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

152 Chapter Five
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Rolling Shear Stress in Plywood Decking
The allowable span length of the plywood based on rolling shear
stress can be calculated.
From Eq. (5-48), l s
= 20F s
(Ib/Q)/w s
= 20(55)(8.05)/143
= 61.9 in.
Bending Deflection in Plywood Decking
For a maximum permissible deflection of l/360, but not greater than
¹⁄16 in., and plywood with three or more spans, Equations (5-57a) and
(5-57b) can be used to calculate the allowable span lengths for bending
deflection.
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From Eq. (5-57a), l d
= [1,743EI/360w d
] 1/3 for ∆ =l/360
= [1,743(1,300,000)(0.278)/360(143)] 1/3
= 23.0 in.
From Eq. (5-57b), l d
= [1,743EI/16w d
] 1/4 for ∆=¹⁄16 in.
= [1,743(1,300,000)(0.278)/16(143)] 1/4
= 22.9 in.
Summary for the 7 / 8 -in.-Thick Plywood Decking
For bending, the maximum span length of the plywood = 20.0 in.
For shear, the maximum span length of the plywood = 61.9 in.
For deflection, the maximum span length of the plywood = 22.9 in.
For this design, bending controls the maximum span length of
the plywood sheathing. The joists that support the plywood must be
placed at a spacing that is no greater than 20.0 in. For uniformity,
choose a joist spacing of 20.0 in.
Joists for Support of Plywood
The joists support the plywood and the stringers must support the
joists. Therefore, the maximum allowable spacing of stringers will be
governed by the bending, shear, and deflection in the joists.
Consider using 4 × 4 S4S joists whose actual size is 3½ in. by
3½ in. From Table 4-1, the physical properties can be obtained for 4 × 4
S4S lumber as follows:
Area, A = 12.25 in. 2
Section modulus, S = 7.15 in. 3
Moment of inertia, I = 12.51 in. 4
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