Bending-Torsion Bearing Capacity of Hollow Sandwich Steel Tube Concrete Members

Literature Overview and Research Significance

This paper investigates the bending-torsion combined bearing capacity of hollow sandwich steel tube concrete (STC) members. The hollow sandwich configuration consists of two concentric steel tubes with a concrete-filled annular space between them, creating a sandwich structure that combines the advantages of both steel and concrete materials. This configuration is particularly relevant for applications where members must simultaneously resist bending moments and torsional moments, such as crane runway beams, helical ramps, bridge cross-beams, and complex pipe rack supports in industrial facilities.

Member Configuration and Material Properties

The hollow sandwich STC member consists of an outer steel tube, an inner steel tube, and a concrete-filled annular space between them. The outer tube provides primary bending resistance and torsional rigidity, while the inner tube contributes to the overall section modulus and provides additional confinement to the concrete core. The concrete fill in the annular space enhances the composite action and provides additional shear transfer between the two steel tubes.

Core Technical Findings on Bending-Torsion Interaction

The research demonstrates that the hollow sandwich configuration significantly enhances the torsional rigidity of the member compared to a single steel tube of equivalent weight. The torsional rigidity is improved by 60-90% due to the presence of the inner tube, which increases the polar moment of inertia without a proportional increase in material weight. The bending capacity is also enhanced by 15-25% compared to a single outer tube, as the inner tube contributes to the overall section modulus.

The bending-torsion interaction curve reveals a non-linear interaction between the two load components. At low torsion ratios (T/Tu < 0.3), the bending capacity is minimally affected, and the member behaves primarily in bending. As the torsion ratio increases, the bending capacity decreases progressively, with a significant reduction occurring at torsion ratios above 0.7. The interaction curve follows an approximate elliptical shape, which is consistent with the interaction curves proposed in design codes for steel members under combined bending and torsion.

Welding and Fabrication Considerations

The fabrication of hollow sandwich STC members involves several critical welding operations. The inner tube must be precisely positioned concentrically within the outer tube, with equal annular spacing maintained along the entire length. This is typically achieved using internal spacers or locating rings that are welded to both tubes at regular intervals (typically every 300-500 mm). The spacer welds must be full-penetration welds to ensure proper load transfer between the tubes.

The concrete filling operation must be performed carefully to ensure complete filling of the annular space without voids or honeycombing. Vibratory compaction is essential, and the concrete mix should have a low water-cement ratio and adequate workability to flow into the annular space. Post-filling inspection using ultrasonic testing (UT) should be performed to detect any voids or incomplete filling areas that could compromise the composite action.

Design Implications and Standards Reference

The design of hollow sandwich STC members under combined bending and torsion should consider the interaction between the two load components. Current design codes such as AISC 360 (Specification for Structural Steel Buildings) and GB 50017 (Standard for design of steel structures) provide interaction equations for steel members under combined bending and torsion, but these equations do not explicitly account for the concrete fill in the annular space. The research findings suggest that the existing interaction equations can be used with an increased section modulus and polar moment of inertia to account for the composite action, but further research is needed to develop specific design provisions for hollow sandwich STC members.

The torsional rigidity of the hollow sandwich member is particularly important for applications where torsional deflection limits are critical, such as crane runway beams and helical ramps. The increased torsional rigidity allows for longer unsupported spans and reduced member sizes, resulting in material savings and improved structural efficiency.

Key Insights and Engineering Recommendations

The most significant finding of this research is that the hollow sandwich configuration provides a synergistic improvement in both bending and torsional capacity, with the combined improvement exceeding the simple sum of the individual improvements. This synergy is attributed to the enhanced composite action between the steel tubes and the concrete fill, which creates a more efficient load distribution mechanism under combined loading.

For engineering practice, the following recommendations emerge from this research. First, the concentricity of the inner and outer tubes should be maintained within a tolerance of ±2 mm to ensure uniform annular spacing and consistent composite action. Second, the spacer welds should be designed to resist the maximum expected shear forces between the tubes, with a design shear strength not less than 200 MPa. Third, the concrete fill should be inspected using ultrasonic testing after curing to verify the absence of voids and incomplete filling areas.

This study provides valuable data for the design of hollow sandwich STC members under combined bending and torsion loading. The key insight is that the hollow sandwich configuration offers a structurally efficient solution for applications requiring simultaneous resistance to bending and torsion, with material savings of 20-35% compared to conventional solid steel sections. Future research should investigate the fatigue behavior of these members under cyclic combined loading, as well as the fire resistance and seismic performance, to enable their broader application in critical infrastructure projects.