A Mixed-Finite Element Approach for Performance-Based Design of Rectangular Concrete-Filled Steel Tube (RCFT) Frames

A comprehensive computational model to simulate the nonlinear seismic response of composite frames with rectangular concrete-filled steel tube (RCFT) beam-columns and steel girders and braces is presented in this work. A three-dimensional fiber-based distributed plasticity mixed finite element formulation is introduced. The finite element formulation of RCFT beam-columns was derived with separate translational degrees-of-freedoms (DOFs) defined for the concrete core and the steel tube to simulate the slip deformation that is typical for composite members. Cyclic constitutive relations were derived accounting for the interactions taking place between the steel tube and the concrete core including slip, confinement, and local buckling. A representative composite frame structure was analyzed under a suite of ground motion records and the demand imposed on the RCFT columns was quantified. A methodology based on the use of the damage function equations was proposed to evaluate the performance of RCFT members under seismic loads. INTRODUCTION Integrating the favorable characteristics of the steel tube and the concrete core in RCFTs provides an efficient design alternative for beam-columns in the lateral load resistance systems of low, moderate, and high rise structures. The advantages offered by RCFTs such as high strength, stiffness, ductility, and economy have long been recognized, which resulted in extensive experimental and computational research studies toward understanding their behavior and developing reliable design methods. The experimental database studies by [Aho and Leon 1997], [Tort and Hajjar 2004], [Kim 2005], and [Gourley et al. 2008] show that there exist significant experimental data in the literature that indicate the superior performance of RCFTs under static and quasi-static loading conditions. In recent experimental work on large scale multistory RCFT frames under dynamic loading, RCFT frames were found to exhibit a stable response under earthquake loading with large energy dissipation and low strength deterioration