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

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Design of Wood Members for Formwork 153
The 4 × 4 S4S lumber selected for the forms is No. 2 grade
Douglas Fir-Larch with no splits. The allowable stresses can be
obtained by adjusting the reference design values in Table 4-3 by
the adjustment factors for size, short-duration loading, and a wet
condition. Table 4-3a provides the adjustments for size, Table 4-4
for short-duration loads, and Table 4-5 for a wet condition application.
Following are the allowable stresses that will be used for the
design:
Allowable bending stress, F b
= C F
× C D
× C M
× (Bending reference
value)
= 1.5(1.25)(0.85)(900)
= 1,434 lb per sq in.
Allowable shear stress, F v
= C D
× C M
× (shear stress reference value)
= 1.25(0.97)(180)
= 218 lb per sq in.
Modulus of elasticity, E = C M
× (reference value of modulus of
elasticity)
= 0.9(1,600,000)
= 1,440,000 lb per sq in.
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Bending in 4 ë 4 S4S Joists
Each joist will support a horizontal strip of decking 20 in. wide, which
will produce a uniform load of 143 lb per sq ft × 20/12 ft = 238 lb per
lin ft along the entire length of the joist. The allowable span length of
the joists, that is, the distance between stringers, based on bending
can be determined as follows:
From Eq. (5-34), l b
= [120F b
S/w] 1/2
= [120(1,434)(7.15)/238] 1/2
= 71.9 in.
Shear in 4 ë 4 S4S Joists
The allowable span length based on shear in the joists can be calculated.
From Eq. (5-36), l v
= 192F v
bd/15w + 2d
= 192(218)(3.5)(3.5)/15(238) + 2(3.5)
= 150.6 in.
Deflection in 4 ë 4 S4S Joists
The permissible deflection is l/360, but not greater than ¹⁄16 in. Using
Eqs. (5-37a) and (5-37b) the allowable span lengths due to deflection
can be calculated as follows.
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