pudulole nment. Use LRFD method or these problems, different values of L, are gi
ID: 1713079 • Letter: P
Question
pudulole nment. Use LRFD method or these problems, different values of L, are given. Dead loads do not include beam If weights. Use both LRFD and ASD methods. termine M, and Mnn for a w21 × 73 used as a beam with an unbraced length e compression flange of 6, 12, and 22 ft. Use A992 steel and C, 1.0. (Ans 12 ft, 536.2 ft-k LRFD:356.6 ft-k ASD) 0.16. Determine the lightest satisfactory W shape to carry a uniform dead load of 2.2 k/ft plus the beam self-weight and a uniform live load of 3.0 k/ft on a simple span of 25 ft. Assume bracing is provided at the ends only. Obtain C, from Fig. 9-10 in the text. 9-17. Select the lightest satisfactory W shape section if F, 50 ksi. Lateral bracing is provided at the ends only. Determine Cb. (Ans. W24 × 117 LRFD and ASD) PL = 40 k PL = 40 k D=1k/ft FIGURE P9-17Explanation / Answer
maximum moment in beam due to dead load = 1*302/8=112.5 kip-ft
Maximum moment in beam due to live load = 40*10=400 kip-ft
Total factored design moment per 1.2DL+1.6LL = (1.2*112.5)+(1.6*400)=775 kip-ft
Determination of Cb
MB=Mmax=775 kip-ft
MA=MC = 581.25 kip-ft
Cb = (12.5*775)/[(2.5*775)+(3*581.25)+(4*775)+(3*581.25)]=1.136
The beam should be selected such that its moment capacity for an unbraced length of 30 ft is 775/1.136=682 kip-ft minimum
The lightest section that satisfies this criteria is W24x117
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