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

126 Chapter Five
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Rearranging the terms in Eq. (5-58), the allowable pressure of
concrete on plywood can be calculated in terms of the permissible
deflection and the physical properties of concrete.
w s
= 1,270E e
I∆ s
/Ct 2 l s
2
(5-59)
Equation (5-58) can also be rewritten to calculate the allowable
span length in terms of the permissible deflection and the physical
properties of plywood.
l s
= [1,270E e
I∆ s
/w s
Ct 2 ] 1/2 (5-60)
Substituting the deflection criteria of l/360 and ¹⁄16 in. for ∆ s
in
Eq. (5-58), the allowable span length for shear deflection of plywood
can be calculated as follows:
For ∆ s
= l/360, l s
= 1,270E e
I/360w s
Ct 2
For ∆ s
= ¹⁄16 in., l s
= [1,270E e
I/16w s
Ct 2 ] 1/2
(5-60a)
(5-60b)
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Plywood is generally used in applications where the loads are
considered uniformly distributed over the plywood and the spans
are normally 30 to 50 times the thickness of the plywood. Tests have
shown that shear deformation accounts for only a small percentage of
the total deflection when the span to thickness is in the range of 30 <
l/t < 50 (see ref. [4]). However, for shorter l/t ratios (15 to 20 or lower),
the shear deflection should be calculated separately and added to the
bending deflection.
Example 5-17
Consider the ¾-in.-thick Plyform Class I panel in Example 5-16 that
is to be supported by 2-in. joists, spaced at 18 in. on centers. Calculate
the shear deflection for the uniformly distributed load of 250 lb
per sq ft.
From Figure 5-10, effective span length for shear deflection is the
clear span:
l s
= 18.0 in. – 1.5 in.
= 16.5 in.
From Table 4-11 for physical properties,
Moment of inertia, I = 0.199 in. 4
From Table 4-12 for allowable stresses,
Modulus of elasticity for shear deflection, E = 1,500,000 lb per sq in.
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