Ultimate Bearing Capacity of External Square Internal Circular Hollow Sandwich CFST Axial Compression Short Columns

Literature Overview

This paper by Zhang Changguang et al. (2008), published in the Journal of Architecture and Civil Engineering, presents a theoretical derivation for the ultimate bearing capacity of external square internal circular hollow sandwich concrete-filled steel tube (CFST) axial compression short columns. The study builds upon the unified strength theory solution for thick-walled cylinders and introduces concrete strength reduction coefficients and equivalent confinement reduction coefficients to account for the geometric characteristics of the square-outer, circular-inner cross-section. The work was supported by the Ministry of Education Doctoral Discipline Point Special Research Fund (Grant No. 20040710001) and the Shaanxi Provincial Natural Science Foundation (Grant No. 2005E204).

Core Technical Approach

The external square internal circular hollow sandwich CFST column is a novel structural section that combines the advantages of square cross-sections (ease of connection, efficient use of space) with the superior confinement characteristics of circular steel tubes. The theoretical analysis proceeds through the following steps:

Unified strength theory solution: The thick-walled cylinder theory provides the stress distribution in the circular steel tube under internal and external pressure conditions.
Equivalent transformation: The square outer tube is equivalently transformed to a circular outer tube using reduction coefficients that account for the geometric differences.
Confinement modeling: The equivalent confinement pressure exerted by the steel tubes on the concrete core is calculated, considering both the inner circular tube and the equivalent outer tube.
Bearing capacity derivation: The ultimate bearing capacity formula is derived by summing the contributions of the steel tubes and the confined concrete core.

The key coefficients introduced in the theoretical framework are:

Coefficient	Symbol	Purpose
Concrete strength reduction coefficient	η_c	Accounts for the reduction in concrete strength due to the hollow sandwich configuration
Equivalent confinement reduction coefficient	η_e	Accounts for the difference in confinement effectiveness between square and circular outer tubes

Interpretation of Key Results

The theoretical formula is validated by comparison with experimental results from existing literature. The comparison demonstrates good agreement between theoretical predictions and experimental measurements, confirming the correctness and feasibility of the proposed analytical approach.

The theoretical framework provides several important insights:

The hollow sandwich configuration enhances the confinement effect on the concrete core compared to conventional solid CFST columns, as both the inner and outer steel tubes contribute to the lateral confinement pressure.
The square outer tube, while less efficient in confinement than a circular tube of equivalent area, provides practical advantages in structural connectivity and architectural integration.
The reduction coefficients effectively capture the geometric effects, enabling the use of circular tube theory for square outer tube configurations.

Integration with Engineering Practice

In steel pipe manufacturing and structural engineering, the hollow sandwich CFST column represents an innovative application that leverages both steel tube fabrication capabilities and concrete technology. The fabrication of such columns requires:

Precision steel tube manufacturing: Both the outer square tube and inner circular tube must be manufactured to tight dimensional tolerances to ensure proper assembly and concrete filling.
Square tube fabrication: The square outer tube can be fabricated through cold rolling, hot rolling, or welding processes. For welded square tubes, the corner welds must be of high quality to prevent stress concentration and cracking under axial compression.
Assembly and concrete filling: The inner circular tube must be precisely positioned within the outer square tube, and the annular space must be completely filled with concrete. Incomplete filling can lead to voids that reduce the confinement effectiveness and bearing capacity.

From a welding perspective, the fabrication of hollow sandwich CFST columns involves several critical welds:

Weld Location	Weld Type	Quality Requirements
Outer square tube corners	Fillet or groove weld	Full penetration, no lack of fusion
Inner circular tube longitudinal seam	Butt weld	Full penetration, NDT inspection
Inner-outer tube connection	Tack welds or structural welds	Adequate strength for assembly stability
Concrete filling openings	Seal welds	Leak-tight to prevent concrete leakage

The welding quality at these locations directly affects the structural performance of the hollow sandwich CFST column, and proper welding procedure qualification (WPQ) and welder qualification (WQ) are essential.

Key Questions and Reflections

The theoretical framework presented in this paper raises several questions for further investigation:

The study focuses on short columns (low slenderness ratio). How does the bearing capacity formula extend to slender columns where buckling effects become significant?
The reduction coefficients are derived theoretically. How do they compare with experimentally calibrated values, and are there material-specific or geometry-specific adjustments needed?
The study considers axial compression only. What is the behavior under combined axial compression and bending, which is more representative of actual structural loading conditions?
How does the hollow sandwich configuration perform under cyclic loading, which is relevant for seismic applications?

These questions point to the need for further experimental and numerical research to validate and extend the theoretical framework to more complex loading and geometric conditions.

Study Insights and Implications

The theoretical derivation for the ultimate bearing capacity of external square internal circular hollow sandwich CFST columns provides a valuable analytical tool for the design of this novel structural section. The use of reduction coefficients to bridge the gap between square and circular geometries is an elegant approach that simplifies the analysis while maintaining accuracy. For engineers in steel pipe manufacturing and structural design, the key implication is that hollow sandwich CFST columns offer enhanced confinement and bearing capacity compared to conventional CFST columns, but their fabrication requires careful attention to dimensional accuracy, weld quality, and concrete filling integrity. The theoretical framework should be further validated through dedicated experimental programs before widespread adoption in structural design practice.