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

332 Chapter Eleven
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jobsite where no adjustment factors are necessary; dry condition,
normal temperature load-duration, etc. For this condition, the allowable
bending stress for 6-in.-wide No. 1 Southern Pine can be obtained
from Table 4-2 as F b
=1,650 lb per sq in.
Rewriting Eq. (5-30) in Table 11-2 to solve for the section modulus
in terms of the uniformly distributed load w, the span length l b
, and
the allowable stress F b
, the equation for the required section modulus
can be obtained as follows. From Eq. (5-30) in Table 11-2:
l b
= [96F b
S/w] 1/2
Rewriting this equation for the section modulus,
S = w(l b
) 2 /96F b
Substituting the values for load, span, and allowable stress,
w = (175 lb per sq ft)(18/12 ft) = 262.5 lb per lin ft
l b
= 7 ft, or 84 in.
F b
= 1,650 lb per sq in.
The required section modulus is
ww.EasyEngineering.n
S = w(l b
) 2 /96F b
= 262.5(84) 2 /96(1,650)
= 11.7 in. 3
Referring to Table 4-1, it will be noted that this section modulus
may be provided by one of the following sizes of lumber:
3 × 6 S4S for which S = 12.60 in. 3
4 × 6 S4S for which S = 17.65 in. 3
Both the 3 × 6 and 4 × 6 are adequate for bending. The d/b is <2
for both members; therefore, no lateral support is required to satisfy
beam stability (refer to Table 4-6). This member should also be checked
for shear and deflection, as illustrated in Chapter 5.
Use of Tables to Determine the Maximum Spans for
Lumber Framing Used to Support Plywood
Tables 11-7 and 11-8 give the maximum spans for lumber framing
members, such as studs and joists that are used to support plywood
subjected to pressure from concrete. The spans listed in Table 11-7 are
based on using grade No. 2 Douglas Fir or grade No. 2 Southern Pine.
The spans listed in Table 11-8 are based on using grade No. 2 Hem-Fir.
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