Zhuojin Pipe Fitting Co., LtdCompression-Bending Performance and Load-Bearing Capacity of Conical Hollow Sandwich Steel Tube Concrete Members
Literature Overview
This study explores the compression-bending behavior of conical hollow sandwich steel tube concrete (HSCC) members and develops a load-bearing capacity calculation method. Conical HSCC members are increasingly used in tower structures, transmission line poles, and variable-height columns where the cross-section tapers along the length. The hollow sandwich configuration provides enhanced confinement to the concrete core while reducing the overall weight of the member.
Core Technical Content and Key Findings
The research involved both experimental testing and theoretical analysis. The conical HSCC specimens were subjected to combined axial compression and bending moments, simulating the loading conditions encountered in real structural applications. The load-bearing capacity calculation method was developed based on the interaction between the axial force and bending moment, considering the non-uniform confinement effect due to the conical geometry.
Test Specimen Parameters
| Parameter | Range / Value |
|---|---|
| Large end outer diameter | 200–300 mm |
| Small end outer diameter | 100–200 mm |
| Large end inner diameter | 100–150 mm |
| Small end inner diameter | 50–100 mm |
| Member length | 1500–2500 mm |
| Concrete grade | C40–C60 |
| Steel grade | Q345, Q420 |
| Eccentricity ratio (e/D) | 0.1–0.5 |
Compression-Bending Performance
The compression-bending capacity of the conical HSCC members was found to be significantly influenced by the eccentricity ratio and the taper ratio (ratio of small end diameter to large end diameter). The interaction curves between axial load and bending moment showed a nonlinear relationship, with the capacity decreasing more rapidly at higher eccentricity ratios. The hollow sandwich configuration provided better load-bearing capacity compared to solid conical steel tube concrete members, with improvements of 10–20% in the bending-dominated region.
The failure mode was typically initiated at the small end of the member, where the cross-sectional area is smallest and the confinement effect is weakest. Local buckling of the outer tube at the small end was observed in most specimens, followed by concrete crushing in the compression zone. The inner tube helped to maintain the integrity of the concrete core, but its effectiveness was reduced at the small end due to the thinner wall thickness.
Load-Bearing Capacity Calculation Method
The developed calculation method considers the following factors:
- The non-uniform confinement pressure along the member length due to the conical geometry.
- The interaction between axial force and bending moment, accounting for the P-Δ effect.
- The contribution of the inner tube to the confinement of the concrete core.
- The degradation of the concrete strength under combined loading conditions.
The method was validated against experimental results, showing good agreement with deviations within 10% for most specimens. The calculation method can be expressed as a function of the axial load, bending moment, eccentricity ratio, and geometric parameters of the conical member.
| Validation Case | Experimental Capacity (kN·m) | Calculated Capacity (kN·m) | Deviation |
|---|---|---|---|
| e/D = 0.1 | 45.2 | 42.8 | -5.3% |
| e/D = 0.2 | 38.6 | 36.1 | -6.5% |
| e/D = 0.3 | 30.5 | 28.2 | -7.5% |
| e/D = 0.4 | 22.1 | 20.3 | -8.1% |
| e/D = 0.5 | 15.8 | 14.2 | -10.1% |
Engineering Practice Implications
The developed calculation method provides engineers with a practical tool for designing conical HSCC members under combined compression and bending. The method should be used in conjunction with appropriate safety factors and design codes to ensure structural reliability. Engineers should pay particular attention to the connection details at the small end of the member, where the failure typically initiates. Reinforcement plates or additional bracing may be required at the small end to enhance the local stability.
The study also highlights the importance of the taper ratio in the design of conical HSCC members. A taper ratio that is too aggressive (i.e., a large difference between the large and small end diameters) can lead to premature failure at the small end. Engineers should aim for a taper ratio that balances weight savings with structural integrity, typically in the range of 0.5–0.7 for most applications.
Study Insights and Conclusions
This research contributes to the understanding of the structural behavior of conical hollow sandwich steel tube concrete members and provides a practical calculation method for their design. The findings are particularly relevant for engineers designing tower structures, transmission line poles, and variable-height columns where combined compression and bending are the dominant loading conditions. The hollow sandwich configuration offers a weight-efficient solution with enhanced load-bearing capacity, but careful attention must be paid to the connection details and the taper ratio to ensure structural reliability.