Biaxial Eccentric Compression Behavior of Square Hollow Sandwich Steel Tube-Concrete Members

Literature Overview

The research by Huang Hong, Zhang Ange, and Zhao Bing, published in Industrial Construction in 2010 (Vol. 40, No. 5, pp. 114-118), presents an experimental and numerical investigation of the mechanical properties of square hollow sandwich steel tube-concrete specimens under biaxial eccentric compression. The work was supported by the Jiangxi Provincial Department of Education Research Funding Project (GJJ08503) and conducted at East China Jiaotong University. Six specimens were tested to examine the effects of slenderness ratio, eccentricity distance, and eccentricity angle on structural performance. This study addresses a gap in the understanding of sandwich-type steel tube-concrete members, which offer potential advantages in weight reduction and cost efficiency compared to conventional solid-filled steel tube-concrete columns.

Experimental Setup and Parameters

The test program consisted of six square hollow sandwich steel tube-concrete specimens subjected to biaxial eccentric compression loading. The key variables investigated include:

The specimens were instrumented to measure axial loads, lateral displacements, and strain distributions across the cross-section. The loading was applied in a biaxial eccentric manner, simulating realistic load conditions encountered in multi-story building frames and bridge piers where loads may not align with the principal axes of the cross-section.

Key Experimental Findings

The experimental results reveal several important characteristics:

1. The plane section assumption holds reasonably well throughout the loading process, indicating that the cross-section remains approximately planar even under significant deformation. This validates the use of classical beam theory for preliminary design calculations.
2. Both slenderness ratio and eccentricity distance have a pronounced effect on load-carrying capacity. As either parameter increases, the ultimate load decreases, with the slenderness ratio generally having the more dominant influence.
3. The eccentricity angle has a relatively minor effect on the overall load-deformation relationship. This finding is significant because it suggests that, for practical design purposes, the orientation of eccentricity may not require overly detailed consideration when evaluating the overall structural response.
4. The load-deformation curves exhibit a characteristic progression from initial linear elastic behavior through yielding to a post-peak descending branch, with the ductility characteristics varying with the slenderness ratio and eccentricity parameters.

Numerical Analysis Using Fiber Model Method

The authors developed a fiber-based computational program to predict the load-deformation behavior of the biaxially eccentric members. The fiber model approach discretizes the cross-section into numerous small elements, each assigned appropriate material constitutive relationships. The calculation results showed good agreement with the experimental data, validating the numerical methodology.

The fiber model method is particularly well-suited for analyzing steel tube-concrete members because it can naturally capture the nonlinear material behavior of both the steel and concrete components, as well as the interaction effects between them. The program accounts for:

• Geometric nonlinearity due to large deformations and P-Δ effects.
• Material nonlinearity of steel (elastic-perfectly plastic or elastic-plastic with hardening).
• Material nonlinearity of concrete (compression and tension softening).
• The confining effect of the steel tube on the core concrete.

The good agreement between numerical predictions and experimental results provides confidence in using the fiber model for parametric studies and design calculations of similar members.

Engineering Practice Considerations

For engineers designing sandwich steel tube-concrete columns subjected to biaxial eccentric loading, several practical considerations emerge from this study:

• The slenderness ratio should be carefully controlled to ensure adequate load-carrying capacity and serviceability.
• The relatively minor influence of eccentricity angle simplifies the design process, as the critical design condition can be identified more straightforwardly.
• The plane section assumption's validity supports the use of simplified design methods for preliminary sizing, though detailed nonlinear analysis is recommended for final design verification.
• The sandwich configuration introduces additional complexity in terms of load distribution between the inner and outer steel tubes, which should be addressed in detailed design.

However, the study's scope is limited to six specimens, which may not fully capture the variability in material properties and construction tolerances encountered in practice. Furthermore, the study does not address cyclic loading behavior, which is critical for seismic design applications.

Study Insights and Reflections

This research provides valuable foundational data on the biaxial eccentric compression behavior of sandwich steel tube-concrete members. The finding that eccentricity angle has minimal influence on the overall response is particularly useful for practical design, as it reduces the number of design cases that need to be evaluated. However, I note that the study could benefit from additional investigation into the interaction between the inner and outer steel tubes, the effect of concrete strength grades on the overall performance, and the behavior under combined axial compression and biaxial bending with varying load paths. The fiber model approach is well-suited for this type of analysis and could be extended to incorporate more sophisticated material models and damage mechanics formulations. For engineers working on innovative composite column designs, this paper offers a solid starting point for understanding the fundamental behavior, but comprehensive design should incorporate additional research on long-term performance, fatigue behavior, and seismic resilience.