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

124 Chapter Five
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l d
= clear span of plywood plus ¼ in. for 2-in. supports,
and clear span plus 5 ⁄8 in. for 4-in. supports, in.
E = modulus of elasticity of plywood, lb per sq in.
I= effective moment of inertia of a 1-ft-wide strip of plywood,
in. 4
Rearranging the terms in Eqs. (5-49) through (5-51), the allowable
pressure of concrete on plywood can be calculated in terms of
the span length, physical properties of plywood, and permissible
deflection.
4
For a single span, w d
= 921.6EI∆ d
/l d
(5-52)
4
For two spans, w d
= 2,220EI∆ d
/l d
(5-53)
4
For three or more spans, w d
= 1,743EI∆ /l d
(5-54)
ww.EasyEngineering.n
Equations (5-52) through (5-54) can be used to calculate the allowable
concrete pressure on plywood with different span conditions for
given deflection criteria. For example, if the permissible deflection is
l/360, then by substituting the value of l/360 for ∆ d
in the equation,
the designer of formwork can calculate the allowable pressure on the
plywood for the stipulated deflection criterion.
Equations (5-52) through (5-54) can be rewritten to calculate the
allowable span length for a given concrete pressure and deflection.
The equations for calculating the allowable the span length for plywood
based on bending deflection are
For a single span, l d
= [921.6EI∆ d
/w d
] 1/4 (5-55)
For two spans, l d
= [2,220EI∆ d
/w d
] 1/4 (5-56)
For three or more spans, l d
= [1,743EI∆ d
/w d
] 1/4 (5-57)
As discussed previously, the deflection criterion is usually
limited to l/270 or l/360, or sometimes not to exceed a certain
amount, such as ¹⁄8 in. or ¹⁄16 in. The value of permissible deflection
can be substituted into the preceding equations to determine the
allowable span length based on the permissible bending deflection
criterion.
The following equations can be used to calculate the allowable
span length for plywood sheathing based on bending deflection for
the stated deflection criterion:
Single spans,
For ∆ d
= l/360, l d
= [921.6EI/360w d
] 1/3
For ∆ d
= ¹⁄16 in., l d
= [921.6EI/16w d
] 1/4
(5-55a)
(5-55b)
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