Experiments On Ultimate Bearing Capacity And Research On Design Method Of Joints Of Concrete-Filled Rectangular Steel Tube Truss

Supported by the Natural Science Foundation of Hunan Province, the behaviors of the joints of the Concrete-Filled Rectangular Steel Tube (CFRST) truss, which is a novel kind of structure, were studied in this paper. The research work aims at offering evidence for the coming Technical Specification for Structures with Concrete-filled Rectangular Steel Tube Members, and solving theoretically and practically the key technical problem of widely application of the CFRST truss. The main research works were concluded as follows:As the first experimental project in China, 37 joints tests are completed, including the ultimate bearing capacity tests on 30 T-, Y-, X-, K- CFRST joints and 7 Y-, X-, K- joints of Rectangular Steel Tube (RST). Comparing the tests of CFRST joints with RST joints, the effect rules of filled-in concrete and geometry parameters of joints on the ultimate bearing capacity were obtained. Based on the test results, the simplified model of transverse local compression to calculate the strength of CFRST joints under compression load was built up. The simplified model took into account the working of the steel tube, and the calculating method of local compression strength of concrete in tube corresponded to that recommended in the Chinese Code for Design of Concrete structures.The nonlinear finite element model of CFRST joints was also built up. Steel tubes, concrete and the interface between steel and concrete were modeled by Shell, 3D Entity and Contact element, respectively; The constitutive model of steel tubes and concrete was elastic-plastic, while the interface was described by Column's Friction Law. The calculating results of nonlinear finite element method kept in with those from tests.Based upon the experimental research and nonlinear finite element calculation, and the analysis of joint failure modes, a set of formula was proposed to predict the ultimate bearing capacity of CFRST joints: 1) The ultimate bearing capacity of T-, Y-, X-compression joints and K-compression branch joints can be predicted according to the failure model of transverse local compression. 2) If the width ratio of branch tube and chord tube β<0.7, steel tube flange yield-line failure mode is used to calculate T-, Y- and X- tension joints. Or else, the minimum of the results obtained by the formula based upon steel tube flange shear failure mode and effective width failure mode is adapted. 3) The gap judging formula for K-CFRST joints is provided. If the gap between two branch members is great, the joint is treated as T- or Y-joint. Or else, theminimum of the results obtained by the formula based upon steel tube flange shear failure mode and effective width failure mode is adapted for the tension branch joints. 4) The ultimate bending capacity of CFRST joints can be considered as branch tube flange effective width failure mode. In the end, the construction requests of CFRST joints are also given in this paper. The formula provided in this paper to calculate the ultimate bearing capacity of CFRST joints are easy to understand in physical concept and simple in practice. The results obtained by these formulas were coincided with those from the tests in this paper, Packer's tests and the finite element numerical simulation calculation.