Zhuojin Pipe Fitting Co., LtdMechanical Performance and Economic Efficiency of Retractable Steel Tube Concrete Supports
Literature Overview
This paper, published in the Journal of Shandong University (Engineering Science) (Vol. 41, No. 5, 2011, pp. 103–107), was authored by Wang Qi, Li Shucai, Wang Hanpeng, Li Weiteng, Li Zhi, Jiang Bei, and Zhang Weijie from Shandong University and Shandong University of Science and Technology. Supported by the National Natural Science Foundation Youth Fund (Grant 50904043) and Shandong Provincial Natural Science Foundation (Grant ZR2009FM009), the study establishes finite element models of four common underground support systems—U-shaped steel arches, lattice steel frames, empty steel tube arches, and steel tube concrete (RTC) arches—and compares their mechanical performance and economic efficiency.
Core Technical Content
Finite Element Modeling and Comparative Analysis
The researchers developed detailed finite element models for each support type, incorporating material nonlinearities, geometric nonlinearities, and soil-structure interaction. The analysis focused on three primary performance indicators: bearing capacity (load-carrying capability), support effect (deformation control), and economic indicators (material consumption per unit of support effect).
The confinement coefficient (套箍系数, defined as the ratio of the cross-sectional area of steel to the cross-sectional area of concrete) emerged as the dominant parameter governing the mechanical behavior of RTC supports. Higher confinement coefficients result in improved concrete compressive strength through the lateral restraint mechanism, enhanced ductility, and increased energy absorption capacity.
Retractable Joint Design and Testing
A critical innovation in this study is the design and experimental validation of retractable (可缩性) joints for RTC supports. These joints enable controlled yielding and displacement accommodation under increasing ground pressure, which is essential for deep soft rock and fractured rock mass conditions. The retractable joints achieve quantitative resistance increase (定量增阻) while simultaneously providing pressure-relief (让压) functionality through a combination of frictional resistance and controlled deformation of specific structural elements.
Technical Parameters and Performance Comparison
| Support Type | Relative Bearing Capacity | Ductility | Material Cost (Relative) | Support Effect per Unit Cost |
|---|---|---|---|---|
| U-shaped steel arch | 1.0 (baseline) | Moderate | 1.0 | 1.0 |
| Lattice steel frame | 0.85 | Low | 0.9 | 0.95 |
| Empty steel tube arch | 1.2 | Moderate | 1.1 | 1.1 |
| RTC arch (optimized) | 1.5–2.0 | High | 1.3–1.5 | 1.5–1.8 |
The confinement coefficient typically ranges from 0.08 to 0.15 for practical RTC support designs, with optimal values around 0.10–0.12 providing the best balance between strength enhancement and material economy.
Engineering Practice Integration
Welding and Fabrication Considerations
The fabrication of RTC support arches involves welding of steel tube segments with specific attention to the following quality requirements:
- Butt welds between tube segments must achieve full penetration with a minimum weld throat thickness equal to the thinner tube wall.
- The welding sequence should follow a symmetric pattern to minimize angular distortion, which is critical for maintaining the arch geometry and ensuring uniform load distribution.
- For the retractable joint components, friction surfaces must be machined to a surface roughness of Ra 3.2–6.3 μm to ensure consistent sliding resistance characteristics.
- Post-weld heat treatment (stress relief annealing at 550–600°C for 2 hours) is recommended for thick-walled tubes (t > 12 mm) to reduce residual stresses that could affect the joint's sliding performance.
Application Scenarios
The study demonstrates that RTC supports with retractable joints are particularly suitable for:
- Deep underground excavations where ground pressure exceeds 2 MPa
- Soft rock conditions with high plastic deformation potential
- Jointed and fractured rock masses requiring deformation accommodation
- Roadway support in mining operations subject to periodic roof pressure bursts
Key Reflections
The economic analysis reveals that while RTC supports have a higher initial material cost compared to traditional U-shaped steel arches, their superior load-carrying capacity and deformation control result in a lower cost per unit of support effect. This economic advantage becomes more pronounced in deep excavations where traditional supports would require multiple reinforcement layers. The retractable joint concept represents a significant advancement in passive support design, transitioning from rigid resistance to controlled yielding—a philosophy that aligns with modern rock mechanics principles advocating for flexible support systems in deformable ground conditions.