Zhuojin Pipe Fitting Co., LtdCreep Effect Analysis on CFST Arch Bridges
Overview of the Study
This 2011 paper by Li Shengyong and colleagues from Fuzhou University examines the time-dependent creep behavior of concrete-filled steel tube arch bridges. The authors derive a creep coefficient formula for axially compressed CFST members based on the age-adjusted effective modulus method and validate it against experimental data. The study then applies the ACI 209R creep model to a 380-meter span CFST railway arch bridge, comparing results with the CEB-FIP 78 model to assess model reliability. Creep is a critical consideration for CFST arch structures because the long-term deformation of the concrete core directly affects the arch rib profile, internal force redistribution, and overall structural safety over the design life.
Creep Coefficient Derivation and Model Comparison
The age-adjusted effective modulus method provides a rigorous approach to modeling creep by accounting for the progressive transfer of load from concrete to steel tube as the concrete creeps under sustained stress. The derived formula for CFST members captures the composite action between the steel tube and concrete core, recognizing that the steel tube constrains the lateral expansion of the concrete and thereby influences the creep strain development. This composite effect means that CFST members exhibit different creep behavior compared to plain concrete members, and using standard concrete creep models without modification can lead to significant prediction errors.
| Creep Model | Source Standard | Prediction Accuracy for CFST | Recommendation |
|---|---|---|---|
| ACI 209R | American Concrete Institute | Most reasonable | Recommended |
| CEB-FIP 78 | European | Overestimates creep | Not preferred |
| CEB-FIP 90 | European | Overestimates creep | Not preferred |
The comparison of three creep models reveals that the ACI 209R model provides the most reasonable predictions for CFST members. The CEB-FIP models, developed primarily for plain concrete, tend to overestimate the creep coefficient because they do not adequately account for the confining effect of the steel tube. This finding has direct implications for the design of CFST arch bridges in regions where CEB or Eurocode standards are adopted, suggesting that a modification factor should be applied when using these models for composite members.
Application to a 380-Meter Span Arch Bridge
The application of the ACI 209R model to a 380-meter span CFST railway arch bridge demonstrates the practical significance of accurate creep prediction. For arch bridges of this scale, even small errors in creep prediction can lead to substantial discrepancies in the calculated arch rib displacement and internal forces over a 50-year design life. The study shows that the long-term creep-induced displacement can reach values that, if unaccounted for, would compromise the clearance requirements for railway operations beneath the arch.
The comparison between ACI 209R and CEB-FIP 78 predictions for the 380-meter bridge reveals differences of up to 20% to 30% in long-term displacement, which translates to significant differences in the required construction camber and the predicted residual deformation at the end of the design life. These differences are not merely academic; they directly influence the construction tolerance requirements and the long-term maintenance strategy for the bridge.
Engineering Practice Implications
For design engineers, the key takeaway is that the choice of creep model has a material impact on the predicted long-term performance of CFST arch bridges. The ACI 209R model should be preferred when designing CFST structures, particularly for long-span arches where creep effects accumulate over decades of service. The construction camber must be calculated using the selected creep model, and periodic monitoring of the arch rib profile during the operational phase should be conducted to validate the predictions and detect any deviations from expected behavior.
The steel tube itself is not immune to time-dependent effects. While steel does not creep in the same manner as concrete, the relaxation of prestress in the concrete core and the potential for corrosion-induced section loss in the steel tube can alter the composite behavior over time. Engineers should incorporate periodic inspection protocols that assess both the concrete core condition and the steel tube integrity, particularly in environments where chloride ingress or carbonation may accelerate the degradation process.
Summary
This study establishes that the ACI 209R creep model provides the most reliable predictions for CFST members and validates this finding through both experimental comparison and application to a major engineering project. The derived creep coefficient formula based on the age-adjusted effective modulus method offers a practical tool for engineers designing CFST arch bridges. The emphasis on model selection and validation underscores the importance of using appropriate time-dependent material models in the design of composite structures, where the interaction between steel and concrete creates behavior that cannot be adequately captured by models developed for plain concrete alone.