Inelastic dynamic analysis of mixed steel-concrete space frames

The aim of this research is to develop analysis methods for simulating the inelastic behavior of three dimensional mixed frame structures comprised of steel, reinforced concrete, and/or composite members subjected to seismic excitation. Two interactive computer programs are developed for this purpose: (1) a fiber element program for the inelastic analysis of cross-sections under axial load and biaxial bending moments including the effects of concrete confinement, concrete tension stiffening, and steel strain hardening, and (2) a frame analysis program for static and dynamic time-history analysis of space frames including the effects of spread-of-plasticity, geometric nonlinearities, and cyclic stiffness degradation.; In the frame analysis program, spread-of-plasticity effects are handled by a stress-resultant bounding surface plasticity model that accounts for the interaction of axial loads and biaxial bending moments in a flexibility based formulation. For steel members, the plasticity model is based on two nested surfaces, whereas for composite and reinforced concrete members the inner surface is degenerated into a point. A detailed investigation of the plastic flow direction considering the physical behavior of beam-column cross-sections shows that it is reasonable to uncouple the plastic axial strain from the plastic curvatures for partially plastified cross-sections. This improved results significantly especially when considering minor axis bending of steel sections.; Geometric nonlinearities are considered through the use of a geometric stiffness matrix. Geometric stiffness terms are expressed as a function of flexibility-dependent member displacement fields that are updated as plasticity spreads. Studies using these terms indicate that geometric stiffness matrices developed using approximate elastic shape functions are reasonably accurate and provide acceptable results for practical purposes.; Comparisons with experimental data and results from more refined analysis methods verify the accuracy of the proposed formulation. The interactive computer programs developed in this research are ideal tools for studying the inelastic behavior of steel, concrete, or mixed systems subjected to dynamic loading and for investigating proposed methods for seismic evaluation and design.