Mechanical Behaviour Of Concrete-filled Thin-walled Steel Tube Members And Connections

Concrete-filled steel tubes (CFST) are increasingly used in the truss structure, such as buildings, bridges, marine structures and transmission line engineering, for their excellent mechanical and economic performance. Steel-concrete composite members will provide a solution to those members with a large diameter-to-thickness ratio because the filled concrete could significantly enhance the local buckling resistance of the thin-walled steel tubes. With the rapid development on UHV power transmission project in China, these structures will have a broad application in future. Unfortunately, there are no specified design standards or methods as well as little research on this area, all of which indicate that the research of thin-walled CFST structures will become extremely urgent and practical significance. The mechanical properties and design method of thin-walled concrete-filled steel tube members were studied herein, and the main contents were list as below:(1) An innovated structure of thin-walled CFST with reinforced lattice angles was proposed and studied in order to solve the problem of insufficient constraint force of inner concrete for those CFST columns with a large diameter-to-thickness ratio. The experimental investigation and finite element analysis (FEA) were carried out. Based on the parametric studies on the composite columns, the rationality of proposed structure was verified. A new design method was finally proposed for CFST columns with reinforced lattice angle under consideration of the interaction between reinforced lattice angle and CFST.(2) Experimental investigation and FEA on Stress Concentration Factor (SCF) of CFST circular hollow section (CHS) connections were carried out. The test specimens included twelve CFST T-, Y-, K-and KT-connections. The effects of the types and chord wall thickness on both values and distributions of SCF along the chord-brace intersection were studied. The comparison between design predictions and test results shown the applicability of current design equations.(3) Mechanical performance of CFST CHS T-, Y-, K-and KT-connections such as failure mode, chord wall deformation, load capacity and so on, under axial tension were conducted by test. A FEA model was established by ABAQUS and validated by the experimental results. Material and geometric nonlinear, steel ductile fracture criteria, concrete-steel contact interaction as well as weld widths were considered in FEA, by which parametric studies were carried out. The general shear stress distribution on punching shear failure face was obtained from the FEA results and was described by proposed equations. A design method based on the actual shear stress distribution for pouching shear failure was proposed.(4) Mechanical performance of CFST CHS T-connections subjected to in-plane bending was studies by both tests and FEA. Parametric studies were also carried out to achieve the effects of inside concrete on mechanical performance as well as shear stress distribution on the punching shear failure face of T-connection. A punching shear failure mechanical model and bearing-capacity design approach of typical failure mode for those T-connections under in-plane bending were proposed and the accuracy was verified.(5) The influence of important parameters such as connections type, chord thickness, chord diameter and plate thickness on both mechanical and deformation performance of CFST gusset plate connections subjected to axial, eccentric tension and in-plane bending were investigated by tests. By test results, ABAQUS analysis models were verified and mechanical characteristics of CFST gusset plate connections were raveled.(6) Parametric studies were performed on CFST gusset plate T-connections subjected to axial tension, eccentric tension and in-plane bending, respectively. The effects of the thickness of gusset plate and chord, chord diameter, eccentricity and material strength on the failure modes and load capacity of the T-connections were considered. Shear stress distributions for the typical failure mode, punching shearing failure, on the failure face of T-connections were summarized, and the corresponding calculation models were established. Furthermore the experimental and FEA results were used to develop the ultimate capacity design method for CFST gusset plate T-connections under each load condition.