Flexural-Compressive Bearing Capacity of Square Steel Tube Concrete Short Columns with Confining Tie Rods

Literature Overview

The study by Long Yueling et al. (2016), published in Industrial Construction, addresses a specific and practically important structural configuration: square steel tube concrete (SSTC) short columns reinforced with external confining tie rods. Ten specimens were tested under quasi-static cyclic loading, comprising two control specimens without tie rods and eight specimens with varying tie rod configurations. The research was supported by the State Key Laboratory of Subtropical Building Science at South China University of Technology and multiple national and municipal funding programs, reflecting the academic rigor and practical relevance of the investigation.

Experimental Configuration and Key Parameters

The experimental program was designed to systematically evaluate the effect of confining tie rods on the flexural-compressive (N-M) bearing capacity of SSTC short columns. The tie rods, which wrap around the square steel tube section, provide additional external confinement to the core concrete, analogous to the confining effect of spiral reinforcement in reinforced concrete columns but applied externally to the steel tube.

The finite strip method was employed to numerically compute the skeleton curves of the specimens under cyclic loading, utilizing a constitutive relationship for the confined concrete that accounts for the additional confinement provided by the tie rods. The calculated results showed good agreement with the experimental data, validating the analytical approach.

Analytical Methodology and Parametric Study

The finite strip method divides the column cross-section into narrow strips, each of which is analyzed independently for stress-strain behavior under the applied load combination. This approach is particularly effective for SSTC columns because the cross-section geometry is regular and the confinement effect can be modeled strip-by-strip. The parametric study examined the influence of four key parameters on the normalized N/Nu-M/Mu interaction curves:

1. Steel tube yield strength: Higher yield strength increases the overall bearing capacity but has a diminishing effect on the interaction curve shape at high axial load ratios.
2. Section steel ratio: Increasing the steel ratio enhances both axial and flexural capacity, with the most significant improvement observed at moderate axial load levels.
3. Concrete strength: Higher concrete strength improves axial capacity more than flexural capacity, shifting the interaction curve upward.
4. Tie rod confinement coefficient: This parameter directly governs the degree of additional confinement, and higher confinement coefficients produce more pronounced improvements in both strength and ductility.

Critical Finding: Inadequacy of Existing Standards

One of the most important conclusions of this study is that the Chinese standard CECS 159:2004, which provides design provisions for rectangular steel tube concrete members, does not account for the confining effect of external tie rods. As a result, the standard underestimates the flexural-compressive bearing capacity of tie-rod-reinforced SSTC columns. The authors developed simplified calculation formulas that incorporate the tie rod confinement effect, and these formulas showed good agreement with experimental results. This finding has direct implications for engineering practice: designers who currently rely on CECS 159:2004 for SSTC columns with tie rods may be producing non-optimal designs that are overly conservative in terms of material usage, or conversely, they may be missing opportunities to leverage the tie rod confinement effect for more efficient structural solutions.

Engineering Practice Integration

From a fabrication and construction standpoint, the installation of external confining tie rods introduces several practical considerations. The tie rods must be anchored to the column using high-strength bolts or welded connections, and these connection details must be designed to develop the full tensile capacity of the tie rods. During construction, the tie rods must be tensioned to the design pre-stress level, which requires careful quality control to ensure uniform tension distribution around the column perimeter. For columns located in seismic zones, the tie rod connections must be detailed to accommodate cyclic deformation without failure, which may require slotted bolt holes or welded cleat connections with adequate ductility.

The study's findings also have implications for the welding processes used in SSTC column fabrication. The steel tube itself may be manufactured using ERW, HFW, or LSAW processes, and the welding quality directly affects the column's load-bearing capacity. Any weld defects, such as lack of fusion, porosity, or undercut, would reduce the effective cross-sectional area of the steel tube and compromise the confinement mechanism. Non-destructive testing (NDT) in accordance with applicable standards such as GB/T 11345 (UT) or GB/T 3323 (RT) should be specified for all critical welds in the steel tube fabrication process.

Study Insights and Concluding Remarks

This research provides a solid experimental and analytical foundation for the design of SSTC short columns with confining tie rods, and the identification of the inadequacy of existing standards is a significant contribution to the field. The development of simplified calculation formulas that account for tie rod confinement offers practical tools for engineers who need to evaluate or design such members. The finite strip method, validated against experimental data, provides a reliable analytical tool that can be extended to more complex loading scenarios and cross-section geometries. Future work should address the long-term performance of tie-rod-confined SSTC columns, including the effects of corrosion on the tie rods and the steel tube, as well as the behavior under combined seismic and fire loading conditions.