Steel Frame Stability Design

Many stability evaluation methods have been proposed over the last thirty years. The SSRC lists five specific methods of analyzing steel frames for stability: the effective-length concept; second-order, inelastic analysis; the PD method; the Merchant-Rankine formula; and the moment amplification method. The SSRC recommends evaluating frame stability separately from individual member analysis, while the AISC includes stability evaluation in the individual member selection process. The AISC requires that second-order effects be considered in the design of frames and that in unbraced frames the effective length factor, K, be not less than unity. The frame stability provisions of the AISC Specifications are based on the effective length concept, using the moment amplification method to account for second-order effects. It should be noted that the AISC does not prohibit the use of methods other than the effective length concept. The effective length concept typically is implemented through the determination of column effective length factors, K, using an alignment chart. The sidesway uninhibited alignment chart provides K factors for use in evaluating the lateral stability of the frame at the individual member level. The alignment chart is based on the assumption all columns in a story are equally critical and equally add to the lateral resistance of the frame. A method for evaluating frames in which all members do not equally contribute to a frame's lateral resistance has been proposed by Yura. However, it has been shown Yura's method is overconservative in frames in which the stability is provided by columns carrying a small portion of the story gravity load. One reason for this conservatism is the application of inelastic buckling allowable stresses where elastic buckling is clearly applicable.