Axial Compression Performance of Square Hollow Section Sandwich High-Strength Steel Tube-Concrete Columns

Literature Overview

This study investigates the axial compressive behaviour of composite columns composed of a square hollow section (SHS) with an internal sandwich layer of high-strength steel encasing high-performance concrete (HPC). The research addresses a critical need in modern high-rise and heavy-load structural engineering, where conventional steel tube-concrete (SRC) columns face limitations in load capacity, ductility, and material utilisation efficiency. By introducing a sandwich configuration of high-strength steel between the outer SHS and the core concrete, the authors explore a novel composite system that leverages the superior confinement effect and material synergy between steel and concrete.

Core Technical Concepts

The sandwich configuration fundamentally alters the stress distribution within the composite column. Unlike conventional SRC columns where a single steel tube confines the concrete core, the sandwich layer introduces an intermediate confinement zone that creates a graded constraint on the concrete. This results in a multi-stage confinement mechanism: the outer SHS provides global stability and lateral resistance, while the inner high-strength steel layer delivers intensified local confinement pressure directly to the concrete surface.

The key mechanical parameters investigated include:

Interpretation of Confinement Mechanism

The confinement pressure in a square section differs significantly from that in a circular section due to the non-uniform stress distribution at corners and flat faces. In the sandwich configuration, the inner high-strength steel layer effectively bridges the corners of the square section, distributing the confinement pressure more uniformly across the concrete surface. This is particularly important because the corner regions of square sections typically experience stress concentrations that can initiate local buckling or concrete crushing at lower load levels.

From a welding and fabrication perspective, the sandwich configuration introduces significant manufacturing challenges. The connection between the outer SHS and the inner steel layer must be designed to ensure composite action while accommodating differential thermal expansion during the concrete casting and curing process. Friction-welded or bolted connections are typically employed, and the residual stresses introduced during fabrication can influence the overall structural performance.

Engineering Practice Implications

For structural engineers designing such columns, several critical considerations emerge:

1. Material compatibility: The high-strength steel used in the sandwich layer must have sufficient elongation capacity to accommodate the concrete's volumetric expansion under confinement. A minimum elongation of 12% is recommended for steels exceeding 600 MPa yield strength.
2. Concrete placement: The narrow gap between the outer tube and inner steel layer requires specialised concrete placement methods. Self-compacting concrete (SCC) with appropriate slump flow (280–320 mm) and low-viscosity paste is essential to ensure full compaction without voids.
3. Interface bonding: The bond strength between the high-strength steel layer and the concrete surface is critical for composite action. Surface preparation of the steel (e.g., shot blasting to Sa 2.5 grade) and the use of bonding agents are recommended to achieve an interface shear strength exceeding 1.5 MPa.
4. Welding considerations: Any welded connections in the sandwich layer must be designed to prevent hydrogen-induced cracking, particularly when using steels above 550 MPa. Preheating to 150–200°C and controlled heat input (below 2.5 kJ/mm) are essential process parameters.

Key Questions and Reflections

The study raises important questions about the long-term durability of the sandwich configuration. The presence of two steel layers in direct contact with concrete creates a larger surface area for potential carbonation ingress and chloride attack. In aggressive environments, the inner steel layer may be more vulnerable to corrosion due to the confined geometry, which limits inspection access.

Furthermore, the cost-benefit analysis of adding a sandwich layer must be carefully evaluated. While the strength improvement of 15–35% is significant, the additional material cost, fabrication complexity, and inspection requirements may not be justified for all structural applications. The technology is most appropriate for critical load-bearing columns in super-tall buildings, long-span bridges, and heavy industrial structures where space constraints limit column dimensions.

Study Insights and Outlook

The sandwich high-strength steel tube-concrete column represents a promising advancement in composite structural engineering. The dual-confinement mechanism provides a synergistic enhancement of both strength and ductility, which is particularly valuable for seismic design where energy dissipation capacity is paramount. Future research should focus on cyclic loading behaviour, fire resistance performance, and long-term creep and shrinkage effects in the sandwich configuration. The integration of such columns into complete structural systems, particularly their interaction with floor slabs and beam-column joints, remains an important area for continued investigation.