Theoretical Analysis and Experimental Study of Concrete-Filled Steel Tube Bearing Capacity Under Double-Shear Unified Strength Theory

Literature Overview

The paper authored by Ma Shufang, Zhao Junhai, and Wei Xueying from the School of Civil Engineering, Chang'an University, published in the Journal of Xi'an University of Architecture and Technology (Natural Science Edition) in 2007, presents a comprehensive theoretical and experimental investigation into the bearing capacity of concrete-filled steel tube (CFST) columns. The research was supported by the Ministry of Education Doctoral Point Fund (Project No. 20040710001) and the Shaanxi Provincial Natural Science Foundation (Project No. 2005E204). The authors fabricated 25 CFST specimens and conducted both axially compressed and eccentrically compressed tests, obtaining load-deformation curves and ultimate loads. Based on the double-shear unified strength theory, they derived a formula for the compressive strength of the core concrete and performed theoretical calculations of CFST bearing capacity. The theoretical results were found to be in good agreement with experimental data, demonstrating the excellent applicability of the double-shear unified strength theory for CFST analysis and calculation.

Core Technical Content and Interpretation

The double-shear unified strength theory, originally proposed by Zhao Junhai, is a unified yield criterion that can degenerate into several classical strength theories such as the Tresca criterion, the von Mises criterion, and the Mohr-Coulomb criterion by adjusting a single parameter. This makes it particularly suitable for materials such as concrete, which exhibit different strength behaviors under different stress states. In the context of CFST members, the core concrete is confined by the surrounding steel tube, creating a complex triaxial stress state that conventional uniaxial strength theories cannot adequately capture.

The authors' key contribution lies in applying this unified strength theory to derive a formula for the equivalent compressive strength of the confined core concrete within the steel tube. This approach accounts for the interaction between the steel tube and the core concrete, recognizing that the confining pressure from the steel tube enhances the concrete's compressive capacity and ductility. The theoretical framework considers the stress state at any point in the core concrete, incorporating both the axial compression and the lateral confinement pressure exerted by the steel tube.

The derivation of the core concrete strength formula is significant because it bridges the gap between the complex triaxial stress state within CFST members and practical engineering calculations. Traditional approaches often rely on empirical confining models such as the Mander model or the Lam and Teng model, which are calibrated from specific experimental datasets. The double-shear unified strength theory offers a more fundamental and theoretically rigorous approach, grounded in the mechanics of materials under multi-axial stress conditions.

Connection with Engineering Practice

In practical steel pipe and pipe fitting engineering, CFST columns are widely used in bridge piers, high-rise buildings, offshore platforms, and industrial structures where high load-bearing capacity and ductility are required. The bearing capacity of these members is a critical design parameter, and the accuracy of theoretical predictions directly affects structural safety and economic efficiency.

From a manufacturing perspective, the steel tubes used in CFST construction must meet specific dimensional and mechanical property requirements. The steel tube's yield strength, wall thickness, and diameter-to-thickness ratio all influence the confinement effect and, consequently, the bearing capacity. For example, a smaller diameter-to-thickness ratio provides greater confinement pressure on the core concrete, leading to higher ultimate bearing capacity. However, this also affects the fabrication process, as thinner walls may be more susceptible to ovalization during the concrete-filling operation.

The eccentric compression tests conducted in this study are particularly relevant for real-world applications where CFST columns rarely experience perfectly axial loads. Eccentric loading introduces bending moments that cause non-uniform stress distribution across the cross-section, with one side experiencing higher compressive stresses and the other potentially experiencing tensile stresses. The theoretical model based on the double-shear unified strength theory must accurately capture this stress redistribution to provide reliable design predictions.

For welding engineers, the connection details between CFST columns and other structural elements are critical. The welding process used to connect the steel tubes must ensure full penetration and adequate heat-affected zone properties. The welding heat input can potentially affect the mechanical properties of the steel tube near the weld, which in turn influences the confinement effect on the core concrete. Therefore, the theoretical framework presented in this paper should be complemented by careful consideration of welding-induced property variations in practical design.

Key Questions and Reflections

One important question that arises from this study is the sensitivity of the theoretical predictions to the choice of the unified strength theory parameter. The double-shear unified strength theory includes a parameter that controls the transition between different classical strength criteria. The authors selected this parameter based on the material properties of the core concrete, but the accuracy of this selection is crucial for the validity of the entire theoretical framework. In engineering practice, the selection of this parameter should be guided by laboratory testing of the specific concrete mix used, rather than relying on generic values.

Another consideration is the scalability of the results. The 25 test specimens used in this study likely represent a limited range of geometric and material parameters. Extrapolating the theoretical framework to significantly different member sizes or material combinations requires careful validation. For instance, the confinement effect may behave differently for very large-diameter CFST columns compared to the laboratory-scale specimens, due to factors such as concrete settlement during curing, differential thermal expansion between steel and concrete, and long-term creep and shrinkage effects.

From a quality control standpoint, ensuring the accuracy of CFST member performance requires rigorous inspection of both the steel tube and the core concrete. The steel tube should be inspected for dimensional accuracy, wall thickness uniformity, and surface defects that could affect the confinement effect. The core concrete should be verified for proper density and strength, as voids or honeycombing within the concrete would significantly reduce the effective confinement and bearing capacity. Non-destructive testing methods such as ultrasonic testing and radiographic testing should be employed to verify the integrity of the composite member.

Study Insights and Implications

This study demonstrates that the double-shear unified strength theory provides a robust and theoretically sound framework for analyzing the bearing capacity of CFST members. The good agreement between theoretical and experimental results validates the approach and suggests that it can be extended to more complex loading conditions and member geometries. The derived formula for the core concrete compressive strength offers a practical tool for engineers to predict the behavior of CFST columns under various loading scenarios.

For the steel pipe manufacturing industry, this research underscores the importance of maintaining precise control over steel tube dimensions and mechanical properties. Variations in wall thickness, yield strength, and geometric accuracy can significantly affect the confinement effect and, consequently, the structural performance of CFST members. Manufacturers should implement stringent quality control measures, including ultrasonic thickness measurement, mechanical property testing of representative samples, and dimensional inspection, to ensure that the steel tubes meet the requirements for CFST applications.

The study also highlights the value of integrating theoretical analysis with experimental validation in the development of design methods. While theoretical models provide a fundamental understanding of structural behavior, experimental testing is essential for validating and calibrating these models. This integrated approach should be adopted in the development of new design codes and standards for CFST structures, ensuring that the theoretical foundations are sound and the practical applicability is demonstrated.

In conclusion, this paper makes a valuable contribution to the field of CFST structural engineering by demonstrating the applicability of the double-shear unified strength theory for bearing capacity analysis. The theoretical framework and derived formulas provide engineers with a powerful tool for the design and analysis of CFST members, while the experimental results offer valuable benchmark data for validating numerical models and design codes. The integration of advanced strength theory with practical engineering considerations represents a significant step forward in the rational design of concrete-filled steel tube structures.