Mechanism of Bending Performance Enhancement in Eccentrically Loaded CFST Members Reinforced with Welded Round Steel Bars

Overview of the Literature

This paper by Liu Dejun, Xia Zhiheng, Wang Jun, Zuo Jianping, and Chang Yongquan, published in the Journal of Harbin Institute of Technology (2023, Vol. 55, No. 5, pp. 122-131), investigates the mechanism by which welding round steel bars to the arch region of a steel tube concrete (CFST) member enhances the bending performance of eccentrically loaded members. The research was supported by the National Natural Science Foundation of China (Grant 51878658) and conducted at the State Key Laboratory of Coal Resource and Safe Mining, China University of Mining and Technology (Beijing). The authors developed a validated ABAQUS finite element model and analyzed moment-deflection curves, longitudinal and circumferential strain distributions, confinement index, and neutral axis shift to elucidate the strengthening mechanism. The study also examines the influence of round steel bar diameter and member slenderness ratio on bending performance.

Core Technical Points

Numerical Model and Validation

The finite element model was built in ABAQUS using the following element types and material models:

The model was validated against experimental data from previous tests, with the maximum deviation between simulated and experimental ultimate moments being less than 8%, confirming the model's reliability for parametric analysis.

Enhancement Mechanism

The primary finding of the paper is that welding round steel bars to the arch region (the compressed zone of the section) of an eccentrically loaded CFST member lowers the neutral axis position, thereby increasing the area of concrete in compression. This, in turn, increases the circumferential strain of the steel tube on the compressed side, enhancing the confinement effect of the steel tube on the concrete core. The enhanced confinement improves both the flexural bearing capacity and the flexural stiffness of the member.

The mechanism can be summarized as follows:

1. The round steel bars act as additional longitudinal reinforcement in the compressed zone.
2. The presence of round steel bars shifts the neutral axis toward the tension side, increasing the compressed concrete area.
3. The increased compressed concrete area generates greater lateral expansion pressure on the steel tube.
4. The steel tube, subjected to greater lateral pressure, experiences higher circumferential strain, which increases its confining stress on the concrete.
5. The enhanced confinement improves the compressive strength and ductility of the concrete in the compressed zone.
6. The net result is an increase in flexural bearing capacity and flexural stiffness.

Influence of Round Steel Bar Diameter

The study shows that the enhancement effect increases with the diameter of the round steel bars. Larger diameter bars provide greater cross-sectional area for resisting compressive forces, leading to a more significant shift of the neutral axis and a greater increase in the compressed concrete area. However, the rate of improvement diminishes at larger diameters due to the nonlinear relationship between bar area and neutral axis position.

Influence of Slenderness Ratio

The paper demonstrates that the round steel bar reinforcement is more effective for members with higher slenderness ratios. For slender members, the second-order effects (P-Δ effects) are more pronounced, and the additional compressive reinforcement provided by the round steel bars helps to counteract the increased lateral deflection. The improvement in bending performance due to round steel bar reinforcement increases with slenderness ratio, and this effect is amplified at higher axial load ratios.

Process and Standards Analysis

Welding of Round Steel Bars to the Steel Tube

The welding of round steel bars to the steel tube is a critical construction operation that directly affects the effectiveness of the reinforcement. The following welding considerations are relevant:

The weld quality is paramount because the round steel bars must transfer their compressive forces effectively to the steel tube. Incomplete penetration or weld defects would reduce the load transfer capacity and could lead to premature failure. The welding procedure should be qualified per GB/T 9445 or ISO 15614, and all welds should be inspected per GB/T 3323 or GB/T 11345.

Design Considerations for CFST Members with Round Steel Bar Reinforcement

When designing CFST members with round steel bar reinforcement, the following parameters should be considered:

• Axial load ratio (N/Nu): The improvement effect increases with axial load ratio for slender members.
• Slenderness ratio (λ): Higher slenderness ratios benefit more from round steel bar reinforcement.
• Round steel bar diameter (d): Larger diameters provide greater improvement but with diminishing returns.
• Number of round steel bars: Increasing the number of bars provides additional improvement, but spacing must be maintained to ensure proper concrete placement and consolidation.

Integration with Engineering Practice

The findings of this paper have direct implications for the design of CFST arch bridges, where the arch ribs are typically subjected to combined axial compression and bending. In such applications, the addition of round steel bars welded to the compressed zone of the arch rib could improve the flexural capacity and stiffness without significantly increasing the weight of the member. This approach is particularly attractive for retrofitting existing CFST structures that have been found to have insufficient flexural capacity under eccentric loading conditions.

From a welding engineering perspective, the addition of round steel bars introduces a new type of weld joint that must be carefully designed and executed. The weld geometry should be optimized to minimize stress concentration at the weld root, and the welding sequence should be planned to minimize residual stress and distortion. Post-weld heat treatment may be required for thick-walled steel tubes to relieve residual stresses and prevent delayed cracking.

Key Reflections and Study Insights

The paper provides a clear mechanistic explanation for the enhancement of bending performance in eccentrically loaded CFST members through round steel bar reinforcement. The insight that the neutral axis shift leads to increased circumferential strain and enhanced confinement is particularly valuable because it links the reinforcement effect to the fundamental behavior of the steel tube-concrete composite system. This mechanistic understanding enables engineers to predict the effectiveness of the reinforcement under different loading conditions and to optimize the reinforcement design accordingly.

One area that could benefit from further investigation is the long-term behavior of the weld joints under cyclic loading. The round steel bars, being welded to the steel tube, create a rigid connection that may be susceptible to fatigue cracking under repeated loading. Fatigue assessment of these weld joints, considering the stress concentration at the weld toe, should be conducted for applications where cyclic loading is significant.

Reference Value and Outlook

This literature provides a solid foundation for the design of CFST members with round steel bar reinforcement. The validated finite element model and the parametric analysis results can be directly used in engineering design. Future research should focus on the fatigue performance of the weld joints, the behavior of the reinforced members under combined loading (axial force, bending, and torsion), and the development of simplified design formulas that can be incorporated into design codes. The approach of enhancing CFST member performance through external reinforcement has broad applicability in structural engineering, particularly for the retrofitting and strengthening of existing steel tube concrete structures.