Stress-Strain Relationship Model for Eccentrically Compressed Steel Tube-Concrete Composite Members

Literature Overview

This paper by Chen Baochun, Chen Youjie, Wang Laiyong, and Han Linhai from Fuzhou University presents a fiber element model for the stress-strain relationship of steel tube-confined concrete (CTC) columns under eccentric compression. Published in China Journal of Highway and Transport in 2004, the research was supported by the National Natural Science Foundation of China (Grant No. 50078016). The study is particularly relevant to bridge engineering applications where steel tube-concrete arch bridges and columns are subjected to combined axial and bending loads. The work builds upon the authors' earlier contributions to the understanding of steel tube-concrete composite behavior and extends it to the eccentric loading condition, which is more representative of real structural behavior.

Core Technical Points

The proposed fiber element model decomposes the cross-section into discrete fibers, each assigned an appropriate stress-strain relationship. The key innovation lies in the development of a core concrete stress-strain model that accounts for the radial stress gradient within the steel tube and its influence on the confining pressure exerted on the concrete. The following table summarizes the constitutive models proposed in the study:

The four-segment linear model for the steel tube is a pragmatic approach that simplifies the complex nonlinear behavior of structural steel while maintaining sufficient accuracy for structural analysis. The segments typically correspond to: (1) elastic region up to proportional limit, (2) elastic-plastic transition, (3) yield plateau and strain hardening, and (4) necking and fracture region. This simplification enables efficient computation while preserving the essential mechanical behavior of the steel tube under combined loading.

The core concrete model addresses a critical aspect of steel tube-concrete composite mechanics: the non-uniform confining pressure distribution across the cross-section. In eccentrically loaded columns, the confining pressure is not uniform; it is higher on the compression side and lower on the tension side. The radial stress gradient model captures this variation, leading to more accurate predictions of the concrete's confined strength and ductility under eccentric loading.

Application to Structural Analysis

The proposed stress-strain relationships were applied to two case studies: (1) eccentrically compressed CTC columns and (2) the complete loading process of steel tube-concrete arch ribs. These applications demonstrate the model's versatility across different structural configurations. For bridge engineering, where steel tube-concrete arches are increasingly employed due to their aesthetic appeal and structural efficiency, the ability to accurately predict the stress-strain behavior under eccentric compression is essential for ensuring structural safety and serviceability.

The following table presents typical design parameters for CTC columns in bridge applications:

Welding and Fabrication Considerations

In the fabrication of steel tube-concrete composite members, the steel tube must be manufactured with high geometric accuracy and surface quality. For seamless steel tubes, the production process involves hot rolling or cold drawing, and the relevant standards include GB/T 8163 for general fluid transport and GB/T 3077 for alloy structural steel tubes. For large-diameter tubes, longitudinal submerged arc welded (LSAW) tubes conforming to GB/T 9711 or API 5L may be used.

The welding quality of any longitudinal or circumferential welds in the steel tube directly affects the composite member's performance. Weld defects such as lack of fusion, porosity, or residual stresses in the heat-affected zone can reduce the confining effectiveness of the steel tube and compromise the composite action between steel and concrete. Non-destructive testing methods including ultrasonic testing (UT) and magnetic particle testing (MT) should be employed to verify weld integrity in accordance with GB/T 11345 and GB/T 26951.

Key Reflections

The fiber element model approach presented in this paper represents a significant advancement in the computational analysis of steel tube-concrete composite structures. The explicit consideration of the radial stress gradient in the confining pressure model addresses a physical phenomenon that was previously neglected or oversimplified in analytical models. This improvement in physical realism translates to more reliable predictions of member behavior, particularly in the post-peak stage where structural safety is most critical.

From a materials engineering perspective, the interaction between the steel tube and the core concrete is governed by frictional contact and mechanical interlock. The confining pressure developed by the steel tube restrains the lateral expansion of the concrete under compression, thereby increasing both the compressive strength and ductility of the concrete. The magnitude of this confining pressure is directly related to the hoop stress in the steel tube, which in turn depends on the steel tube's geometric properties (D/t ratio) and material properties (yield strength, elastic modulus).

The eccentric compression condition introduces additional complexity because the bending moment creates a non-uniform stress distribution across the cross-section. On the tension side, the steel tube may yield in tension while the concrete cracks and loses its contribution. On the compression side, the concrete is confined by the steel tube and exhibits enhanced strength and ductility. The transition zone between these regions is where the most complex interaction occurs, and the proposed model's ability to capture this behavior through the fiber element approach is a notable strength.

Summary and Outlook

This research provides a rigorous and physically grounded stress-strain model for eccentrically compressed steel tube-concrete composite members, with particular emphasis on the non-uniform confining pressure distribution. The fiber element formulation enables efficient integration into structural analysis software, making it accessible to practicing engineers for design verification and performance assessment. The model's application to both column and arch rib analysis demonstrates its versatility across different structural configurations. Future work should extend the model to include cyclic loading conditions relevant to seismic design, consider the effects of corrosion on the steel-concrete interface over time, and validate the model against full-scale experimental data from full-scale bridge structures. The continued refinement of analytical models for steel tube-concrete composites will support the wider adoption of this efficient structural system in bridge and building engineering.