High-strength concrete-filled steel tube columns with tie bars: Seismic behavior and design

Abstract

The current study reports the utilization of tie bars to connect the opposite faces of steel tubes in the plastic hinge regions to strengthen the deformability of square high-strength concrete (HSC)-filled steel tube (CFST) columns. To evaluate the seismic performance of square HSC-filled steel tube columns with tie bars, four CFST columns with tie bars (TBCFST) and four CFST counterparts with concrete strength of about 125 MPa were examined. The test variables included the tie bar spacing, steel tube strength, and axial load ratio. It was found that when the height of the tie bar-confined region was equal to the steel tube width and the tie bar confinement index (λm) was in the range of 0.09 to 0.17, the local buckling of steel tube and concrete crushing were primarily concentrated in the unconfined zone adjacent to the tie bars. Finite element analysis was also conducted and showed that the ultimate drift ratio (θu) of the columns enhanced linearly with the increase in λm and confinement index of CFST (ξs), but diminished swiftly with an increase in the axial load ratio (n). For a given value of λm, θu increased with the increase in the tie bar columns (ntb). The flexural moment capacity ratio (Mtu/M0u) of the TBCFST column to the CFST counterpart increased as λm increased. The ratio Mtu/M0u did not follow a clear trend with the variation of ξs and n, but increased slightly with the increase in ntb. The minimum requirements for the design of tie bars were proposed.