Behavior of three-dimensional steel moment frame structures under the influence of bidirectional ground excitation.

Abstract

This dissertation presents an investigation of nonlinear response of three dimensional steel moment frame structures under the influence of bidirectional ground excitation of different intensities and characteristics. The study is comprised of three topics: first, development of analytical model of three dimensional beam column, second, investigation of three-dimensional symmetrical and asymmetrical steel moment frame structures subjected to unidirectional and bidirectional ground excitations, and third, investigation of dynamic instability of three-dimensional symmetrical steel moment frame structures subjected to bidirectional ground excitations and large column axial loads. The development of a three dimensional beam column element analytical model uses a prismatic fiber model under finite displacements. The model includes material nonlinearity due to plasticity with strain hardening under monotonic or cyclic loading, and geometric nonlinearity due to large deflections and rotation. The torsional behavior of three-dimensional symmetrical and asymmetrical steel moment frame structures subjected to unidirectional and bidirectional ground excitations with different intensities was studied by comparing bidirectional earthquake response with combinations of single directional responses, for both doubly symmetric as well as for torsionally sensitive building configurations. Results indicate that torsional response of the initially symmetric structure was not important and that the nonlinear response remained primarily translational. Inelastic torsional response of the torsionally sensitive building was smaller than that predicted from elastic response results. Torsional motion was less significant for increased ground motion intensities. The presence of large axial column loads affects the load-displacement hysteresis curves when the story drifts are large. Nonlinear static analysis shows that the inverted triangular floor force profile can be used as a lower bound to actual earthquake response ultimate capacity. Large drift and yielding in the columns of a story under combined biaxial bending and axial force can induce frame instability and collapse of the entire structure. Structures with a weak story and limited structural redundancy should be avoided for seismic resistance.