Zhuojin Pipe Fitting Co., LtdAxial Compression Bearing Capacity of Square Steel Tube Confined Ultra-High Strength Concrete Short Columns
Literature Overview
This paper, published in Highway and Transportation Research in 2024 by Shi Bochao from the Fuzhou Metro Construction Engineering Quality and Safety Supervision Station, investigates the axial compression behavior and ultimate bearing capacity of square steel tube confined ultra-high strength concrete (UHSC) short columns. The research was supported by the Fujian Provincial Department of Science and Technology Industry-University-Research Cooperation Program (2022H6009). The study combines experimental testing and finite element analysis, with concrete compressive strength, steel yield strength, and steel ratio as the primary variables, and proposes a confinement-based calculation method for the ultimate bearing capacity.
Core Technical Findings
The research reveals that the confinement coefficient is the primary factor governing the failure mode of square steel tube confined UHSC short columns. When the confinement coefficient is less than 0.83, the columns exhibit shear failure; when it is greater than or equal to 0.83, they exhibit crushing failure. The load-displacement and load-strain curves indicate that the hoop confinement capacity of the steel tube is not fully utilized before the ultimate load, with the confinement effect becoming more pronounced after the ultimate load is reached.
| Confinement Coefficient | Failure Mode | Post-Peak Behavior |
|---|---|---|
| < 0.83 | Shear failure | Sudden load drop, limited ductility |
| ≥ 0.83 | Crushing failure | Gradual load reduction, improved ductility |
The study also finds that increasing the steel ratio effectively improves the post-failure compressive ductility of the specimens. Specimens with the lowest steel ratio all exhibit sudden load drops, highlighting the importance of matching the confinement strength to the concrete compressive strength. Furthermore, the ultimate load increase for UHSC-filled columns is smaller than for columns filled with conventional concrete, attributed to the different compressive-to-tensile strength ratio of UHSC, which limits the plastic deformation capacity and reduces the interaction between the steel tube and the concrete core.
Material and Confinement Analysis
The confinement mechanism in steel tube confined concrete columns relies on the lateral pressure exerted by the expanding concrete core on the steel tube walls, which in turn provides a confining pressure that enhances the concrete's compressive strength and ductility. For UHSC, which typically exhibits compressive strengths exceeding 100 MPa and a high compressive-to-tensile strength ratio (often greater than 10:1), the confinement mechanism operates differently compared to conventional concrete.
The lower plastic development of UHSC near the ultimate load means that the steel tube does not receive as much lateral expansion force from the concrete core, resulting in less effective confinement. This fundamental material difference explains why the Mander model and Chinese standard calculation methods fail to accurately predict the behavior of UHSC-filled specimens with confinement coefficients greater than 0.3. The Chinese standard method demonstrates smaller calculation errors compared to the Mander model, and by introducing a confinement reduction coefficient, the calculation deviation can be controlled within 10%.
Welding Quality and Steel Tube Manufacturing
The performance of square steel tube confined UHSC columns is intimately connected to the manufacturing quality of the steel tubes. The following welding and fabrication aspects are particularly critical:
- Square steel tubes are typically fabricated by welding four flat plates or by forming and welding a single plate into a square section. The weld seams must achieve full penetration and be free of defects such as porosity, slag inclusion, and incomplete fusion.
- The welding process must be carefully controlled to minimize heat-affected zone (HAZ) softening, particularly for high-strength steel grades where the HAZ may exhibit reduced yield strength compared to the base metal.
- Residual stresses from welding create a non-uniform stress state in the steel tube walls, which can affect the local buckling behavior under axial compression. The interaction between welding residual stresses and the confining pressure from the concrete core should be considered in the design.
- For UHSC applications, where the concrete compressive strength is extremely high, the steel tube must be of sufficient strength to provide effective confinement. This often requires the use of high-strength structural steel grades (e.g., Q460, Q550, or equivalent), which present additional welding challenges due to their susceptibility to hydrogen-induced cracking in the HAZ.
Engineering Practice Implications and Study Insights
This research makes a significant contribution to the understanding of steel tube confined UHSC columns, a structural system that combines the high strength of UHSC with the confinement provided by steel tubes. The proposed confinement-based calculation method, with the introduction of a reduction coefficient, provides a practical tool for design engineers, with calculation deviations controlled within 10%.
The finding that the confinement coefficient threshold of 0.83 separates shear failure from crushing failure provides a clear design criterion for ensuring ductile behavior. Engineers designing UHSC-filled steel tube columns should aim for confinement coefficients at or above this threshold to ensure adequate ductility and post-peak load-carrying capacity.
From a welding and fabrication standpoint, the use of high-strength steels for the square tubes requires rigorous welding quality control, including pre-heat treatment, controlled interpass temperatures, and post-weld heat treatment to prevent hydrogen-induced cracking. Non-destructive testing (NDT) of all welds should be performed according to relevant standards, with acceptance criteria tailored to the criticality of the application. The introduction of the confinement reduction coefficient in the calculation method acknowledges the practical limitations of idealized confinement models and provides a more realistic design approach that accounts for material nonlinearity and welding-induced imperfections.
This study demonstrates that the combination of UHSC and steel tube confinement offers a promising structural system for applications requiring high compressive strength in compact cross-sections, such as bridge piers, high-rise building columns, and metro station structural elements, provided that appropriate material selection, welding quality control, and design methodology are employed.