Restoring force characteristic model for bending moment–rotation angle relationship of concrete‐filled square steel tube beam‐columns

This paper proposes a new simplified restoring force characteristic model of the bending moment–rotation angle relationship for square concrete‐filled steel tube (CFT) beam‐columns to estimate the elastoplastic flexural‐shear behavior from short to slender beam‐columns. The square CFT beam‐column model incorporates a reduction in strength after the ultimate strength. The proposed simplified model is multilinear and involves a cracking strength point, a yield strength point, an ultimate strength point, and strength reduction points. In the proposed model, each flexural strength is calculated by a general superposed strength method, and new empirical expressions for each rotation angle are proposed from the results of previous experimental cyclic and monotonic loading tests. The proposed model for the bending moment–rotation angle relation is found to agree approximately with experimental results up to large rotation angles.