RESEARCH ON THE EFFECTIVE LENGTH OF CONCRETE-FILED STEEL TUBE FRAME COLUMNS

The effective length of concrete-filled steel tubular (CFST) frame columns is a key parameter to determine its ultimate strength rationally and correctly, and is also an important parameter to perform structural internal force analysis and design for CFST frame. Currently this parameter is determined based on the steel structural columns because of the lack of research. This method is unreasonable and illogical because the CFST frame columns and steel frame columns are different in mechanical characteristic. This paper presents a simplified stable model of frame with CFST columns to perform the nonlinear buckling analysis using ABAQUS. The material and geometric nonlinear are considered to derive the effective length of CFST columns of the sway frame and nonsway frame, respectively. An effective length factors table of the CFST columns of sway frame and nonsway frame is obtained based on a large number of parametric analyses, while a regressive simplified formula is proposed. A typical 4-bay and 4-story plane CFST frame is selected to compare the results of different methods. It is shown that the effective length factors derived by the proposed method in this paper are larger than the conventional approaches for steel frames, because the proposed method takes into account the initial imperfection and nonlinear behavior of the CFST columns. The error range is from 0.97% to 4.05% for nonsway frame, and from 9.95% to 27.17% for sway frame. This means the initial imperfection and material variance are the key issue and cannot be ignored for the effective length factors.