Axial Compression Behavior of High-Strength Square Steel Tube-Concrete-Circular Steel Tube Composite Short Columns

Research Overview and Structural Innovation

The paper by Li Xiaoli and Chen Li, published in the "Journal of Northeast Petroleum University" in 2021, investigates the axial compression performance of high-strength square steel tube-concrete-circular steel tube (HSCS) composite short columns. This hybrid structural system combines an outer high-strength square steel tube, a concrete infill, and an inner circular steel tube, creating a multi-layered composite cross-section that offers enhanced load-bearing capacity and ductility. The study employs a comprehensive parametric analysis using finite element modeling, examining the influence of various geometric and material parameters on the structural response. For engineers familiar with steel pipe manufacturing and structural engineering, this research represents an innovative approach to maximizing the efficiency of steel-concrete composite systems.

Parametric Study Design and Material Constitutive Models

The research designed 27 test specimens with controlled variations in key parameters, including the ring concrete axial compressive strength (fck), the tensile strength of the outer square steel tube, wall thickness, and width of the outer tube, the tensile strength, wall thickness, and diameter of the inner circular tube, as well as the slenderness ratio and hollow ratio of the specimens. The finite element models were validated against existing experimental data to ensure predictive accuracy before being used for parametric analysis.

The constitutive models used for steel and concrete were carefully selected to accurately capture the nonlinear behavior under axial compression. The steel material model accounts for the bilinear elastic-plastic behavior with strain hardening, while the concrete model incorporates the confinement effect from both the outer square tube and the inner circular tube. The validation of the finite element model against experimental data confirmed a prediction accuracy within ±10% of measured values, which is acceptable for engineering design purposes.

Key Findings on Axial Compression Performance

The parametric analysis revealed several important trends in the axial compression behavior of HSCS composite short columns:

1. The axial compression load-bearing capacity increases significantly with increasing width, wall thickness, inner tube tensile strength, outer tube tensile strength, and concrete compressive strength. This confirms that all material and geometric parameters contribute positively to structural performance.
2. The axial compression load-bearing capacity decreases as the hollow ratio increases. The study recommends an optimal hollow ratio range of 0.1 to 0.3, which provides a good balance between structural performance and material economy.
3. The load-bearing capacity calculation formula developed in the study demonstrates high predictive accuracy and is suitable for practical engineering applications.

The interaction between the outer square tube and the inner circular tube creates a unique confinement mechanism that differs from conventional CFST columns. The outer square tube provides primary confinement to the concrete, while the inner circular tube contributes additional confinement and also acts as a secondary load-bearing element. This dual-confinement mechanism results in improved ductility and energy absorption capacity compared to single-tube CFST systems.

Hollow Ratio Optimization and Design Implications

The hollow ratio, defined as the ratio of the hollow area to the total cross-sectional area, is a critical design parameter that affects both structural performance and economic efficiency. The study findings indicate that a hollow ratio between 0.1 and 0.3 provides the optimal balance. Below 0.1, the additional material does not contribute proportionally to load-bearing capacity due to diminishing returns from increased concrete confinement. Above 0.3, the reduction in concrete volume leads to significant loss of load-bearing capacity and confinement effectiveness.

From a manufacturing perspective, the hollow ratio has implications for the selection of steel pipe sizes and the design of the composite assembly. A higher hollow ratio requires a larger inner circular tube relative to the outer square tube, which affects the manufacturing process, welding procedures, and quality control requirements. The welding joints between the outer square tube and the inner circular tube must be carefully designed and executed to ensure proper load transfer and structural integrity.

Engineering Practice and Quality Control Considerations

For the practical implementation of HSCS composite columns, several quality control measures are essential:

The welding of the inner circular tube to the outer square tube is a critical process that requires careful procedure qualification. The weld geometry, heat input, and cooling rate must be controlled to avoid excessive distortion and residual stress. For high-strength steel grades, preheating and post-weld heat treatment may be necessary to maintain the mechanical properties in the heat-affected zone.

Study Insights and Conclusions

This research demonstrates the significant potential of HSCS composite short columns as an advanced structural system for high-rise buildings, industrial structures, and infrastructure projects. The dual-confinement mechanism provided by the combination of outer square and inner circular steel tubes offers superior load-bearing capacity and ductility compared to conventional CFST systems. The parametric study provides valuable design guidance, particularly regarding the optimal hollow ratio range of 0.1 to 0.3, which should be incorporated into future design codes and standards. Engineers should consider the HSCS system as a viable option for applications requiring high load capacity and energy absorption capacity, provided that appropriate manufacturing quality control and welding procedures are implemented throughout the construction process.