Modeling analytical moment-rotation curves of semi-rigid connections for CFT square columns and steel beams

In a structural analysis for building frames, beam-to-column connections are assumed generally as a rigid and/or pinned condition for analysis simplicity. It gives some discrepancy for an overall behavior of building frames having some rotational rigidity in all connections. Many researchers have been interested in, and studied, structural behavior for building frames with semi-rigid connections. Despite many structural merits of concrete filled steel tubular (CFT) columns, such merits are not very clear for exact structural behavior between CFT columns and steel beams. This article is focused on a moment-rotation relationship for CFT frames composed of beam-column connections of exterior diaphragm with four rectangular plates. It is known from previous experimental results that CFT frames with exterior diaphragm connections have been represented as a semi-rigid condition, with a low stiffness due to out-of-plane deformations of square tubes. The main purpose of this study is to get a function formula of the moment-rotation curve for the connections. There are not sufficient test results of the connections to get a function. Therefore, an analytical approach needs to be utilized using a finite element program, ANSYS. The analysis models are made from existing test specimens and additional models are made for the implementation of insufficient test results. The function of a moment-rotation curve is proposed, using a three-parameter power model and a least-mean-square technique. The initial rotational stiffness of beam-to-column connection is determined by slope from zero to the analytical results.