Question number 9-27 9.11 Problems for Solution 311 9-27. A W18 x 40 beam of A99

Question

Question number 9-27 9.11 Problems for Solution 311 9-27. A W18 x 40 beam of A992 steel is fixed at one support and simply supported at the other end. A concentrated load of dead load of 9.25 k and live load of 6.50k is applied at the center of the 32 ft span. Assume lateral bracing of the compression flange is provided at the pinned support, the load point, and the fixed support. You may neglect the beam self-weight, assume that C, 1.0 and the moments may be determined from Table 3-23 in the AISC Manual. Is the W18 adequate? (Ans. LRFD OK, 129.0 ft-k s 130.2 ft-k; ASD NG, 94.5 ft-k 86.7 ft-k)

Explanation / Answer

Per table 3-23 of AISC, the maximum mone t developed in the beam = moment at fixed end = 3Pl/16

Maximum moment in beam due to dead load = 3*9.25*32/16=55.5 kip-ft

Maximum moment in beam due to live load = 3*6.5*32/16=39 kip-ft

Total ASD moment in beam = 55.5+39=94.5 kip-ft

Total LRFD moment in beam = (1.2*55.5)+(1.6*39)=129 kip-ft

Unbraced length of beam=32/2=16ft

moment capacity of W18x40 per ASD = 86.7 kip-ft

moment capacity of beam per LRFD = 130.2 kip-ft

Therefore, beam is adequate per LRFD but inadequate per ASD

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