Analysis and Design of Strength Interaction Diagrams for Concrete-Encased CFST Columns After Fire Exposure

Concrete-encased concrete-filled steel tubular (CFST) columns generally exhibit excellent fire resistance and maintain substantial residual load-bearing capacity after fire exposure. Evaluating their structural safety under compression and bending commonly employs strength interaction diagrams, which serve as essential tools in cross-sectional analysis and design. However, the post-fire cross-sectional behavior and material nonlinearity are significantly altered by the historical temperature distribution after fire exposure, resulting in non-uniform material degradation and complicating the accurate construction of the interaction diagrams. To address this challenge, this study develops a fiber-based cross-sectional analysis method to generate 3D interaction diagrams for these columns after fire exposure. The method is evaluated against limited available experimental results and its predictions are further evaluated through comparisons with numerical studies reported in the literature. A comprehensive parametric study involving 162,000 cases is conducted to investigate the influence of key parameters, including fire exposure time, aspect ratio, concrete compressive strength, confinement factor, and concrete cover thickness. Based on the results, a simplified design method for 3D interaction diagrams of concrete-encased CFST columns after fire exposure is developed. Furthermore, the proposed method has been implemented in the novel structural analysis software, MSASect2, to facilitate practical application and improve the reproducibility of this study.