Reliability-based performance-based design of rectangular concrete-filled steel tube (RCFT) members and frames

A computational study was conducted to develop a reliability-based performance-based design methodology for three-dimensional composite frames consisting of steel girders and braces framing into RCFT beam-columns. A new mixed finite element formulation was developed to simulate the geometrically and materially nonlinear response of both RCFT beam-columns and steel members. The RCFT beam-column element was derived with 18 degrees-of-freedom to account for the differential slip displacement between the steel tube and the concrete core. The interface of the steel tube and concrete core comprised a layer of nonlinear springs allowing shedding of force between the two media. Comprehensive constitutive relations were derived to model the material inelasticity of the steel tube, concrete core, and the steel and concrete interface under random cyclic loadings. A corresponding formulation was developed for hot-rolled wideflange members to enable modeling of braced and unbraced composite frame structures. The constitutive relations were calibrated and verified with respect to experimental tests in the literature to account for key nonlinear phenomena such as bond breakage at the interface, cracking and confinement of concrete core, residual stress distributions, hardening, and local buckling of cold-formed steel tubes and hot-rolled steel members. The accuracy of the mixed finite element formulation along with the material constitutive relations were tested through analyzing a series RCFT and hot-rolled steel specimens from the literature and comparing the computational and experimental response parameters. The verification results confirm that the mixed finite element formulation has the capability of producing realistic simulations for RCFT beam-columns, hot-rolled steel girders, and RCFT frames subjected to three-dimensional static or transient dynamic loading. The mixed finite element formulation was then utilized to perform demand assessment and capacity assessment studies of RCFT frames. For this purpose, a series of RCFT frames were designed according to the up-to-date design specifications. Utilizing nonlinear time history analysis, the demand and capacity of these structures were quantified, documenting the key response parameters critical for composite structures. The dispersions in the analysis results due to randomness and uncertainties were then evaluated to derive the demand and capacity factors to implement performance-based design of RCFT frames within a reliability framework.