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Formwork Design Tables
Professor Kamran M. Nemati
CM 420 Temporary Structures
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Formwork Design Tables
Temporary
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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
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
Professor Kamran M. Nemati
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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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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
