Zhuojin Pipe Fitting Co., LtdSectional Stress Analysis of Dumbbell-Shaped Steel Tube Concrete Arch Ribs During Concrete Pouring
Literature Overview
This paper, published in China Journal of Highway and Transport in 2005 by Chen Baochun and colleagues from Fuzhou University and Fuzhou Planning and Design Institute, addresses a critical construction safety issue in steel tube concrete (STC) dumbbell-shaped arch ribs: the risk of tube bursting during concrete pouring. The research is supported by the Fujian Provincial Higher Education Science and Technology Project (JA03016). The study employs finite element analysis to investigate the sectional stresses in the steel tube during concrete pouring under various pouring sequences and conditions, providing practical recommendations to prevent bursting accidents.
Core Technical Content
Problem Identification
Dumbbell-shaped STC arch ribs consist of two outer steel tubes connected by a web plate, forming a dumbbell-like cross-section. During construction, concrete is poured into the hollow sections to create the composite STC member. However, the internal pressure generated by the wet concrete can exceed the hoop strength of the steel tubes, leading to bursting or bursting-type failures. This is a significant construction safety hazard that has resulted in accidents and project delays in bridge construction.
Finite Element Modeling
The authors established a finite element model to analyze the sectional stresses in the steel tube during concrete pouring. The model accounts for the geometry of the dumbbell-shaped cross-section, the material properties of the steel tube and concrete, and the loading conditions associated with concrete pouring. The analysis considers various pouring sequences and conditions to identify the most critical scenarios.
Key Findings on Stress Distribution
The analysis revealed that during concrete pouring into the web section (the central cavity between the two outer tubes), very high stresses develop at the junction between the steel tubes and the web plate. This stress concentration is identified as the primary cause of bursting accidents. The stress concentration occurs because the web plate acts as a constraint on the deformation of the steel tubes, creating a complex stress state at the junction.
| Pouring Sequence | Stress Level | Risk Assessment |
|---|---|---|
| Web first, then tubes | Highest | Most unfavorable |
| Tubes first, then web | Lower | More favorable |
The pouring sequence significantly influences the stress state in the steel tube. Pouring the web section first creates the highest stress concentration at the tube-web junction, while pouring the outer tubes first reduces the stress concentration and provides a more favorable stress distribution. This finding has direct implications for construction planning and sequencing.
Preventive Measures
The study recommends several measures to prevent bursting accidents:
- Tie rods: Installing tie rods across the web section to provide additional restraint against bursting
- Steel section reinforcement: Reinforcing the web plate with steel sections to increase its stiffness and reduce stress concentration
- Multi-stage pouring: Dividing the pouring process into multiple stages to reduce the internal pressure at any given time
Engineering Practice Implications
Construction Planning
The findings have significant implications for construction planning and sequencing:
- Pouring sequence optimization: The pouring sequence should be planned to minimize stress concentrations. Pouring the outer tubes first, followed by the web section, is the recommended sequence.
- Pouring rate control: The rate of concrete pouring should be controlled to avoid excessive internal pressure. Slow pouring allows the concrete to set and gain strength gradually, reducing the risk of bursting.
- Temperature control: The temperature of the concrete should be controlled to avoid thermal stresses that could contribute to bursting. Hot concrete should be cooled before pouring, and cold weather precautions should be taken in winter.
Steel Tube Design and Fabrication
From a steel pipe manufacturing and fabrication perspective, the study highlights several important considerations:
- Tube thickness: The steel tube thickness should be adequate to resist the internal pressure from concrete pouring. The required thickness depends on the tube diameter, the concrete pressure, and the allowable stress in the steel.
- Web plate design: The web plate should be designed to provide adequate restraint without creating excessive stress concentrations. The junction between the tubes and the web plate should be carefully detailed to minimize stress concentrations.
- Weld quality: The welds connecting the tubes and the web plate must be high quality, with full penetration and minimal defects. Weld defects at the stress concentration zone could initiate bursting.
Welding Procedures
The welding of dumbbell-shaped STC arch ribs requires careful attention to several factors:
- Weld procedures should be qualified for the specific steel grades and thicknesses used
- Preheating may be necessary for thicker sections to reduce residual stresses
- Interpass temperature should be controlled to avoid excessive thermal cycling
- Post-weld heat treatment may be required for high-strength steels to ensure adequate toughness
- NDT should include visual inspection, magnetic particle testing, and possibly ultrasonic testing for critical welds
Key Questions and Reflections
The study raises several important questions. First, the long-term durability of the preventive measures, such as tie rods and steel section reinforcement, has not been assessed. Second, the effect of concrete mix design on the internal pressure during pouring is not discussed, which could be an important factor in preventing bursting. Third, the applicability of the finite element model to different dumbbell-shaped cross-section geometries should be investigated.
The finding that the web-first pouring sequence is the most unfavorable is intuitive but warrants further investigation. The stress concentration at the tube-web junction is a complex phenomenon that depends on the relative stiffness of the tubes and the web plate, the pouring rate, and the concrete properties. Engineers should use the findings as a guide but also conduct site-specific analysis for each project.
Study Insights and Implications
This research provides valuable insights into the construction safety issues associated with dumbbell-shaped STC arch ribs. The identification of the tube-web junction as the critical stress concentration zone and the recommendation of favorable pouring sequences are practical and actionable findings. The proposed preventive measures, including tie rods, steel section reinforcement, and multi-stage pouring, provide engineers with tools to mitigate the risk of bursting accidents. The work represents a significant contribution to bridge engineering practice and highlights the importance of construction phase analysis in the design and construction of STC structures. Engineers working on STC arch ribs should incorporate these findings into their construction planning and risk management strategies.