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

Literature Overview

This study by Zeng Yong et al. (2021), published in China Railway Science, investigates the long-term creep behavior of steel tube concrete (STC) arch ribs in extra-large span arch bridges. The research combines a 1:2.5 scale model test on the arch foot chord member, finite element analysis incorporating a novel STC creep coefficient that accounts for steel tube confinement, and full-scale field measurements from an actual bridge. The investigation spans one year of sustained loading at 180 t, providing valuable long-term creep data for a critical structural component.

Key Experimental Results

The model test results are striking and demonstrate the profound influence of internal force redistribution over time:

The study also identifies two critical parameters governing the STC creep coefficient: the section steel ratio (the ratio of steel tube cross-sectional area to total section area) and the ratio of elastic moduli between steel and concrete (Es/Ec). Both parameters have a substantial influence on the magnitude of creep-induced stress redistribution.

Creep Coefficient and Confinement Effect

A central contribution of this paper is the introduction of a new STC creep coefficient that explicitly accounts for the confinement provided by the steel tube. In conventional concrete creep models, the confinement effect of steel reinforcement is either ignored or approximated through empirical adjustments. The authors' approach provides a more physically grounded framework by recognizing that the steel tube restricts the lateral expansion of concrete under sustained axial load, thereby reducing the creep strain compared to unconfined concrete.

For steel pipe manufacturing engineers, this research highlights a critical material interaction that must be understood at the component level. The steel tube in an STC arch rib is not merely a formwork or a passive reinforcement; it actively participates in load sharing throughout the service life. The creep of the infill concrete progressively transfers load to the steel tube, which means that the long-term stress in the steel tube may be significantly higher than the initial construction-stage stress. This has direct implications for:

• The selection of steel grade for the arch rib tube (higher yield strength may be required to accommodate long-term stress increases)
• Weld quality requirements at splice joints (welded connections must resist the redistributed forces)
• Residual stress management during manufacturing (residual stresses from forming and welding superimpose on the operational stresses)

Finite Element vs. Field Measurement Comparison

The finite element analysis results for steel tube stresses show good agreement with field measurements in terms of trend and approximate magnitude, with both showing an increase as construction progresses. However, the concrete stress predictions diverge more significantly from field data. The field-measured concrete stresses are consistently lower than the FEA predictions, and in some cases, tensile stresses were observed in the field where the model predicted compression. This discrepancy suggests that the concrete-steel bond behavior and the actual confinement effectiveness may be more complex than captured by current constitutive models.

Engineering Practice Considerations

For the design and construction of extra-large span STC arch bridges, the creep-induced stress redistribution must be explicitly accounted for in the structural analysis. The 1.2 times increase in steel tube load after one year represents a substantial additional demand that, if ignored, could compromise the long-term structural integrity. Engineers should consider the following in practice:

1. Use of high-strength steel tubes (e.g., Q345qE or Q390qE per Chinese bridge specifications) with adequate margin for creep-induced stress increases
2. Implementation of rigorous weld inspection protocols at arch rib splices, as these are critical stress concentration zones where redistributed forces accumulate
3. Long-term monitoring programs that include both steel tube and concrete stress measurement to validate design assumptions
4. Consideration of construction sequencing effects, as the time-dependent load path significantly affects the final stress state

Study Insights and Reflections

This research provides a compelling demonstration of why long-term behavior studies are essential for STC structures. The magnitude of creep-induced load transfer is far greater than what might be assumed from short-term tests. For steel pipe suppliers and manufacturers involved in bridge construction, understanding these long-term effects is crucial for specifying appropriate material grades, wall thickness tolerances, and quality assurance requirements. The discrepancy between FEA and field data for concrete stresses also reminds us that numerical models must be continuously validated against real-world observations, and that the complex interaction between steel tubes and infill concrete remains an area requiring further research.