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

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Forms for Beams and Floor Slabs 355
applied parallel to face grain will apply. The physical properties for a
1-ft-wide strip of the ¾-in.-thick plywood installed with the face grain
across supports from Table 4-11 are as follows:
Approximate weight = 2.2 lb per sq ft
Moment of inertia, I = 0.199 in. 4
Effective section modulus, S e
= 0.455 in. 3
Rolling shear constant, Ib/Q = 7.187 in. 2
Table 4-12 gives the allowable stresses for the Plyform Class I as
follows:
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Allowable bending stress, F b
= 1,930 lb per sq in.
Allowable rolling shear stress, F s
= 72 lb per sq in.
Modulus of elasticity, E = 1,650,000 lb per sq in.
Bending Stress in Plyform Decking
Because the plywood will be installed over multiple supports, three
or more, Eq. (5-41) can be used to determine the allowable span length
of the plywood based on bending.
From Eq. (5-41), l b
= [120F b
S e
/w b
] 1/2
= [120(1,930)(0.455)/(130)] 1/2
= 28.5 in.
Rolling Shear Stress in Plyform 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(72)(7.187)/(130)
= 79.6 in.
Bending Deflection in Plyform Decking
For a maximum permissible deflection of l/360, but not greater than
1/16 in., and plywood with three or more spans, Eqs. (5-57a) and
(5-57b) can be used to calculate the allowable span lengths for bending
deflection.
From Eq. (5-57a), l d
= [1,743EI/360w d
] 1/3 for ∆ = l/360
= [1,743(1,650,000)(0.199)/360(130)] 1/3
= 23.0 in.
From Eq. (5-57b), l d
= [1,743EI/16w d
] 1/4 for ∆ = 1/16-in.
= [1,743(1,650,000)(0.199)/16(130)] 1/4
= 22.9 in.
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