Zhuojin Pipe Fitting Co., LtdSeismic Performance of Circular and Square Hollow Steel Tube Concrete Columns
Literature Overview
The paper by Wang Hongwei, Lu Dehui, and Zhou Yun, published in Earthquake Engineering and Engineering Dynamics (2014, Vol. 34, No. 2, pp. 129-136), presents a comprehensive experimental study on the seismic performance of circular and square hollow steel tube concrete (HSTC) columns. Funded by the Guangdong Provincial Natural Science Foundation (Grant No. 10451009101006260) and the National Natural Science Foundation of China for Young Scholars (Grant No. 51108106), the study investigates the influence of axial compression ratio, hollow ratio, and cross-sectional shape on the hysteresis curves, skeleton curves, and ductility coefficients of HSTC columns.
Twelve HSTC column specimens were tested under low-cycle reversed loading to simulate seismic loading conditions. The study also evaluates the rationality of the axial compression ratio limits specified in the design code CECS 254-2011 for solid and hollow steel tube concrete structures.
Core Technical Findings
The experimental results reveal several important findings regarding the seismic behavior of HSTC columns. First, both the axial compression ratio and the hollow ratio have a significant adverse effect on the hysteresis performance. Higher axial compression ratios and higher hollow ratios lead to degraded hysteresis loops, indicating reduced energy dissipation capacity and increased stiffness degradation under cyclic loading.
| Parameter | Effect on Hysteresis Performance | Effect on Ductility |
|---|---|---|
| Axial compression ratio (increasing) | Degrades hysteresis loops | Reduces ductility coefficient |
| Hollow ratio (increasing) | Degrades hysteresis loops | Reduces ductility coefficient |
| Circular cross-section | Better hysteresis performance | Higher ductility than square |
| Square cross-section | Worse hysteresis performance | Lower ductility than circular |
The circular cross-section exhibits better hysteresis performance compared to the square cross-section. This is attributed to the more uniform stress distribution and the absence of corner stress concentrations in circular sections. The square section has corners where stress concentrations develop, leading to earlier local buckling and concrete crushing, which degrades the hysteresis performance.
All specimens failed at the bottom of the column, with the failure mode characterized by crushing of the core concrete and outward buckling of the outer steel tube. This failure mode is consistent with the expected behavior of steel tube concrete columns under combined axial and lateral loading. The axial compression ratio has a significant influence on the failure mode: higher axial compression ratios result in more pronounced outward buckling at the bottom of the column. Similarly, lower hollow ratios (i.e., more solid sections) result in more pronounced buckling, likely due to the greater confinement effect provided by the solid concrete core.
Methodology and Test Program
The low-cycle reversed loading test is a standard method for evaluating the seismic performance of structural members. The loading protocol typically involves cyclic loading at increasing displacement amplitudes, simulating the progressive damage accumulation under earthquake loading. The hysteresis curves, obtained by plotting the lateral load against the lateral displacement at each loading cycle, provide information on the energy dissipation capacity, stiffness, and strength degradation of the member.
The skeleton curve, obtained by connecting the peak points of the hysteresis loops, represents the monotonic load-displacement relationship and is used to characterize the overall strength and stiffness of the member. The ductility coefficient, defined as the ratio of the displacement at ultimate load to the displacement at yield, quantifies the deformation capacity of the member beyond yielding.
The test program design, with twelve specimens covering various combinations of axial compression ratio, hollow ratio, and cross-sectional shape, provides comprehensive data for evaluating the influence of these parameters on seismic performance. The comparison between circular and square sections is particularly valuable for design recommendations regarding cross-sectional shape selection.
Engineering Practice Implications
For the seismic design of HSTC columns, the findings have several practical implications. First, the axial compression ratio should be limited to ensure adequate hysteresis performance and ductility. The study provides recommendations for axial compression ratio limits for different hollow ratios, which can be used to update or supplement the existing design code provisions.
Second, the cross-sectional shape should be considered in the selection of HSTC columns for seismic applications. Circular sections are preferred for their superior hysteresis performance and ductility, although square sections may be used where architectural or structural layout requirements dictate. When square sections are used, additional measures may be needed to improve the seismic performance, such as increased steel ratio or reduced axial compression ratio.
Third, the hollow ratio should be carefully selected to balance the benefits of reduced weight and improved economy against the potential degradation of seismic performance. The study provides guidance on the acceptable range of hollow ratios for different seismic design categories.
Study Insights and Reflections
This study provides valuable experimental data on the seismic behavior of HSTC columns, which are an emerging structural system offering the advantages of reduced weight and improved economy compared to solid steel tube concrete columns. The findings on the influence of axial compression ratio, hollow ratio, and cross-sectional shape are directly applicable to the design of HSTC structures in seismic regions.
The study also contributes to the validation and refinement of the design code CECS 254-2011, which provides provisions for the design of solid and hollow steel tube concrete structures. The recommended axial compression ratio limits based on the experimental results offer a more rational basis for design than the existing code provisions, which may be overly conservative or, in some cases, insufficiently conservative.
The comparison between circular and square sections is particularly insightful. The superior seismic performance of circular sections is attributed to the more favorable stress distribution and the absence of corner stress concentrations. This finding reinforces the traditional preference for circular cross-sections in seismic design and provides quantitative evidence to support this preference.
Summary
The research by Wang et al. provides essential experimental insights into the seismic performance of circular and square hollow steel tube concrete columns. The findings on the adverse effects of axial compression ratio and hollow ratio on hysteresis performance and ductility, as well as the superior seismic behavior of circular sections, have direct implications for the design of HSTC structures in seismic regions. The recommended axial compression ratio limits offer a rational basis for design, and the comparison between cross-sectional shapes provides guidance for structural selection. Engineers designing HSTC columns for seismic applications should incorporate these findings into their design procedures, ensuring adequate energy dissipation capacity, ductility, and overall seismic resilience.