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

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Design of Wood Members for Formwork 155
There is no load-duration adjustment factor in the compression perpendicular
to grain.
Allowable compression stress perpendicular to grain is
F c⊥
= (0.67)(625 lb per sq in.)
= 418.7 lb per sq in.
The allowable compressive unit stress of 418.7 lb per sq in. is greater
than the applied unit stress of 77.8 lb per sq in. Therefore, the unit stress
is within the allowable value for this species and grade of lumber.
Size of Stringer Based on Selected 48-in. Spacing
Although the loads transmitted from the joists to the stringers are
concentrated, it is generally sufficiently accurate to treat them as uniformly
distributed loads having the same total value as the concentrated
loads when designing forms for concrete slabs.
With a 48-in. spacing and a 143 lb per sq ft pressure, the uniform
load on a stringer will be 143 lb per sq ft × 48/12 ft = 572 lb per lin ft.
Consider using 4 × 6 S4S stringers.
From Table 4-1, the physical properties for 4 × 6 S4S lumber are
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A = 19.25 in. 2
S = 17.65 in. 3
I = 48.53 in. 4
The allowable stresses for the No. 2 grade Douglas Fir-Larch 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)
= 1.3(1.25)(0.85)(900)
= 1,243 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 modulus of elasticity)
= 0.9(1,600,000)
= 1,440,000 lb per sq in.
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