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

324 Chapter Eleven
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bending F b
and shear F s
are in pounds per square inch. The unit for
allowable span length is inches.
Because the slab decking is normally placed over multiple joists
(more than four) the equations for allowable span lengths of three or
more spans will apply. The physical properties of plywood and Plyform
sheathing are shown in Tables 4-9 and 4-11 in Chapter 4. Allowable
stresses are shown in Table 4-9 for plywood and Table 4-11 for Plyform.
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Example 11-1
Determine the maximum spacing of joists for a concrete floor slab
that will be formed with ¾-in.-thick Plyform decking. The total design
load on the slab will be 150 lb per sq ft. The deflection criterion limits
the permissible deflection to l/360 but not to exceed ¹⁄16 in.
The face grain of the Plyform will be placed across the joists; therefore,
the physical properties for stress applied parallel to face grain
will apply. From Tables 4-9 and 4-11, the physical properties and allowable
stresses for ¾-in.-thick Class I Plyform can be obtained.
The physical properties of ¾-in. Plyform are
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
The allowable stresses of Class I Plyform are
Bending, F b
= 1,930 lb per sq in.
Shear, F s
= 72 lb per sq in.
Modulus of elasticity, E = 1,650,000 lb per sq in.
Because the Plyform will be placed over multiple supports, the
equations for three or more spans will apply. The spacing of the joists
is governed by the maximum span length based on bending, shear,
and deflection of the Plyform decking as follows.
For bending stress in the Plyform decking, the maximum span
length from Table 11-1,
l b
= [120F b
S e
/w b
] 1/2
= [120(1,930)(0.455)/150] 1/2
= 26.5 in.
For rolling shear stress in Plyform decking, the maximum span
length from Table 11-1,
l s
= 20F s
(Ib/Q)/w s
= 20(72)(7.186)/150
= 68.9 in.
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