Theoretical Analysis of Steel Tube-Confinement Concrete Bearing Capacity Based on Double-Shear Unified Strength Theory

Literature Overview

The paper by Ma Shufang, Zhao Junhai, and Wei Xueying (2007), published in the Journal of Xi'an University of Architecture and Technology, presents a systematic experimental and theoretical investigation into the load-bearing capacity of concrete-filled steel tubular (CFST) members. Twenty-five CFST specimens were fabricated and subjected to both axial and eccentric compression tests. The core contribution lies in deriving a formula for the confined concrete compressive strength based on the double-shear unified strength theory, and validating this theory against experimental results. The work was funded by the Ministry of Education Doctoral Fund Project (20040710001) and the Shaanxi Provincial Natural Science Foundation (2005E204).

Core Technical Content

The double-shear unified strength theory, originally developed by Zhao Junhai, provides a more comprehensive framework than traditional Mohr-Coulomb or von Mises criteria for modeling the strength behavior of materials under complex stress states. In the context of CFST members, the confining pressure exerted by the steel tube on the core concrete creates a triaxial stress state that fundamentally alters the compressive behavior of the concrete. The authors derived an analytical expression for the confined concrete compressive strength by incorporating the confining pressure into the double-shear yield criterion.

Key Derivation Approach

The confined concrete compressive strength formula was obtained through the following logical chain:

1. The confining pressure from the steel tube was calculated based on equilibrium conditions at the steel-concrete interface.
2. The double-shear unified strength criterion was applied to the triaxial stress state of the confined concrete.
3. A parameter $\theta$ was introduced to unify different failure criteria, allowing the model to transition between tensile and compressive failure modes.
4. The bearing capacity of the CFST member was then calculated by integrating the confined concrete strength over the cross-section, combined with the contribution of the steel tube.

The theoretical results showed good agreement with the experimental load-deformation curves and ultimate loads, confirming the applicability of the double-shear unified strength theory for CFST member analysis.

Engineering Relevance to Steel Pipe Selection

From a steel pipe engineering perspective, this research has several important implications for pipe material selection and specification:

The study implicitly demonstrates that the mechanical properties of the steel tube material directly influence the overall member capacity. In engineering practice, when selecting steel pipe grades for CFST applications, engineers should consider not only the axial strength contribution of the steel but also the confining action it provides. High-strength steel grades (e.g., Q355, Q420, or API 5L X65/X70 equivalent) offer dual benefits: increased direct load-bearing capacity and enhanced confinement effect due to higher yield stress.

Practical Considerations for Pipe Fabrication and Welding

For CFST columns and members fabricated from welded steel pipes, the following quality considerations arise from the theoretical framework presented:

• Weld integrity at the steel-concrete interface: Any loss of bond between the steel tube and core concrete due to poor surface preparation, incomplete filling, or corrosion at the interface will reduce the effective confining pressure and degrade the actual bearing capacity below the theoretical prediction.
• Welded pipe seam quality: For ERW or HFW welded pipes used as the outer shell, the longitudinal weld seam represents a potential weak zone. Under the high triaxial compressive stress state of confined concrete, any micro-defects in the weld seam could propagate, leading to premature tube rupture and loss of confinement.
• Dimensional accuracy: The confinement ratio (d/t) is a critical parameter. Pipe wall thickness tolerance deviations, particularly for thin-walled pipes, can significantly alter the confining pressure and thus the member capacity.

Study Insights and Reflections

This research provides a valuable theoretical foundation that bridges materials science and structural engineering for CFST members. The double-shear unified strength theory offers a more accurate representation of concrete behavior under confinement than simpler empirical models. However, from a manufacturing and quality control standpoint, the theoretical predictions are only achievable when the steel tube is fabricated to tight dimensional tolerances, the weld quality is consistently high, and the steel-concrete bond is properly maintained throughout the service life.

The practical takeaway for pipe engineers is that the mechanical performance of CFST members is highly sensitive to the quality and properties of the outer steel tube. This reinforces the importance of rigorous incoming inspection, dimensional control, and weld quality assurance in the supply chain for structural steel pipes intended for composite construction applications. The theoretical framework also suggests that future research should investigate the effect of weld seam metallurgical properties and heat-affected zone characteristics on the long-term confinement effectiveness of CFST members under cyclic or fatigue loading conditions.