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

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Design of Wood Members for Formwork 123
From Table 4-9, for a 3/4-in.-thick plywood panel installed across
supports, the rolling shear constant Ib/Q = 6.762 in. 2 /ft. From Table
4-10, the allowable rolling shear stress F s
= 53.0 lb per sq in. The allowable
pressure can be calculated from Eq. (5-45).
w s
= 20F s
(Ib/Q)/l s
= 20(53.0)(6.762)/(14.5)
= 494 lb per sq ft
For the 3/4-in. plywood, the allowable pressure of 494 lb per sq ft
due to shear is greater than the allowable pressure of 270 lb per sq ft
due to bending, therefore bending governs the panel for this particular
condition.
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Allowable Pressure on Plywood Based on
Deflection Requirements
Plywood used for formwork must have adequate rigidity to resist
deflection of the plywood between supporting joists or studs. As presented
in previous sections, the permissible deflection is usually
specified as l/270 for structural concrete formwork and l/360 for
architectural concrete. To prevent waves in the surface of concrete
that is formed with plywood, it is common to limit the deflection to
l/360, 7 ⁄8 but not greater than ¹⁄16 in.
There are two types of deflections that apply to plywood,
bending deflection and shear deflection. The APA has conducted
numerous tests to evaluate the deflection of plywood. Plywood
sheathing is generally used where the loads are uniformly distributed
and the span lengths are normally less than 30 to 50 times the
thickness of the plywood. For small l/d ratios, tests have shown
that shear deformation accounts for only a small percentage of the
total deflection.
Allowable Pressure on Plywood due to Bending Deflection
The following equations are recommended by the Plywood Design
Specification for calculating deflection due to bending:
For a single span, ∆ d
= w d
l d4
/921.6EI (5-49)
For two spans, ∆ d
= w d
l d4
/2220EI (5-50)
For three or more spans, ∆ d
= w d
l d4
/1743EI (5-51)
where ∆ d
= deflection of plywood due to bending, in.
w d
= uniform pressure of concrete on plywood, lb per sq ft
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