf load line, the 3 x 4

stud has an allowable span of 37 in.

„ Support for studs (wale or ties) would be needed at

about 3-ft intervals.

„ Placing top and bottom wales 6 in. above bottom of

form and 6 in. below top of form would permit use of

four wales spaced 3 ft apart.

Formwork Design Tables- EXAMPLE

Wall Forms

Formwork Design Tables

Professor Kamran M. Nemati

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Formwork Design Tables- EXAMPLE

Wall Forms

1 in.

thick

Plywood

3 ft.

1 ft.

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„ WALES

„ If double wales are spaced 3 ft apart, the

equivalent uniform load per lineal ft is

36 / 12 x (600) = 1800 lb per ft.

„ Assuming continuity of wales, the left side of

Table 7-8.1 would be used to determine

spacing of wale supports.

„ Entering the table from left on the 1800 lb/lf

load line, a convenient span and double

member combination may be chosen from the

left side of the table where adjusted bending

stresses are applicable for long term loading

of Douglas fir-Larch of Southern Pine.

Formwork Design Tables- EXAMPLE

Wall Forms

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„ WALES (Cont’d)

„ For example, if double 2x6 wales are used the

spacing between ties that support the wales can be a

maximum of 33 in.

„ A check of the load capacity of available ties might

help in confirming the wale selection.

„ If the double 2x6 were used with supporting ties

spaced at 33 in., the average tie load would be

33 / 12 x 36 / 12 x 600 = 4950 lb

„ A tie with a safe working load of 5000 lb should be

selected.

„ With a tie spacing of 24 in., the necessary tie capacity

will be 3600 lb. [ 24/12 x 36/12 x 600 = 3600 lb]

Formwork Design Tables- EXAMPLE

Wall Forms