Zhuojin Pipe Fitting Co., LtdBending Performance of Concrete-Filled Steel Tube Members with Internal Lattice Steel Cores
Literature Overview
The paper by Wang Xiantie and colleagues from Xi'an University of Architecture and Technology, published in the Journal of Civil and Environmental Engineering in 2012, presents a systematic investigation into the bending behavior of concrete-filled steel tubes (CFST) incorporating internal lattice-shaped steel cores. The research was supported by the National Natural Science Foundation of China (Grant No. 51678474) and represents a meaningful contribution to the field of composite structural members, particularly in the context of seismic design and high-strength construction where conventional CFST members may not provide sufficient bending capacity or ductility.
The study examines four groups of specimens with different internal configurations, including the presence or absence of stiffeners and lattice steel cores, subjected to four-point bending tests. Finite element analysis using ABAQUS was conducted to validate experimental findings, and the results were further benchmarked against Chinese, American, and European codes for both bending capacity and stiffness calculations.
Core Technical Findings
The experimental results demonstrate that the lattice steel core effectively delays concrete cracking in the tension zone and postpones the yielding of the steel tube in the compression zone, thereby significantly enhancing the overall bending performance of the composite member. This is fundamentally consistent with the composite action mechanism in CFST members, where the steel tube provides lateral confinement to the concrete core, while the concrete resists the outward buckling of the steel tube wall.
The parametric study revealed several important relationships between geometric and material parameters and bending performance:
| Parameter | Influence on Bending Capacity | Influence on Stiffness |
|---|---|---|
| Steel tube diameter-to-thickness ratio (D/t) | Significant — lower D/t improves capacity | Significant — lower D/t improves stiffness |
| Lattice steel core size | Significant — larger cross-section increases capacity | Moderate effect |
| Lattice steel core strength | Significant — higher yield strength increases capacity | Minor effect |
| Concrete strength | Minor effect on capacity | Minor effect on stiffness |
| Lattice steel leg spacing | Minor effect on capacity | Minor effect on stiffness |
The Chinese code (GB 50017) provides conservative estimates of bending capacity for such members, while the American code (AISC 360) and European code (EN 1993-1-1) yield conservative stiffness predictions. The Chinese code, however, exhibits relatively large errors in stiffness calculation, suggesting that current code provisions for this specific composite configuration require further refinement.
Engineering Practice Implications
From a practical standpoint, the lattice steel core concept offers an attractive solution for applications requiring high bending resistance within limited section dimensions. In pipeline engineering, where large-diameter steel pipes are frequently used as structural columns or piers in offshore platforms and bridge piers, the incorporation of internal steel cores can substantially improve bending capacity without increasing the outer diameter. This is particularly relevant for pipe rack supports in petrochemical plants, where the pipe itself serves as a structural member.
The parametric findings have direct implications for design optimization. Engineers should prioritize selecting a steel tube with a lower diameter-to-thickness ratio (typically D/t ≤ 30 for plastic sections per GB 50017) and a larger lattice steel core cross-section to maximize bending capacity. The spacing of lattice legs and the concrete strength, while important for other design considerations such as shear resistance and fire performance, have limited impact on bending capacity and can be selected based on constructability and cost optimization.
A noteworthy observation is the conservative nature of the Chinese code for bending capacity calculations. This conservatism, while ensuring structural safety, may lead to uneconomical designs. Engineers considering lattice-core CFST members should be aware that experimental-based capacity estimates may exceed code predictions by a significant margin, and this discrepancy should be addressed through proper safety factor calibration in future code revisions.
Key Reflections
The study raises an important question regarding the interaction mechanism between the lattice steel core and the surrounding concrete-steel tube composite. The lattice configuration, unlike a solid steel core, allows for better bond development and composite action due to the interlocking of concrete into the lattice voids. This is analogous to the confinement effect observed in reinforced concrete columns with transverse reinforcement, where the spacing and configuration of stirrups critically influence the confinement efficiency.
For practitioners in the steel pipe and welding industry, the implications extend to fabrication considerations. The lattice steel core requires precise welding of multiple intersecting members, introducing potential weld defects such as lack of fusion, porosity, and residual stresses. Quality control during fabrication should include ultrasonic testing (UT) of weld joints in the lattice core, dimensional inspection of leg spacing, and verification of steel tube straightness to ensure proper fit-up.