Question number 11-18 -18 and 11-19. Design problems in braced frames-using load

Question

Question number 11-18 -18 and 11-19. Design problems in braced frames-using loads and moments obtained using the requirements of the effective length method. 1-18.) Select th e lightest W12 beam-column member in a braced frame that sup- ports service loads of PD = 70 k and PL = 105 k. The service moments are Dx 30 ft-k, Mix 45 ft-k, Mpy 10 ft-k, and My 15 ft-k. The member is t long and moments occur at one end while the other end is pinned. There are 16 f no transverse loads on the member and assume Cb = 1.0. Use 50 ksi steel.

Explanation / Answer

factored axial load on column = 1.2*70 + 1.6*105=252 kips

factored moment about strong axis = 1.2*30 + 1.6*45=108 kip-ft

factored moment about weak axis = 1.2*10 + 1.6*15=36 kip-ft

effective length of column = 16 ft

Let us consider W12x65

Axial load capacity per effective length of 16 ft = 640 kips

moment capapcity of W12x65 about major axis = 356-(356-231)/(35.1-10.7)*(16-10.7) = 328.8 kip-ft

moment capacity of W12x65 about minor axis = 165.4 kip-ft

From intercation check, (252//640)+(8/9)(108/328.8 + 36/165.4) = 0.88<1

Section is adequate and can be used

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