Experimental Investigation of Stress-Strain Behavior under Eccentric Compression in Concrete-Filled Steel Tubes

Research Background and Scope

Chen Baochun, Wang Laiyong, Ouyang Zhijing, and Han Linhai from Fuzhou University and Fujian Provincial College of Architecture conducted a systematic experimental study on 18 eccentrically loaded CFST short columns. Published in Engineering Mechanics (Volume 20, Issue 6, 2003, pages 154–159), this work was supported by the National Natural Science Foundation of China (Grant No. 50078016) and the State Key Laboratory of Disaster Prevention in Civil Engineering. The experimental parameters include eccentricity ratio, steel ratio, and concrete compressive strength, providing a comprehensive dataset for understanding the interaction between bending and axial loading in CFST members.

Key Experimental Findings

The study systematically examines how eccentricity ratio affects the mechanical performance of CFST columns. The primary findings are summarized as follows:

The data indicate that as the eccentricity ratio increases, the confining pressure generated by the steel tube becomes less effective at enhancing the concrete strength. This occurs because the bending component introduces a non-uniform stress state in the concrete core, with one side under tension and the other under compression, thereby reducing the overall confinement efficiency.

Technical Analysis and Constitutive Implications

The stress-strain relationship under eccentric compression differs fundamentally from that under pure axial compression. Under axial loading, the steel tube provides uniform lateral confinement to the concrete, resulting in a triaxial stress state that significantly enhances both the strength and ductility of the concrete core. Under eccentric loading, however, the confinement becomes non-uniform, and the concrete on the tension side may experience cracking that further degrades the confinement mechanism.

The finding that the ductility coefficient remains relatively insensitive to eccentricity is particularly significant for seismic design. It suggests that CFST columns retain their deformation capacity even under combined flexural-axial loading, which is the typical loading condition in earthquake-resistant structures. This observation supports the use of CFST members in moment-resisting frames where large plastic deformations may be required during seismic events.

Engineering Practice Integration

From a design perspective, the results underscore the importance of accounting for the reduced confinement effectiveness under eccentric loading. Design codes that simply scale axial compression strength by a confinement factor derived from concentric loading tests may be unconservative for members subjected to significant bending moments. Engineers should apply interaction curves that properly capture the degradation of concrete strength enhancement as eccentricity increases.

The increased compressive edge strain at ultimate load under higher eccentricity also has implications for steel tube wall thickness selection. Thinner-walled tubes may experience local buckling at the highly strained compression edge before reaching their full plastic capacity, particularly in members with high D/t ratios. This consideration is especially relevant for CFST columns used in bridge substructures where combined axial and bending loads are common.

Study Insights and Outlook

This experimental study provides essential data for refining the analytical models used in CFST design. The clear trend of reduced confinement effectiveness with increasing eccentricity should be incorporated into fiber-section models and interaction diagram formulations. Future research should extend these findings to long columns where second-order effects further complicate the stress distribution, and to cyclic loading conditions where the bond degradation and steel tube plasticity evolve over multiple load reversals.