Question number 11-15 11-15 Analysis problem in an unbraced frame-using loads an

Question

Question number 11-15 11-15 Analysis problem in an unbraced frame-using loads and moments obr ed w the requirements of the effective length method. A W10 × 77 section consisting of 50 ksi is used for a 20 ft long beam-column in a one-story building frame. It is unbraced in the plane of the frame (-r axi)b is braced out of the plane of the frame o-y axis) so that Ky 1.0. K, has been determined to equal 1.25. A first-order analysis has been completed and the results yielded a Pui load of 100 k and a Pt, load of 150 k. The factored Mnt moment was found to be 60 ft-k and a factored Mitx moment of 90 ft-k. The moments given are at the top of the member. The lower end has moments that are three-quarters of these values and bend the member in single curvature. There are no trans- verse loads between the ends and Cb-1.0. The Pstory-4800 k and the ratio of Pmf/Ptory-1.0. The story shear, H, is equal to 120 k and the drift index (A/L) is 0.0025. Using the LRFD method, is the member adequate? (Ans. OK 0.939 LRFD) 1

Explanation / Answer

Total axial load on column=100*150=250 kips

Factored moment in section=60+90=150 kip-ft

Effective length about minor axis=20*1=20 ft

Axial loas capacity of section=548 kips

Moment capacity of beam for 20 ft Unbraced length=

366 - (366-225)/(45.3-9.18)*(20-9.18) =323.8 kip=ft

Interaction check=(250/548)+(150/323.8) =0.919<1

Section is adequate

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