Nonlinear Analysis of Rectangular Concrete-Filled Steel Tubular Columns at Elevated Temperatures

Rectangular thin-walled concrete-filled steel tubular (CFST) slender columns under axial and eccentric loads may undergo local and global interaction buckling when exposed to fire. Computational studies on the fire and post-fire behavior of rectangular short and slender CFST columns including local buckling effects have been extremely limited. This thesis presents new computational models for predicting the responses of rectangular and square CFST short and slender columns under fire exposure and after being exposed to fire. The models incorporate important features, which include local and global interaction buckling, air gap between the steel tube and concrete, concrete moisture content, emissivity of exposure surfaces, initial geometric imperfections, second-order, and material nonlinearities at elevated temperatures. Computational models are formulated by using the fiber approach for simulating the fire resistance, fire behavior and post-fire performance of rectangular CFST short and slender columns loaded concentrically and eccentrically. The progressive local buckling of steel tube walls at elevated temperatures is included in the formulation by using the local and post-local buckling models proposed. Computer simulation procedures sequentially coupling the nonlinear thermal and stress analyses are developed. The temperature distribution within a CFST column exposed to fire is determined by the thermal analysis. The modeling procedures capture the axial load-strain behavior, axial load-deflection responses, and fire-resistance of loaded CFST columns exposed to fire. Numerical solution algorithms implementing Muሷller’s method are developed to solve the nonlinear equilibrium equations of loaded CFST columns under fire exposure. The existing experimental and numerical results are utilized to validate the fiber-based computational models, which are employed to study the fire and post-fire responses of CFST short and slender columns with various important parameters. It is shown that the computational models are capable of predicting well the responses of rectangular CFST short and slender columns exposed to fire and after being exposed to fire. The computed results on the fires resistance and fire and post-fire behaviors of CFST rectangular columns with local buckling effects are given in the thesis for the first time. The research findings presented provide a better understanding of the fire and post-fire performance of short and slender CFST columns incorporating local buckling, and are valuable to structural designers and composite code writers