Semibifurcation and bifurcation analysis of flexibly connected steel frames

The critical load of a steel frame structure is dependent on joint details and flexibility. For a conventional type of skeletal structure made up of beam and column elements, the geometrical change is normally negligible. However, the variation of the joint stiffness may be significant near the ultimate limit state. In such cases, the rigorous second-order nonlinear analysis for determining the maximum load is cost-ineffective, while the bifurcation load approach is erroneous, because the change in connection properties with respect to the applied loads is not known. This paper introduces a numerical procedure called the semibifurcation method for the determination of the buckling load of steel frames that does not suffer from the drawbacks of either method. In the proposed technique, the variation of the joint stiffness is taken into account in the calculation of the buckling load of a steel frame. This leads to a significant saving of computer time without a notable reduction in accuracy for most conventional and braced steel buildings.