A Mixed Finite Element Approach for Nonlinear Dynamic Analysis of Composite Rectangular Concrete-filled Steel Tube (rcft) Frames

The worldwide progress to date in experimental and computational studies on composite frame structures having rectangular concrete-filled steel tube (RCFT) beam-columns and steel girders and braces has revealed their exceptional seismic resistance. In this paper, a computational study to develop reliability-based performance-based seismic design guidelines of RCFT members is presented. A three-dimensional geometrically and materially nonlinear mixed distributed-plasticity finite element formulation is developed for RCFT beam-columns as part of complete composite frame structures. The element stiffness matrices and internal forces are derived using a two-field mixed formulation, where deformation and stress-resultant fields along the element length are interpolated independently. Translational degrees-of-freedom for the steel tube and the concrete are defined separately to allow for differential slip displacement along the element length between the constituent materials, modeling cyclic interface and load transfer response ranging from perfect slip to complete bond. The material nonlinearity of the steel tube and the concrete core is accounted for through assigning comprehensive cyclic nonlinear stress-strain relations. Using an experimental database developed by the authors that documents detailed progression of damage in RCFT members and frames, a comprehensive verification study for the mixed finite element formulation is conducted on RCFT members under various loading conditions that highlights the complex interrelationship between the constitutive materials and the progression of damage with these composite members.