Simplified Calculation Method for Axial Compression Bearing Capacity of Square Steel Tube Concrete Short Columns

Literature Overview

This paper by Ma Licheng, Shi Qingxuan, and Wang Qiuwei from Xi'an University of Architecture and Technology, published in 2022 in Building Structures, presents a simplified calculation method for the axial compression bearing capacity of square steel tube concrete (CFST) short columns. The research is supported by the National Natural Science Foundation of China (Grants 51878540 and 51878543) and the National Key R&D Program (Grant 2017YFC0703406). The authors collected 167 test data points from domestic and international experiments on square CFST short columns, analyzed the failure mechanisms under two different loading conditions, and derived a new bearing capacity formula based on limit equilibrium theory. The proposed formula is applicable to columns with a width-to-thickness ratio (B) of 20 or greater.

Failure Mechanism and Confinement Effect Analysis

A key contribution of this paper is the clarification of why the ultimate axial compression bearing capacity of square CFST short columns is not influenced by the loading method. The authors examined two loading conditions: loading applied directly to the concrete core and loading applied through the steel tube. In both cases, the failure mechanism involves progressive yielding of the steel tube walls, followed by concrete crushing under confinement. The confinement provided by the steel tube increases the effective compressive strength of the concrete, and this effect is governed by the geometric and material properties of the tube rather than the path of load introduction.

The concept of a confinement effect coefficient is introduced to quantify the influence of three parameters on the composite action between steel and concrete: concrete compressive strength, steel tube yield strength, and the width-to-thickness ratio (B) of the tube. The confinement effect is strongest when the tube is relatively thin (low B ratio) and when both the concrete and steel have high strength. As the B ratio increases, the tube walls become more susceptible to local buckling, which reduces the effectiveness of confinement and limits the composite action.

Derivation of the Simplified Calculation Formula

Starting from the ideal axial compression limit state, the authors apply limit equilibrium theory to derive the bearing capacity formula. The derivation considers the force equilibrium between the confined concrete core and the steel tube, accounting for the non-uniform distribution of confinement pressure across the concrete cross-section. In a square steel tube, the corners of the concrete cross-section receive less confinement than the center, because the steel tube walls are farther from the center and the constraint geometry is less favorable at the corners. The authors address this non-uniformity by modifying the basic formula to account for the reduced confinement effectiveness near the corners of the square section.

The resulting formula is compared with the calculation methods prescribed by Chinese and American codes. The comparison reveals that the proposed method yields results slightly more conservative than the Chinese code method but more economical than the American code method. This positioning is significant because it suggests that the proposed method provides a balanced level of safety and economy, making it suitable for practical engineering applications where both safety and cost-effectiveness are important considerations.

Engineering Practice and Quality Control Implications

For steel pipe manufacturing and structural engineering practice, this paper has several important implications. First, the emphasis on the width-to-thickness ratio (B) as a critical parameter reinforces the importance of precise dimensional control during steel pipe manufacturing. For square steel tubes used in CFST columns, the wall thickness and cross-sectional dimensions must be controlled within tight tolerances to ensure that the B ratio remains within the range for which the confinement model is valid. Deviations in wall thickness—whether due to rolling variations, cutting tolerances, or welding distortion—can significantly affect the confinement effectiveness and, consequently, the structural capacity.

Second, the paper's analysis of failure mechanisms provides valuable guidance for non-destructive testing (NDT) protocols. Since the confinement effect depends on the integrity of the steel tube walls, any defects such as cracks, delaminations, or thickness variations can compromise the composite action. Ultrasonic testing (UT) for wall thickness measurement and magnetic particle testing (MT) for surface and near-surface defect detection should be considered essential quality control steps for square steel tubes intended for CFST applications.

From a welding perspective, the connections between steel tube segments in CFST columns—typically butt-welded or fillet-welded—must be designed and executed to maintain the continuity of the confinement action. Welding residual stresses in the steel tube walls can interact with the compressive stresses imposed by the concrete infilling, potentially leading to premature local buckling or weld cracking. Proper welding procedures, including preheating, interpass temperature control, and post-weld heat treatment, are essential to minimize residual stress effects.

The proposed formula's positioning between Chinese and American code methods suggests that it could serve as a bridge between different design philosophies. Chinese codes tend to be more conservative, reflecting a philosophy of safety margin prioritization, while American codes are more performance-based, allowing higher utilization of material capacity. The proposed method's balanced approach may be particularly suitable for projects that require compliance with multiple code systems or for applications where the cost of over-design is significant. Engineers should, however, validate the formula against project-specific test data before applying it to critical structural elements, particularly when the B ratio approaches or exceeds the upper limit of the validated range.