Creep Effect Analysis of Extra-Large Span Steel Tube Concrete Arch Bridges

Literature Overview

Zeng Yong, Li Yongqi, Sun Xudong, Li Xueqin, Xue Xiaofang, Gu Anbang, and Tan Huimei from the State Key Laboratory of Mountain Bridge and Tunnel Engineering at Chongqing Jiaotong University conducted this comprehensive study on the creep effects in extra-large span steel tube concrete (STC) arch bridges. The research was funded by multiple sources including the National Natural Science Foundation of China (project 51908093), the China Scholarship Council (project 201308505149), and the Chongqing Overseas Talent Return Startup Innovation Support Fund. The study was published in the journal "China Railway Science" in 2021, Volume 42, Issue 5, pages 76-84.

Experimental and Analytical Approach

The research employs a multi-faceted approach combining physical model testing, finite element analysis, and field measurements. A 1:2.5 scale model of the arch rib and arch foot upper chord tube STC member was constructed for creep testing. The model was subjected to a sustained load of 180 tonnes for one year, providing valuable long-term creep data. The study introduces a new STC creep coefficient that accounts for steel tube confinement effects, which represents a significant advancement over conventional creep models that do not adequately consider the interaction between steel and concrete.

The key experimental findings after one year of sustained loading are remarkable:

• The load borne by the steel tube increased by 1.2 times
• The load borne by the concrete core decreased by 53%
• Significant internal force redistribution occurred between the steel tube and concrete core

These results demonstrate that creep in STC members is not merely a time-dependent deformation phenomenon but involves substantial stress redistribution between the steel and concrete components. The conventional assumption of proportional strain compatibility between steel and concrete is inadequate for long-term analysis of STC structures.

Creep Coefficient and Stress Redistribution

The study introduces a modified STC creep coefficient that considers the following parameters:

The stress redistribution phenomenon observed in the model test has profound implications for the design of extra-large span STC arch bridges. The progressive transfer of load from concrete to steel over time means that the steel tube stress increases continuously, potentially approaching or exceeding design limits if not properly accounted for. This stress increase occurs even under constant external loading conditions, making it a critical design consideration.

Welding and Fabrication Implications

For extra-large span STC arch bridges, the steel tubes are typically fabricated from large steel plates through heavy-section welding. The welding quality and residual stress distribution in these steel tubes directly affect the long-term creep behavior. Key welding considerations include:

• Welding process selection: SAW is typically used for longitudinal welds of large-diameter tubes, while FCAW or SMAW may be used for transverse welds
• Weld residual stress: High residual stresses in weld zones can accelerate creep damage through stress superposition
• Heat-affected zone properties: The HAZ of the steel tube may have different creep resistance than the base metal
• Weld quality verification: NDT methods including UT, RT, and MT should be used to ensure weld integrity

The welding residual stress in the steel tube interacts with the creep-induced stress redistribution. In the long term, the superposition of welding residual stress and creep-induced stress may lead to localized stress concentrations that could initiate fatigue cracks or creep damage. Engineers should consider stress relief treatments for critical welds in STC arch bridge steel tubes.

Field Measurement and Finite Element Validation

The study compares finite element analysis results with field measurements from an actual extra-large span STC arch bridge. The comparison reveals important discrepancies:

• Steel tube stress: The finite element calculation results and field measurements show generally consistent trends, with numerical values close to each other, and both increase with construction progress.
• Concrete core stress: The finite element results show some differences from field measurements; the field-measured concrete stresses are smaller, and even tensile stresses are observed in some locations.

The discrepancy in concrete core stress is particularly significant. The field observation of tensile stresses in the concrete core suggests that the actual stress state is more complex than predicted by conventional finite element models. This may be due to:

• Differential creep between the steel tube and concrete core
• Shrinkage effects not fully captured in the model
• Construction sequence effects and temporary loading conditions
• Interface debonding between steel tube and concrete core

Engineering Design Recommendations

Based on the comprehensive study, the following recommendations are proposed for the design of extra-large span STC arch bridges:

1. Use the modified STC creep coefficient that accounts for steel tube confinement effects in long-term stress analysis
2. Perform time-dependent stress analysis considering the progressive stress redistribution between steel and concrete
3. Monitor steel tube stress during construction and service life to validate analytical predictions
4. Design steel tube capacity with adequate margin to accommodate long-term stress increases due to creep
5. Consider the possibility of tensile stresses in the concrete core and provide appropriate reinforcement

The 180-tonne sustained load test on the 1:2.5 scale model provides valuable data for calibrating creep models. However, the extrapolation from model scale to full scale requires careful consideration of size effects on creep behavior. Engineers should use the modified creep coefficient in conjunction with appropriate safety factors to account for uncertainties in long-term behavior prediction.

Study Insights and Concluding Remarks

This comprehensive study represents a major advancement in the understanding of creep effects in extra-large span STC arch bridges. The combination of long-term model testing, finite element analysis, and field measurements provides a robust basis for design recommendations. The finding that steel tube load increases by 1.2 times while concrete load decreases by 53% over one year is particularly significant for design engineers who must ensure long-term structural safety.

For steel pipe manufacturers and welding engineers, this study highlights the importance of producing steel tubes with consistent material properties and high-quality welds for STC arch bridge applications. The long-term creep behavior of the steel tube is influenced by its material composition, microstructure, and fabrication quality. Welding residual stresses can significantly affect the long-term stress state, and proper stress relief treatments should be considered for critical components.

The modified STC creep coefficient developed in this study should be validated through additional long-term testing and field monitoring before being incorporated into design codes. The discrepancy between finite element predictions and field measurements for concrete core stress indicates that further research is needed to improve the accuracy of numerical models. Engineers should adopt a cautious approach in designing extra-large span STC arch bridges, using the modified creep coefficient with appropriate safety margins and implementing comprehensive monitoring programs to ensure long-term structural integrity.