Inelastic torsional seismic analysis of frame buildings

In the response of three dimensional frame structures to seismic loading, it has been observed that columns common to intersecting frames are especially prone to damage. This presumably results from the combination of forces from the frames; the axial loads are additive, but the moments interact biaxially. Two procedures for analysis of three dimensional frames are studied: time step analysis under given seismic records, using the programme DRAINTABS; and a pseudo-nonlinear modal analysis based on the substitute structure method of Shibata and Sozen, using the programme PITSA. Both these programmes are extended to include biaxial response, which they had not previously done. The evaluation of damage in the two programmes was felt to be unsatisfactory, and procedures are developed to estimate the curvature ductility demand in each case, which is considered to be the basic indicator of potential damage. PITSA is based upon a "substitute damping ratio" which is intended to represent the loss of energy due to hysteresis. The relationship of this factor to the shape of the hysteresis loop is studied. It was hoped that the two programmes, which are aimed at different classes of problems, would nevertheless provide mutual confirmation. Unfortunately, the detailed assumptions are sufficiently different to prevent close agreement, and only general trends are confirmed. Thus it may be concluded that further work on this topic is needed.Applied Science, Faculty ofCivil Engineering, Department ofGraduat