Cyclic Evolution of Damage and Beam-column Interaction Strength of Concrete-filled Steel Tube Beam-columns

The assessment of response of composite seismic force resisting systems requires accurate nonlinear formulations for the composite members. In this work, reporting the first phase of a NEES project, a three-dimensional distributed plasticity formulation is presented for both circular and rectangular composite concrete-filled steel tubes (CFTs) that includes new constitutive models for the concrete and steel materials to enable tracking of the critical cyclic response of CFTs, including confinement of the concrete core leading to concrete cracking and crushing, and cyclic local buckling of the steel tube. The formulation is validated against an extensive series of new experiments of full-scale circular and rectangular concrete-filled steel tube beam-columns that were tested cyclically under three-dimensional loading (compression plus biaxial flexure). The loading histories to which the specimens were subjected were chosen to catalog the beamcolumn interaction strength, to document the evolution of damage to CFTs subjected to different types of cyclic loading histories, and to explore the evolution of the stress-resultant loading surface (i.e., the beam-column interaction strength surface) under cyclic loading. Experimental observations of the evolution of the loading surface indicate a change in position and shape as well as a significant decrease in size of the beam-column strength surface under high levels of inelastic loading. This paper addresses the comparison between the computational formulation and experimental results and discusses the implications of the experimental findings on nonlinear modeling of composite seismic force resisting systems.