Axial Compressive Capacity of Irregular Multi-Chamber CFST Columns Based on Unified Theory

Literature Overview

This paper by Wu Haipeng, Cao Wanlin, and Dong Hongying from Beijing University of Technology, published in Engineering Mechanics (2019, Vol. 36, No. 8, pp. 114-121), extends the well-known "Unified Theory" of confined concrete to irregular multi-chamber concrete-filled steel tube (CFST) columns. The study investigates how the steel tube's confining effect on concrete varies as the cross-section shape transitions from triangle, square, and regular n-gon to circular. The authors introduce the concept of "shape efficiency" and divide the concrete cross-section into effective and ineffective confinement zones, then develop a unified calculation method applicable to any irregular multi-chamber cross-section.

Core Technical Framework

The foundational principle of the Unified Theory is that the confining pressure exerted by a steel tube on the enclosed concrete is not uniform across the cross-section; it varies with the local geometry. For a circular section, the confinement is uniform and the theory is straightforward. However, for polygonal or irregular shapes, the steel tube's ability to restrain lateral concrete expansion diminishes near corners and edges where the curvature is low or discontinuous.

The authors partition the concrete cross-section into two regions:

• Effective confinement zone: the region where the steel tube provides meaningful lateral restraint, and the concrete benefits from confinement strengthening.
• Ineffective confinement zone: the region near corners or flat segments where the steel tube cannot effectively restrain concrete dilation, and the concrete behaves closer to unconfined.

A key innovation is the introduction of shape efficiency, which acts as a reduction factor on the confining effect based on the geometric characteristics of the section. The paper establishes that the initial tangent angle of the quadratic curve bounding the effective and ineffective confinement zones is linearly related to the interior angle of the regular polygon. This linear relationship provides a practical geometric criterion for zone demarcation.

Methodology for Irregular Multi-Chamber Sections

The calculation method proceeds in three stages:

1. Decomposition: The irregular multi-chamber cross-section is decomposed into individual simple polygonal chambers.
2. Individual calculation: For each chamber, the axial compressive capacity is computed using the Unified Theory, incorporating both shape efficiency (based on the polygon geometry) and regularity factors (accounting for deviations from regular polygon shapes, such as unequal side lengths or non-uniform angles).
3. Superposition: The axial capacities of all chambers are summed to obtain the total capacity of the multi-chamber section.

The authors demonstrate that for quadrilateral sections, the regularity of the shape significantly influences the confining effectiveness. A square section achieves better confinement than a rectangle of similar area because the square's more uniform curvature distribution allows more uniform stress transfer from the steel tube to the concrete.

Engineering Practice Implications

From a steel pipe manufacturing and structural engineering perspective, this study has several practical implications:

• Pipe selection: When designing CFST columns with non-circular cross-sections, engineers can now quantitatively assess how much confinement benefit is lost compared to circular sections. A square section, for example, retains approximately 85-90% of the confinement efficiency of an equivalent circular section, which is useful for preliminary design decisions.
• Multi-chamber design: The decomposition method allows engineers to evaluate complex built-up sections (such as those formed by welding multiple square tubes together) by treating each chamber independently. This is particularly relevant for large-diameter columns where a single tube may be impractical.
• Welding considerations: Multi-chamber CFST sections typically involve welding of individual tube segments. The quality of internal welds directly affects the structural integrity and confinement performance. Engineers should pay special attention to weld quality at chamber boundaries, as weld defects could compromise the load transfer between steel tubes and concrete.

Study Insights and Reflections

The strength of this research lies in its systematic approach: starting from the well-established Unified Theory for circular sections and progressively extending it to polygonal and irregular shapes through clearly defined geometric criteria. The linear relationship between the interior angle and the confinement zone boundary tangent angle is elegant and practical. However, the method relies on the assumption that chambers can be treated independently, which may underestimate interaction effects between adjacent chambers in tightly packed multi-chamber sections. Future work should investigate the interaction between chambers through shared walls or interfaces, and validate the method with additional experimental data on irregular cross-sections beyond simple polygons.

The paper represents a significant advancement in the theoretical treatment of CFST columns and provides a valuable tool for engineers designing non-circular and multi-chamber CFST structures. The methodology bridges the gap between the simplicity of circular CFST design and the complexity of real-world irregular sections, enabling more accurate and economical structural design.