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Example 2
Select the
lightest W section available in the ASD manual for the beam ABCDE shown in
Fig. 5.12 using A36 steel. Lateral supports are provided at points B and D specified on the figure. The weight of the beam is included
in the distributed load of 1 K/ft. Consider bending only.
Solution
To start
the iterative design process, initially assume the allowable bending stress
to be Fb = 0.60Fy. (An experienced
designer will most probably assume a somewhat lesser value because of the relatively
long unbraced lengths.)
Maximum
bending moment along the beam:
Try
W16 x 31.
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Sx = 47.2 in.3
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d/Af = 6.53 1/in.
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rT = 1.39 in.
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bf = 5.525 in.
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a. Check
for lateral support from A to B.
(The
bending moment at a point within the unbraced length is larger than at both
ends.)
Check
whether Fb = 0.66Fy. From Eq.
(5.9):
Try
W14 x 34.
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Sx = 48.6 in.3
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d/Af = 4.56 1/in.
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rT = 1.76 in.
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bf = 6.745 in.
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O.K.
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b. Check
for lateral support from B to C
c. Check for
lateral support from C to D.
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O.K.
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Note that for
all W sections given in the ASD made of A36 steel except W6x15, the local flange and web buckling
requirements for compactness are always satisfied.
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