Experimental Study on Hysteretic Performance of Steel Tube Confined Concrete Compression-Bending Members

Literature Overview and Research Significance

The study by Yao Guohuang and Han Linhai (2004), published in Journal of Earthquake Engineering and Engineering Dynamics, presents experimental results on the cyclic load-deformation behavior of 12 steel tube confined concrete (STCC) compression-bending members. The research was supported by the National Science Fund for Distinguished Young Scholars (Grant No. 50425823) and the Fujian Province High-Level Talent Research Startup Project, and was conducted at the College of Civil Engineering, Fuzhou University.

This work addresses a critical gap in understanding the seismic performance of composite steel-concrete structural members subjected to combined axial compression and flexural loading, which represents the dominant loading condition for columns in moment-resisting frames.

Experimental Program and Test Configuration

The experimental program consisted of 12 specimens divided into two groups:

The primary variable investigated was the axial compression ratio (N/Nu), which governs the transition between flexure-dominated and compression-dominated failure modes. Each specimen was subjected to quasi-static cyclic loading to capture the full hysteresis behavior including strength degradation, stiffness deterioration, and ductility characteristics.

Key Experimental Findings

The experimental observations revealed several important characteristics of STCC members under cyclic loading:

Failure Modes

• Low axial compression ratio specimens: Exhibited flexure-dominated failure with visible yielding of the steel tube at the mid-span region and progressive concrete crushing. The circular specimens demonstrated superior confinement effectiveness due to uniform radial pressure distribution.
• High axial compression ratio specimens: Showed compression-flexure failure with sudden concrete crushing and potential local buckling of the steel tube. The square specimens were more susceptible to corner region failure due to stress concentration effects.

Hysteresis Curve Characteristics

The load-deformation hysteresis curves exhibited the following patterns:

• Initial elastic response followed by progressive stiffness degradation
• Pinching behavior became more pronounced with increasing axial load ratio
• Energy dissipation capacity decreased as the axial compression ratio increased
• The circular STCC members demonstrated more stable and fuller hysteresis loops compared to square counterparts

Stiffness Degradation

The stiffness degradation followed a predictable pattern: initial elastic stiffness was maintained until first yielding of the steel tube, after which a rapid degradation occurred. The rate of degradation was directly correlated with the axial compression ratio, with higher ratios producing more severe stiffness loss.

Calculation Methods for Ultimate Capacity and Flexural Stiffness

The authors proposed preliminary calculation methods for:

1. Ultimate bearing capacity: Based on the confined concrete strength model, considering the interaction between the steel tube confining pressure and the concrete core strength enhancement. The proposed formula accounts for the concrete confinement effectiveness factor, which depends on the steel tube geometry and material properties.
2. Flexural stiffness: Derived from the initial tangent stiffness considering the composite action of the steel tube and concrete core, with degradation factors calibrated against experimental data.

Engineering Practice Implications

From a materials and welding perspective, the STCC member fabrication involves several critical considerations:

• Steel tube quality: The steel tube must be manufactured to high dimensional accuracy standards (per GB/T 8162 or EN 10219) to ensure uniform confinement pressure. Wall thickness tolerances should not exceed ±10%.
• Weld integrity: Any longitudinal or circumferential welds in the steel tube must achieve full fusion and be inspected by magnetic particle testing (MT) or penetrant testing (PT) to detect surface and near-surface defects.
• Concrete placement: The quality of concrete placement inside the steel tube directly affects the confinement effectiveness. Compaction methods must ensure complete filling without voids, particularly in regions near the tube walls where the confinement effect is most critical.
• Material compatibility: The steel tube material should be selected to provide adequate ductility at elevated temperatures, considering that the steel tube may yield before the concrete crushes under cyclic loading.

Study Insights and Reflections

This experimental study provides valuable baseline data for the design of STCC columns in seismic structures. The systematic variation of axial compression ratio across both circular and square geometries enables direct comparison of confinement effectiveness between different tube shapes. The finding that circular STCC members outperform square counterparts in terms of hysteresis stability and energy dissipation is consistent with theoretical expectations based on uniform confinement pressure distribution.

However, several limitations should be acknowledged: the study focuses on quasi-static cyclic loading, which does not fully capture the rate-dependent material behavior observed under dynamic seismic loading. Additionally, the specimens were tested under controlled laboratory conditions that may not represent the complex boundary conditions encountered in actual structures. Future research should extend to dynamic testing and full-scale member testing to validate the proposed calculation methods under realistic loading scenarios.

The proposed calculation methods for ultimate capacity and flexural stiffness provide practical tools for engineers, though further refinement through additional experimental data and parametric studies would enhance their predictive accuracy and reliability for design applications.