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

158 Chapter Five
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Design Summary of Forms for Concrete Slab
The allowable stresses and deflection criteria used in the design are
based on the following values.
For 7 / 8 -in.-thick plywood from Group II species with S-2 stress
rating:
Allowable bending stress = 1,025 lb per sq in.
Allowable rolling shear stress = 55 lb per sq in.
Modulus of elasticity = 1,300,000 lb per sq in.
For 4 × 4 S4S joists, No. 2 grade Douglas Fir-Larch lumber:
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Allowable bending stress = 1,434 lb per sq in.
Allowable shear stress = 218 lb per sq in.
Modulus of elasticity = 1,440,000 lb per sq in.
Permissible deflection less than l/360, but not greater than ¹⁄16 in.
For 4 × 6 S4S stringers, No. 2 grade Douglas Fir-Larch lumber:
Allowable bending stress = 1,243 lb per sq in.
Allowable shear stress = 218 lb per sq in.
Modulus of elasticity = 1,440,000 lb per sq in.
Permissible deflection less than l/360, but not greater than ¹⁄16 in.
The forms for the concrete slab should be built to the following
conditions:
Item
Decking
Joists
Stringers
Shores
Nominal Size and Spacing
7⁄8-in.-thick plywood
4 × 4 S4S at 20 in. on centers
4 × 6 S4S at 48 in. on centers
60 in. maximum spacing along stringers
This design example describes the calculations for determining the
minimum size and spacing of members using 4-in.-wide S4S lumber.
The purpose of the example is to illustrate the procedures and calculations
for designing components of slab forms for concrete roofs and
floors. The 4-in.-wide lumber was selected because workers can easily
walk and work on a 4-in.-square member, compared to a 2-in.-wide
member. Also, the 4-in.-wide member will not roll over, or fall sideways,
as it is being placed compared to a 2-in.-wide member. Furthermore, the
larger size 4-in. lumber will not require the amount of bridging or lateral
bracing, compared to 2-in. size lumber. Figure 5-19 summarizes the
design, including the size and spacing of decking, joists, and stringers.
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