Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

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Design of Wood Members for Formwork 117
Combining the applied bending moment M = wl 2 /120 with the
allowable bending stress F b
= M/S and rearranging terms, the following
equation can be used to calculate the allowable span length due
to bending for multiple-span wood beams:
l b
= [120F b
S/w] 1/2 in. (5-34)
Combining the shear force V = 5wl/96 with the allowable shear
stress F v
= 3V/2bd and rearranging terms, the allowable span length
due to shear in a multiple-span beam will be:
l v
= 192F v
bd/15w (5-35)
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Combining the modified shear force V = 5wl/96[1 – 2d/l] with the
allowable shear stress F v
= 3V/2bd, the allowable span length due to
modified shear in multiple-span beams is
l v
= 192F v
bd/15w + 2d (5-36)
The general equation for calculating the allowable span length
based on the deflection of wood beams with multiple-span lengths
can be obtained by rearranging the terms in Eq. (5-23) as follows:
l ∆
= [1,743EI∆/w] 1/4 (5-37)
If the permissible deflection is l/360, substituting l/360 for ∆ in
Eq. (5-28) for multiple-span beams, the allowable span length due to
a deflection of l/360 is
l ∆
= [1,743EI/360w] 1/3
(5-37a)
If the permissible deflection is ¹⁄16 in., substituting ¹⁄16 in. for ∆ in
Eq. (5-28) for multiple-span beams, the allowable span length based
on a permissible deflection of ¹⁄16 in. is
l ∆
= [1,743EI/16w] 1/4
(5-37b)
The techniques described in this section can be used for other load
and support conditions of beams to determine the allowable length
for a specified bending stress, shear stress, or permissible deflection.
Stresses and Deflection of Plywood
Plywood is used extensively as sheathing in formwork, to support
lateral pressures against wall or column forms or vertical pressures
on floor forms. The loads on plywood are usually considered as being
uniformly distributed over the entire surface of the plywood. Thus,
the equations for calculating stresses and deflections in plywood are
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