Experimental and Numerical Study of Double-Walled Thin-Walled Square Steel Tube Confining Concrete Short Columns

Literature Overview

This paper by Deng Xuhua and colleagues from Xiangtan University investigates the axial compressive behavior of double-walled thin-walled square steel tube confined concrete (DSTC) short columns with varying stiffener configurations. The research was funded by the Hunan Provincial Department of Education (21A0073) and the Hunan Provincial Natural Science Foundation (2020JJ4579). Three specimens with different stiffener forms were fabricated and tested under axial compression, complemented by ABAQUS finite element simulations and parametric analysis. The study addresses a practical concern in structural engineering: how to enhance the load-bearing capacity and ductility of thin-walled steel tube concrete (STC) columns, which are increasingly used in high-rise buildings and bridge piers due to their high strength-to-weight ratio.

Core Technical Findings

The primary conclusion is that stiffeners effectively delay local buckling of the outer steel tube and improve specimen ductility. Compared to unstiffened specimens, stiffened specimens show an ultimate bearing capacity improvement of approximately 25%. Multi-chamber configurations demonstrate superior ductility compared to conventional single-rib forms. A particularly noteworthy finding is the sensitivity of ultimate capacity to interstitial concrete strength: for every 20 MPa increase in interstitial concrete strength, the ultimate bearing capacity rises by approximately 17%. In contrast, changes in steel grade have a relatively smaller influence on ultimate capacity compared to concrete strength variations.

Key Technical Parameters and Design Considerations

From a welding and fabrication perspective, the stiffener-to-tube connection represents a critical quality control point. The stiffeners are typically attached to the outer steel tube via fillet welds or groove welds, and the quality of these welds directly governs the composite action between the stiffener and the tube wall. Common defects at such joints include incomplete penetration, undercut, and lack of fusion at the weld root, particularly when the stiffener thickness differs significantly from the tube wall thickness. According to GB/T 19804 (welded structural parts) and relevant welding procedure specifications, the weld preparation, preheat temperature, and interpass temperature must be carefully controlled. For thin-walled tubes (typically t/D ratios below 1/30), the heat input during stiffener welding can induce local distortion and residual stresses that may reduce the effective buckling resistance.

Engineering Practice Integration

In practice, double-walled STC columns are attractive for applications where space is constrained but high axial capacity is required, such as in densely packed urban high-rise foundations or bridge pier columns with limited cross-sectional dimensions. The multi-chamber design concept can be extended to include internal steel tubes of varying wall thicknesses, creating a graded stiffness profile. However, the fabrication of such columns presents significant challenges:

1. Inner tube placement and centering: Ensuring concentricity between the inner and outer tubes during concrete placement requires precision alignment fixtures and continuous monitoring.
2. Concrete placement in the interstitial gap: The narrow gap between tubes makes vibration and compaction difficult, risking honeycombing and voids. Pumpable concrete with appropriate slump and admixture design is essential.
3. Weld quality inspection: All stiffener welds should undergo visual inspection (VT) and magnetic particle testing (MT) per GB/T 26512 or equivalent. For critical joints, phased array ultrasonic testing (PAUT) is recommended to detect planar defects.
4. Residual stress management: Post-weld stress relief or controlled cooling may be necessary for thick-section stiffeners to prevent hydrogen-induced cracking in high-strength steels.

Standards and Material Selection

The outer and inner steel tubes may be manufactured per GB/T 8163 (seamless tubes for fluid transport), GB/T 3091 (welded tubes for fluid transport), or ASTM A500 (welded structural hollow sections). For seismic applications, the steel grade should meet the elongation and reduction-of-area requirements of GB/T 1499.2 or ASTM A992. The interstitial concrete, given its critical role in capacity enhancement, should be designed with strength grades ranging from C40 to C80, with careful attention to shrinkage and creep properties to minimize long-term residual stresses in the composite section.

Study Insights and Reflections

This research highlights an important design philosophy: in composite steel tube concrete structures, the concrete strength in the interstitial zone is a more efficient lever for capacity improvement than increasing the steel grade. This insight has direct implications for cost optimization in engineering projects. However, the limited specimen count (three physical specimens) warrants cautious extrapolation. Future research should expand the parameter matrix to include slenderness ratios, eccentric loading, cyclic loading for seismic assessment, and fire resistance. The ABAQUS parametric study, while valuable, relies on validated constitutive models for both steel and confined concrete, and the accuracy of these models under multi-axial stress states should be continuously benchmarked against experimental data. From a fabrication standpoint, engineers should pay particular attention to the dimensional tolerances of the double-walled configuration and the quality of all internal welds, as these factors can significantly influence the actual composite action achieved in service.