Bearing Capacity Calculation of Steel Tube Confined Reactive Powder Concrete Short Columns

Literature Overview

This paper, authored by Wang Qiuwei and colleagues from Xi'an University of Architecture and Technology, investigates the axial compressive behavior of steel tube confined reactive powder concrete (RPC) short columns through systematic experimental testing and theoretical modeling. Published in the Chinese Journal of Applied Mechanics (2020, Vol. 37, No. 4), the study addresses a critical challenge in ultra-high-strength concrete applications: the inherent brittleness of RPC when confined within steel tubes. The research was supported by the National Natural Science Foundation of China (Grants 51878543, 51878540), reflecting its significance in the field of composite structural engineering.

Core Technical Content and Key Findings

The experimental program involved 20 circular steel tube confined RPC short columns with an outer diameter of 133 mm. Two loading configurations were employed: full-section loading (load applied to both the steel tube and the concrete core simultaneously) and core-only loading (load applied exclusively to the concrete core). The steel tube wall thicknesses varied at 4.5 mm, 6 mm, 8 mm, and 10 mm, creating different diameter-to-thickness ratios (D/t) ranging from approximately 11.3 to 27.1.

The study established several critical observations regarding failure modes and structural behavior:

• When the confinement coefficient exceeds 0.9, specimens exhibit bulging (drumming) failure patterns
• When the confinement coefficient is below 0.9, specimens fail in shear
• Steel tube wall thicknesses of 4.5 mm and 6 mm produce load-displacement curves with pronounced descending branches
• Steel tube wall thicknesses of 8 mm and 10 mm yield stable or continuously ascending load-displacement curves
• The overall axial compressive bearing capacity exceeds the simple superposition of steel and concrete contributions by approximately 12% under full-section loading and 24% under core-only loading

Technical Parameters and Process Analysis

From a steel pipe manufacturing perspective, the wall thickness selection is particularly significant. The transition from brittle shear failure to ductile bulging failure at a confinement coefficient of 0.9 has direct implications for pipe specification. Thinner-walled tubes (4.5 mm and 6 mm) with higher D/t ratios are more susceptible to local buckling and cannot adequately restrain the expansive lateral strains of RPC under compression. This is consistent with well-established principles in steel tube concrete (CFST) design where the D/t ratio governs the local buckling behavior of the steel shell.

Theoretical Model and Calculation Method

The authors developed a bearing capacity calculation method based on the double-shear unified strength theory, establishing separate formulations for the two loading conditions. The model accounts for the interaction mechanism between the steel tube and the concrete core, recognizing that the composite action produces a synergistic enhancement beyond simple material superposition. The calculated results deviate from experimental values by less than 10%, demonstrating good applicability.

The working mechanism analysis reveals that under axial compression, the concrete core exerts lateral pressure on the steel tube, which in turn provides confining pressure to the concrete. This interaction creates a triaxial stress state in the concrete that significantly enhances its compressive strength and ductility. The confinement effect is more pronounced when the load is applied only to the concrete core, as the steel tube initially bears no direct axial load and responds primarily through hoop stress development.

Connection with Steel Pipe Engineering Practice

From the steel pipe manufacturing and quality control standpoint, this research has several practical implications:

1. Pipe wall thickness tolerance: The dramatic difference in post-peak behavior between 6 mm and 8 mm wall thicknesses underscores the importance of maintaining tight dimensional tolerances during pipe manufacturing. Variations in wall thickness directly affect the D/t ratio and consequently the confinement efficiency.
2. Surface quality requirements: The bond between the steel tube inner surface and the concrete core is critical for effective load transfer. Surface treatments, including controlled roughness or mechanical keying, become increasingly important for RPC applications where the concrete strength is extremely high.
3. Material grade selection: The steel grade used for the confining tube must have sufficient yield strength to provide effective confinement without itself failing prematurely. For RPC applications, the steel tube yield strength should be carefully matched to the concrete strength to ensure compatible deformation behavior.
4. Welding considerations: If the steel tubes require field fabrication or connections, welding procedures must be carefully controlled to avoid residual stresses that could compromise the confinement mechanism. The heat-affected zone (HAZ) properties near welds should be verified through hardness testing and microstructural examination.

Key Questions and Reflections

The study raises several important questions for engineering practice. First, the transition from shear failure to bulging failure at a confinement coefficient of 0.9 suggests a critical design threshold that should be incorporated into design codes. Second, the 24% improvement in bearing capacity under core-only loading compared to 12% under full-section loading indicates that the confinement mechanism is more effective when the steel tube is not directly loaded in compression, which has implications for connection design and load path optimization.

The practical significance of this research extends to applications where ultra-high-strength materials are combined with steel confinement, such as in bridge piers, nuclear containment structures, and offshore platforms where compact, high-capacity columns are required. The steel pipe specifications derived from this study should be considered alongside relevant standards including GB/T 8162, GB/T 8163, and API 5L for seamless pipe manufacturing.

Study Insights and Implications

This research contributes meaningfully to the understanding of composite behavior between ultra-high-strength concrete and steel confinement. For steel pipe manufacturers and structural engineers, the key takeaway is that the effectiveness of steel tube confinement is not solely determined by the steel properties but by the interaction ratio between steel and concrete, quantified through the confinement coefficient. The experimental evidence supports the design philosophy that thinner steel tubes can be used more efficiently when the concrete strength is appropriately matched, potentially reducing material costs while maintaining structural safety. The theoretical framework based on the unified strength theory provides a robust tool for predicting the behavior of these composite members under various loading conditions, facilitating more economical and reliable structural design.