Zhuojin Pipe Fitting Co., LtdCompression-Torsion Performance of Hollow Sandwich Steel Tube Concrete Members
Literature Overview
Published in Progress in Steel Building Structures in 2021, this study by Liao Xu, Li Jiren, and Wang Qingli from Liaoning University of Science and Technology presents experimental and numerical investigation of square hollow sandwich steel tube concrete members under combined compression and torsion. The research was funded by the Liaoning Provincial Department of Education (2017LNQN12) and the Liaoning University of Science and Technology Talent Introduction Fund (2017RC09). Twelve physical specimens were tested under quasi-static compression-torsion loading, with additional finite element analysis extending the parametric study to material strength and slenderness ratio effects.
Experimental Results and Key Findings
The hollow sandwich configuration—consisting of an outer square steel tube, an inner square steel tube, and a concrete layer sandwiched between them—creates a unique structural system with potential advantages in weight efficiency and confinement effectiveness. The experimental program reveals critical behavior under compression-torsion interaction that differs substantially from pure torsion or pure compression responses.
| Test Condition | Axial Compression Ratio | Torsional Capacity Relative to Pure Torsion | Post-Peak Behavior |
|---|---|---|---|
| Pure torsion | 0 | 100% (baseline) | Gradual capacity degradation |
| Low axial load | ≤ 0.5 | Significantly reduced | Sharp capacity drop after peak |
| Moderate axial load | 0.6 | 70% of pure torsion | Ductility markedly reduced |
| Higher axial load | > 0.6 | Further reduced | Brittle failure mode |
The most striking finding is that when the axial compression ratio reaches 0.6, the torsional capacity has already degraded to only 70% of the pure torsion capacity. This represents a severe interaction effect that would be catastrophic in structural applications where members are subjected to simultaneous axial and torsional loading, such as in bridge deck systems or storage tank walls under wind loading.
Material Strength and Slenderness Effects
The finite element parametric study reveals differential sensitivity to material properties. The outer steel tube strength has a pronounced effect on torsional capacity, which is consistent with the mechanical understanding that torsional resistance in a hollow section is primarily provided by the outer wall shear flow. Concrete strength has comparatively minor influence on overall member capacity, suggesting that the concrete core functions primarily as a stabilizing element rather than a primary load-bearing component in torsion.
Regarding slenderness ratio effects, torsional stiffness decreases significantly with increasing slenderness, while load capacity remains relatively insensitive. This distinction between stiffness and strength behavior is important for serviceability limit state design, where excessive torsional deformation may be more critical than ultimate capacity.
Welding and Fabrication Considerations
From my expertise in steel pipe fabrication and welding quality control, the hollow sandwich configuration presents several manufacturing challenges that directly affect the structural performance characterized in this study. The inner and outer square steel tubes must be precisely aligned during assembly, and the concrete sandwich layer must be cast with controlled thickness to ensure uniform confinement pressure distribution.
The welding connections between the inner tube, outer tube, and any internal stiffeners create a complex residual stress field that interacts with the applied compression-torsion loading. Specifically:
- Longitudinal welds in the square tubes introduce directional residual stresses that bias the torsional response
- Transverse stiffener welds create localized stress concentrations at the tube wall
- The interface between the steel tube walls and the sandwich concrete layer may experience debonding under torsional shear
- Corner regions of the square cross-section experience elevated triaxial stress states under torsion that promote crack initiation
The observed brittle failure mode at higher axial compression ratios suggests that the weld HAZ regions may be contributing to premature failure. The combination of compressive axial stress and torsional shear creates a complex stress state in the HAZ that exceeds the local ductility capacity of the weld metal and heat-affected zone microstructure.
Engineering Application Implications
The compression-torsion interaction behavior documented in this study has direct implications for the design of steel tube concrete members in applications where torsional loading is present. These include:
- Bridge deck girders subjected to eccentric vehicle loading
- Storage tank walls under wind and thermal loading
- Offshore platform jackets under combined gravity and vortex-induced vibration
- Transfer towers in wind turbine support structures
The finding that outer steel tube strength is the dominant parameter for torsional capacity suggests that material selection for the outer tube should prioritize high yield strength grades such as Q355 or Q420 per GB/T 1591, or equivalent ASTM A572 Grade 50/65 specifications. The relatively minor influence of concrete strength on torsional capacity implies that high-performance concrete (HPC) with compressive strengths above 60 MPa may not provide proportional benefit for torsional applications, and that cost-effective conventional concrete may be adequate for the sandwich layer.
Study Reflection and Recommendations
This research contributes valuable experimental data on a structurally innovative member type that has received limited investigation in the literature. The hollow sandwich configuration offers theoretical advantages in terms of weight efficiency and composite action, but the experimental results reveal significant limitations under combined loading conditions that must be carefully considered in design practice.
The critical finding regarding the severe degradation of torsional capacity under moderate axial compression demands that engineers exercise extreme caution when applying this member type to structures where compression-torsion interaction is anticipated. Design codes and standards do not currently provide explicit provisions for hollow sandwich steel tube concrete members under combined loading, which creates a gap between research findings and practical design methodology.
For future work, I recommend extending the experimental program to include cyclic compression-torsion loading to characterize fatigue behavior, conducting microstructural analysis of failure surfaces to identify fracture mechanisms, and developing design equations that explicitly account for the compression-torsion interaction effects documented in this study. The welding procedure specifications for these members should incorporate preheat and interpass temperature controls to minimize HAZ brittleness, particularly for the outer tube material grades that dominate torsional performance.