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

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Design of Wood Members for Formwork 119
FIGURE 5-11
Effective span length for design of plywood.
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The allowable pressure based on fiber stress bending of plywood
was presented in Chapter 4. The following equations are restated
here for clarity:
w b
= 96F b
S e
/(l b
) 2 for one or two spans (5-38)
w b
= 120F b
S e
/(l b
) 2 for three or more spans (5-39)
where w b
= allowable pressure of concrete on plywood, lb per sq ft
F b
= allowable bending stress in plywood, lb per sq in.
S e
= effective section modulus of a 1-ft-wide plywood strip,
in. 3 /ft
l b
= center-to-center distance between supports, in.
Equations (5-38) and (5-39) can be rewritten to calculate the allowable
span length in terms of the uniformly distributed pressure, the
allowable bending stress, and the section properties of plywood. The
equations written in this manner are useful for determining the joist
spacing necessary to support a given concrete pressure on the forms.
Rearranging the terms in the equations, the span lengths can be calculated
as follows:
l b
= [96F b
S e
/w b
] 1/2 for one or two spans (5-40)
l b
= [120F b
S e
/w b
] 1/2 for three or more spans (5-41)
Using the equations in this manner allows the designer to determine
the permissible pressure that can be applied on plywood panels.
The following examples illustrate the bending stress equations for
plywood.
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