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

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Forms for Beams and Floor Slabs 359
spacing and a 130 lb per sq ft pressure, the uniform load on a stringer
will be 130 lb per sq ft × 48/12 ft = 520 lb per lin ft. Consider using
4 × 6 S4S stringers.
From Table 4-1, the physical properties for 4 × 6 S4S lumber are
A = 19.25 in. 2
S = 17.65 in. 3
I = 48.53 in. 4
The allowable stresses for the 4 × 6 No. 2 grade Southern Pine can
be obtained by adjusting the reference design values in Table 4-2 by
the adjustment factors for short-duration loading and a wet condition.
Table 4-4 provides the adjustments for short-duration loads, and
Table 4-5 gives adjustment factors for wet condition. Following are
the allowable stresses that will be used for the design:
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Allowable bending stress, F b
= C D
× C M
× (reference bending stress)
= (1.25)(0.85)(1,250)
= 1,328 lb per sq in.
Allowable shear stress, F v
= C D
× C M
× (reference shear stress)
= (1.25)(0.97)(175)
= 212 lb per sq in.
Modulus of elasticity, E = C M
× (reference modulus of elasticity)
= (0.9)(1,600,000)
= 1,440,000 lb per sq in.
Bending in 4 ë 6 S4S Stringers
Based on bending, the span length of the stringers must not exceed
the value calculated.
From Eq. (5-34), l b
= [120F b
S/w] 1/2
= [120(1,328)(17.65)/520] 1/2
= 73.5 in.
Shear in 4 ë 6 S4S Stringers
Based on shear, the allowable span length of the stringers must not
exceed the value calculated.
From Eq. (5-36), l v
= 192F v
bd/15w + 2d
= 192(212)(3.5)(5.5)/15(520) + 2(5.5)
= 111.4 in.
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