Structural behavior of beam-to-column connections of rectangular CFST columns using mixed diaphragms

The Concrete Filled Steel Tube (CFST) is a structure made of a steel tube filled with concrete, in which the rigidity of the member is increased, since the steel tube confines the concrete and the concrete reduces the local buckling of the steel tube. The subject of this study is a mixed diaphragm in which a penetrated diaphragm and an exterior diaphragm are used in combination at the beam-to-column connections of rectangular CFST columns to improve the concrete filling property of the columns. In order to verify that the resistance force of the mixed diaphragm is the same as or superior to that of the current diaphragm type, the study was conducted based on the optimal dimensions of the mixed diaphragm. The validity and superiority of the mixed diaphragm at the connections of the rectangular CFST structure were then confirmed. The size of the rectangular CFST column, of which the yield moment is larger than that of the beam, was selected. Finite element analysis (FEM) was used to select the type of rectangular CFST that exceeds the yield and full plastic moments of the beam, and that satisfies the need to maintain the initial rigidity and the maximum resistance force. Regression analysis was conducted to obtain an approximate expression for calculating the optimal diaphragm. The reliability and validity of the obtained approximate expression have been confirmed through a statistical theory based analysis and approximate plane analysis.