Bearing Capacity Calculation of Circular Steel Tube Reinforced Concrete Members

Literature Overview

Wang Tingwei and colleagues from Dalian University of Technology conducted experimental and analytical research on circular steel tube reinforced concrete (STRC) members, combining the confinement benefits of steel tubes with the tensile reinforcement capacity of conventional rebar. Eight circular cross-section specimens were tested under both concentric and eccentric compression. The study employed both finite element analysis and fiber model methods for bearing capacity calculation, and proposed a simplified calculation method that treats the steel tube as conventional reinforcement while modeling the concrete as doubly confined concrete. The research was supported by the National Natural Science Foundation (51278081).

Core Technical Findings

The experimental results confirm that circular STRC members exhibit characteristics consistent with both steel tube concrete and reinforced concrete behaviors. The finite element method yields conservative (lower) bearing capacity predictions, while the fiber model method produces results in good agreement with experimental values. The proposed simplified method treats the steel tube as ordinary reinforcing steel and the core concrete as doubly confined concrete, applying conventional reinforced concrete bearing capacity calculation procedures.

The simplified calculation approach is particularly valuable for engineering design because it leverages existing reinforced concrete design codes and procedures, reducing the barrier to adoption. By treating the steel tube as equivalent reinforcement, designers can use familiar interaction diagrams and design tables with appropriate modification factors.

Welding Quality and Steel Tube Selection

For circular STRC members, the steel tube is typically fabricated from seamless pipe, ERW pipe, or spiral welded pipe depending on diameter and specification requirements. The bearing capacity calculation assumes the steel tube acts as continuous reinforcement, which requires that any longitudinal welds achieve full mechanical continuity. For spiral welded pipes used in large-diameter STRC members, the weld quality directly affects the confinement effectiveness.

The welding process selection should consider the following criteria: seamless pipes provide uniform properties but are limited in diameter; ERW and HFW pipes offer good properties for moderate diameters; spiral welded pipes are suitable for large diameters but introduce a helical weld that may affect circumferential confinement. For submerged arc welded (SAW) spiral pipes, the weld HAZ should be inspected by ultrasonic testing to ensure no laminar defects exist that could compromise confinement.

Design Methodology and Code Integration

The proposed simplified method enables integration into existing design codes by treating the steel tube as additional reinforcement area. This approach requires determining the equivalent reinforcement ratio contribution of the steel tube based on its yield strength and cross-sectional area relative to the concrete core. The doubly confined concrete model accounts for the additional confinement provided by the steel tube beyond what conventional spiral reinforcement provides.

Study Insights and Conclusions

This research provides a practical and code-compatible methodology for designing circular STRC members, bridging the gap between advanced analytical methods and conventional design practice. The validation through both experimental testing and multiple numerical methods gives confidence in the simplified approach. For welding engineers, the key implication is that steel tube weld quality directly affects the confinement effectiveness assumed in the design calculations; any weld defects that reduce the tube's effective yield strength will proportionally reduce the calculated bearing capacity. The fiber model method serves as a valuable verification tool for complex loading scenarios where the simplified method may be less accurate.