Creep Effect on Stress Redistribution in Concrete-Filled Steel Tube Arch Bridge Ribs

Literature Context and Research Motivation

The paper by Xie Xiaoli, Qin Rong, and Xie Kaizhong, published in Guangxi Science in 2001 (Vol. 8, No. 1, pp. 22-25), addresses a critical structural engineering issue: the time-dependent stress redistribution in concrete-filled steel tube (CFST) arch ribs due to concrete creep. Funded by the National Natural Science Foundation of China, this research is significant because arch bridges are long-span structures where time-dependent effects can significantly alter the load distribution between the steel tube and concrete core over the service life of the structure.

Theoretical Approach and Analytical Framework

The authors developed a semi-analytical approach based on concrete creep theory and the finite element method. The methodology incorporates two key aspects of concrete behavior: elastic after-effect (also known as creep compliance) and the creep characteristics of concrete under multi-axial confinement. The latter is particularly important because the steel tube provides lateral confinement to the concrete core, which modifies the creep behavior compared to unconfined concrete.

The analytical framework proceeds as follows: first, the short-term stress distribution under external loading is computed using conventional elastic analysis. Second, the time-dependent creep deformation of the concrete is calculated using the age-adjusted effective modulus method or a similar viscoelastic approach. Third, the compatibility conditions between the steel tube and concrete are enforced to determine the stress redistribution. The resulting semi-analytical expressions provide closed-form solutions for stress redistribution at arbitrary cross-section shapes.

Key Technical Parameters and Analysis Results

The following table summarizes the key parameters and their influence on stress redistribution:

The analysis demonstrates that creep causes a significant transfer of stress from the concrete core to the steel tube over time. Under sustained loading, the concrete gradually deforms more than the steel due to creep, and the compatibility requirement forces additional stress onto the steel tube. This redistribution can be substantial, potentially increasing the steel tube stress by 10-20 percent of the initial elastic value over decades of service.

Engineering Practice and Design Considerations

For the design of CFST arch bridges, this research has several important implications. First, design codes that consider only elastic stress distribution may underestimate the long-term stresses in the steel tube, potentially leading to inadequate steel tube thickness or excessive corrosion protection requirements.

Second, the stress redistribution affects the long-term deflection of the arch rib. Since the steel tube bears more of the load over time, the overall stiffness of the section decreases, leading to increased arch deflection. This must be considered in the design of the bridge deck and the bearing conditions at the arch supports.

Third, for existing CFST arch bridges, the creep-induced stress redistribution affects the residual capacity assessment. When evaluating the load-bearing capacity of an aged CFST arch bridge, the current stress state must account for the time-dependent redistribution that has occurred since construction.

Study Insights and Recommendations

This research makes a valuable contribution to the understanding of long-term structural behavior of CFST arch bridges. The semi-analytical approach developed by the authors provides a practical tool for engineers who need to assess creep effects without resorting to full time-dependent finite element analysis for every design scenario.

One area for further development is the consideration of shrinkage effects in addition to creep. Shrinkage can cause additional stress redistribution, particularly in the early service life when the concrete is still curing. Additionally, the temperature-dependent creep behavior should be considered for bridges in regions with significant seasonal temperature variations.

This paper provides essential guidance for the long-term design and assessment of CFST arch bridges, and its methodology is applicable to other CFST structural applications where time-dependent effects are significant.