Bending Performance of Self-Stressed Steel Tube Concrete Members

Research Background and Significance

The study by Lin Haixiao and colleagues from Henan Polytechnic University and Dalian University of Technology investigates the bending performance of self-stressed concrete-filled steel tube (SCFST) members. Self-stressed concrete is a type of expansive concrete that develops internal compressive stresses upon hardening, which can improve the structural performance of concrete members. When used in conjunction with steel tubes, the self-stress effect can potentially enhance the bending capacity and stiffness of CFST members beyond what is achievable with conventional concrete. This research is significant for engineers seeking to optimize the design of CFST structural members by exploiting the beneficial effects of self-stressed concrete.

Experimental Program

The experimental program involved 15 SCFST specimens subjected to bending tests. The specimens were designed to represent typical structural applications, with variations in steel tube dimensions, concrete mix design, and self-stress level. The testing followed standard procedures for flexural testing of CFST members, with careful measurement of load, deflection, and strain at critical locations.

Parameter	Description	Typical Range
Steel tube outer diameter	D	Multiple sizes
Steel tube wall thickness	t	Multiple values
Steel grade	f_y	Common structural grades
Concrete self-stress level	σ_ss	Multiple levels
Span-to-depth ratio	L/h	Typical structural values

The experimental results showed that SCFST members consistently exhibited higher bending capacity than conventional CFST members with equivalent dimensions and material properties. The increase in bending capacity was proportional to the level of self-stress in the concrete, confirming the beneficial effect of expansive concrete on structural performance.

Finite Element Modeling and Analysis

A three-dimensional finite element model was developed to complement the experimental results and to extend the parametric study beyond what was feasible with physical testing. The model employed a concrete interface model to capture the bond behavior between the steel tube and the self-stressed concrete, and an elastic-plastic material model for the steel to account for yielding and strain hardening. The boundary conditions and loading were applied to replicate the experimental setup, and the model was calibrated against the experimental results.

Model Component	Model Type	Key Parameters
Concrete	Interface model	Self-stress level, compressive strength, tensile strength
Steel tube	Elastic-plastic model	Yield strength, elastic modulus, hardening modulus
Steel-concrete interface	Bond model	Bond strength, slip capacity

The finite element results showed good agreement with the experimental data, validating the model and confirming the influence of self-stress on bending performance. The analysis also revealed that the self-stress effect is more pronounced at higher levels of loading, where the concrete has begun to crack and the steel tube has yielded. At this stage, the self-stress provides additional confinement and load redistribution, delaying the progression of failure.

Empirical Formula and Stiffness Recommendation

Based on the combined experimental and analytical results, the authors proposed an empirical formula for calculating the bending capacity of SCFST members. The formula incorporates the self-stress level as an additional parameter beyond those used in conventional CFST design formulas. The formula takes the form:

M_u = M_u,CFST · (1 + k · σ_ss / f_c)

where M_u is the bending capacity of the SCFST member, M_u,CFST is the bending capacity of the equivalent conventional CFST member, k is a modification factor determined from regression analysis, σ_ss is the self-stress level, and f_c is the concrete compressive strength.

For the calculation of bending stiffness, the authors recommended using the AIJ (Association for Japanese Industries) calculation method, which provides a well-established framework for CFST member stiffness that can be extended to SCFST members with appropriate modifications for the self-stress effect.

Engineering Practice Considerations

From a steel pipe manufacturing perspective, the use of self-stressed concrete in CFST members requires careful attention to the concrete placement process. The expansive properties of self-stressed concrete can cause increased internal pressures during hydration, which must be accommodated by the steel tube without causing distress. The welds in the steel tube must be of high quality to withstand the additional internal pressures, and the fabrication tolerances must be tight enough to ensure proper concrete placement and compaction. For welding engineers, the potential for increased residual stresses due to the expansive concrete must be considered in the weld design and inspection criteria.

Summary

This study demonstrates that self-stressed concrete can significantly enhance the bending performance of CFST members, providing a pathway for more efficient structural design. The proposed empirical formula and the recommendation for stiffness calculation using the AIJ method offer practical tools for engineers. The research also highlights the importance of controlling concrete placement quality and weld integrity when using self-stressed concrete in steel tube structures.