Recent investigations by the authors have shown that existing displacement predictive procedures (including push-over analysis) could grossly underestimate the inelastic storey-drift of a multi-storey building under ultimate conditions if the collapse mechanism bifurcates from one principal displacement shape into two principal displacement shapes following the formation of certain plastic hinges. Such bifurcation behaviour has been demonstrated in buildings which have not been designed to fail in a ductile mechanism This behaviour is further studied in this paper with the analyses of a number of two-degree-of-freedom models (comprising a "lower" lumped mass and an "upper" lumped mass). A plastic hinge is assumed to form above the lower lumped mass. It has been found that the displacement of the lower lumped mass decreases while the strength reduction factor (defining the yield strength of the plastic hinge) increases. An important feature of the developing procedure of storey drift prediction is the use of the "floor" displacement response spectrum (derived in accordance with the motion of the lower lumped mass) to predict the inelastic drift behaviour of the upper lumped mass.
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