Seismic Behavior of concrete filled steel Tubular Built-up columns

With the advantages of CFST built-up columns, including the higher confinement in the concrete, delay of the steel local buckling, higher compressive and flexural strength, earthquake and fire resistance, rapid construction, savings in the construction costs, etc. CFST built-up columns are increasing adopted in structural members with larger load eccentricity ratio and slenderness ratio, such as stadium, industrial buildings, bridge pier and pillar, and electrical transmission tower. However, the research is mainly focused on static performance, seldom research has been reported on the dynamic behavior of CFST built-up columns. The present research investigates the seismic behavior of CFST built-up columns. A detailed literature survey on the CFST built-up structures, including mechanical characteristics, applications, ductility in seismic design, previous experimental researches, and finite element formulation, is firstly illustrated. Six specimens with different grades of concrete and brace arrangements are designed and tested subjected to cyclic loading. The hysteretic behavior, such as failure mode, deformed shape, displacement ductility, rigidity and strength degradation, and energy dissipation capacity of test specimens are discussed. The corresponding validated finite element model (FEM) simulations are developed for parametric analysis, to discuss the hysteretic behavior, affected by axial load ratio, chord spacing, brace spacing, diameter to thickness ratio, and steel yield strength. Results indicate that the hysteretic characteristics of specimens are saturated and exhibited good ductility. The concrete strength and steel yield strength played a slight effect to the displacement ductility factor. While the ductility will be significantly affected by axial load ratio and geometrical types. Based on extended parametric analysis and regression analysis, a simplified method, consisted by equivalent slenderness ratio, axial load ratio and steel yield strength, is proposed to calculate the displacement ductility factor of CFST battened columns and laced columns, respectively. The accuracy is validated with test results. After that, to investigate the seismic performance of built-up columns used in practice, an innovative lightweight bridge with CFST composite truss girder and CFST lattice pier is studied as case study. For the purpose, FEM simulation and shaking table test are carried out. The FEM results agree with experimental data. In addition, the plastic hinges were predicted under transverse and longitudinal excitation respectively, revealed that CFST built-up columns has a favorable seismic performance.