Eccentric Compression Performance of Steel Tube Concrete Composite Columns

Literature Overview

This paper by Ke Xiaojun and colleagues from Guangxi University and Northeast Electric Power University, published in the Engineering Mechanics (2018, Vol. 35, No. 12), investigates the compression-bending performance of steel tube concrete (CFST) composite columns through eccentric compression tests. The research was supported by the National Natural Science Foundation of China (Grants 51508112, 51668007, 51468003) and Guangxi provincial research programs. The study examines failure modes, confinement mechanisms, and develops improved bearing capacity calculation formulas that account for the contribution of the steel tube and its confining action.

Experimental Program and Test Parameters

The experimental program involved eccentric compression tests on CFST composite columns, with the eccentricity ratio as the primary variable. The test specimens included steel tubes filled with concrete and reinforced with longitudinal rebars, representing a composite column configuration that combines the advantages of steel tube confinement with the ductility of reinforced concrete.

Core Technical Findings

The experimental results confirm that CFST composite columns exhibit both large eccentric and small eccentric compression failure modes, consistent with conventional reinforced concrete column behavior. The boundary failure criterion is defined by the simultaneous yielding of the tensile longitudinal reinforcement and the attainment of the ultimate compressive strain at the compressed concrete edge. The plane section assumption holds for the cross-sectional strain distribution, which validates the application of conventional section analysis methods.

A key finding of this research is that the transverse strain of the steel tube increases as the eccentricity ratio decreases, indicating that the confinement effect of the steel tube becomes more pronounced as the loading approaches pure axial compression. This observation demonstrates that the steel tube contribution to the bearing capacity cannot be neglected even in eccentric compression conditions, contrary to the simplified approach adopted in the current code CECS 188:2005, which ignores the steel tube effect in compression-bending calculations.

Improved Bearing Capacity Calculation

The paper proposes a new section compression-bending bearing capacity calculation formula that explicitly accounts for the steel tube and its confining action. The formula incorporates the enhanced concrete strength due to steel tube confinement, the direct contribution of the steel tube to axial and flexural resistance, and the interaction between these components under eccentric loading. Validation against 40 sets of experimental data demonstrates that the proposed method provides reasonable and reliable predictions.

The improved calculation method addresses a significant deficiency in the current design code, which underestimates the bearing capacity of CFST composite columns under eccentric compression by neglecting the steel tube contribution. This underestimation leads to conservative design that may result in unnecessary material usage and increased construction costs. The proposed formula provides engineers with a more accurate and economical design tool that better utilizes the structural potential of CFST composite columns.

Welding and Fabrication Quality Considerations

From a steel pipe fabrication and welding perspective, the CFST composite columns described in this study require careful attention to several fabrication details. The steel tubes used for these columns are typically round, square, or rectangular welded steel pipes, and the quality of the longitudinal and transverse welds directly affects the structural performance.

The welding of longitudinal rebars to the internal reinforcement cage and to the steel tube is a critical fabrication step. These welds must be designed to ensure full composite action between the steel tube, concrete, and reinforcement. The weld metal should have adequate ductility to accommodate the large plastic deformations expected at the ultimate limit state. The heat-affected zone of these welds must maintain sufficient toughness to prevent brittle fracture under combined compressive and tensile stresses.

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