Comparative Axial Compression Tests of Square Hollow Sandwich Steel Tube Concrete Short Columns

Literature Overview

Huang Hong and colleagues from East China Jiaotong University conducted comparative axial compression tests on four different cross-sectional configurations of steel tube concrete specimens: solid square STC, square-in-circle hollow sandwich STC, and two variants of square-in-square hollow sandwich STC with different inner tube placement orientations. The research was funded by the National Natural Science Foundation of China (51378206, 51008122) and Jiangxi Provincial Youth Science Fund. The specimens were tested to failure, with full load-deformation curves recorded and finite element analysis performed using ABAQUS for verification.

Core Technical Findings

All specimens exhibited failure through external tube weld cracking and inward buckling of the inner tube after reaching ultimate bearing capacity. The critical finding is that when the hollow ratio is not excessively large, the square hollow sandwich STC columns demonstrate higher axial compressive bearing capacity than solid square STC columns of equivalent outer dimensions. This counterintuitive result is attributed to the enhanced confinement effect provided by the sandwich geometry, which creates a more uniform stress distribution in the core concrete.

The lateral strain development rate follows a clear hierarchy: solid square specimens develop lateral strain the fastest, followed by square-in-circle hollow sandwich specimens, and finally the two square-in-square hollow sandwich configurations which exhibit the slowest lateral strain development. This indicates superior confinement efficiency in the square-in-square geometry, as the inner tube provides direct lateral support to the core concrete across all four faces.

Welding and Fabrication Considerations

From a manufacturing standpoint, the hollow sandwich STC configuration introduces significant welding challenges. The inner tube must be precisely positioned within the outer tube before concrete placement, requiring temporary fixturing that does not compromise the final weld quality. The failure mode of external tube weld cracking indicates that the longitudinal and circumferential welds of the outer tube are critical load-bearing elements. For rectangular or square tubes fabricated by LSAW or UOE processes, the longitudinal weld must achieve full penetration and uniform HAZ properties.

The inward buckling of the inner tube under compression suggests that the inner tube's own buckling resistance becomes a limiting factor in the sandwich configuration. This has direct implications for inner tube wall thickness selection and any welding operations performed on the inner tube. If the inner tube is fabricated from welded pipe, the weld location relative to the outer tube loading direction becomes a design parameter. Welds oriented parallel to the compression axis may exhibit different buckling behavior than circumferential welds.

Finite Element Verification and Engineering Application

The ABAQUS finite element analysis reproduced the full load-deformation process with good agreement to experimental results, validating the numerical modeling approach. This is significant for engineering practice because it enables parametric studies of hollow ratio, inner tube dimensions, and material properties without the cost and time of physical testing. For design purposes, the hollow sandwich configuration offers a weight reduction opportunity while maintaining or improving bearing capacity, provided the hollow ratio is kept within the experimentally validated range.

Study Insights and Conclusions

This research demonstrates that hollow sandwich STC columns can outperform solid STC columns in axial compression when the hollow ratio is appropriately controlled. The superior confinement behavior of the square-in-square geometry, evidenced by slower lateral strain development, makes it particularly attractive for applications where both strength and ductility are required. For welding engineers, the primary concern is ensuring that outer tube welds achieve full structural integrity, as weld cracking is the initiating failure mechanism. The validated finite element model provides a reliable tool for optimizing sandwich STC column design, enabling engineers to balance weight savings against bearing capacity requirements while accounting for welding quality variability.