Bending Bearing Capacity of Multi-Cavity Round-End Steel Tube Concrete Components

Literature Overview

This research investigates the flexural behavior of steel tube concrete members with multi-cavity round-end cross-sections. The multi-cavity configuration introduces internal voids within the concrete-filled steel tube, creating a lightweight structural element with potentially improved bending efficiency. The steel tube is typically a seamless tube per GB/T 8163 or a welded tube per GB/T 9711, with a round or oval cross-section that is modified to incorporate internal cavities. The cavities reduce the weight of the member while maintaining structural integrity through the confinement provided by the steel tube and the remaining concrete.

Core Technical Content and Cross-Sectional Geometry

The multi-cavity round-end cross-section consists of an outer steel tube with a circular or elliptical profile, containing multiple internal cavities that are also circular or elliptical in shape. The cavities are arranged symmetrically within the tube cross-section, typically in a triangular or rectangular pattern. The steel tube wall thickness ranges from 8 mm to 16 mm, and the overall outer diameter ranges from 300 mm to 600 mm.

The bending tests are conducted on simply supported beam specimens with a span-to-depth ratio of 8 to 12, loaded at mid-span or under three-point loading. The load is applied through a loading spreader beam to avoid localized bearing stresses. Strain gauges are mounted on the steel tube surface at critical locations, including the mid-span section, support regions, and cavity boundaries.

The moment-curvature relationship reveals that the multi-cavity configuration reduces the initial stiffness compared to a solid concrete-filled tube but maintains a comparable ultimate moment capacity for moderate cavity ratios. The ultimate moment reduction is approximately 10% to 20% for a cavity area ratio of 20% to 30%, which is less than the proportional reduction in concrete cross-sectional area. This indicates that the steel tube contributes significantly to the flexural capacity, and the cavities primarily affect the concrete contribution.

Failure Mode and Strain Distribution

The failure mode of multi-cavity steel tube concrete beams is characterized by the following sequence:

1. Tensile cracking of the concrete at the bottom fiber under increasing load
2. Steel tube yielding at the extreme fibers
3. Progressive concrete crushing at the compression zone
4. Steel tube local buckling at the compression zone
5. Ultimate failure by combined concrete crushing and steel tube buckling

The strain distribution measurements reveal that the presence of cavities creates localized strain concentrations at the cavity boundaries. The strain at the cavity boundary in the tension zone can reach 1.5 to 2.0 times the average strain in the beam section. In the compression zone, the strain concentration is less pronounced but still significant, with values reaching 1.3 to 1.6 times the average compression strain.

The stress redistribution caused by the cavities is most critical at the section with the highest moment, typically the mid-span for simply supported beams. The cavities reduce the effective concrete area in both the tension and compression zones, but the effect is more pronounced in the compression zone where the concrete is already at or near its crushing strain. The steel tube must compensate for the lost concrete contribution, which increases the demand on the steel tube and may lead to earlier local buckling.

Steel Tube Material and Manufacturing Quality

The steel tube material properties are critical for the bending performance of multi-cavity concrete-filled tubes. The steel grade should have adequate ductility to accommodate the large plastic deformations at the extreme fibers. Q345 steel with a yield strength of 345 MPa and an elongation of at least 21% is suitable for most applications. Higher-grade steels such as Q460 can be used for increased capacity, but the reduced ductility may limit the energy dissipation capacity.

The manufacturing process of the steel tube affects its bending performance. Seamless tubes provide uniform material properties throughout the cross-section, which is advantageous for bending applications where the stress distribution is non-uniform. Welded tubes, particularly those with longitudinal welds, may exhibit reduced bending capacity if the weld is located at the extreme fiber. The weld procedure must be qualified to ensure that the HAZ has adequate toughness and ductility.

The dimensional tolerances of the steel tube are also important. Out-of-roundness, wall thickness variation, and straightness deviations all affect the bending performance. The tolerances per GB/T 8163 specify that the out-of-roundness should not exceed 1% of the outer diameter, and the wall thickness deviation should not exceed ±0.5 mm for tubes up to 6 mm thick.

Quality Control and Inspection Requirements

The quality control of multi-cavity steel tube concrete beams requires attention to several critical aspects:

Study Insights and Engineering Implications

This research demonstrates that multi-cavity steel tube concrete members offer a promising approach to lightweight structural design without significant loss of bending capacity. The cavities reduce the weight of the member by 15% to 25% while maintaining 80% to 90% of the original moment capacity. This weight reduction has significant implications for transportation, handling, and foundation design. However, the increased complexity of the cross-section requires careful attention to fabrication quality, particularly the positioning of the cavities and the integrity of the steel tube. The research also highlights the importance of the steel tube contribution to flexural capacity, which is often underestimated in design calculations that focus primarily on the concrete contribution. Future research should explore the optimization of cavity geometry and arrangement to maximize the weight reduction while minimizing the capacity loss, and develop simplified design formulas that account for the multi-cavity configuration.