Zhuojin Pipe Fitting Co., LtdCreep Coefficient Model Comparison for Concrete-Filled Steel Tube Arch Bridges
Literature Overview
This paper by Lai Xiuying and Chen Baochun from Fuzhou University (2014) compares two widely used creep coefficient models—ACI 209R-92 and CEB-FIP MC90—for the analysis of concrete-filled steel tube (CFST) arch bridges. The research was funded by the National Natural Science Foundation of China (Grant No. 814466) and was published in the Journal of Fuzhou University (Natural Science Edition), Volume 42, Issue 5, pages 737–743. The study analyzes 11 actual CFST arch bridges, evaluating the creep effects from the stage of bare steel tube closure to 10 years after completion, and compares the results obtained from the two models.
Core Technical Content
Creep Models Compared
The two models are the most commonly referenced international approaches for predicting concrete creep in structural engineering:
| Model Feature | ACI 209R-92 | CEB-FIP MC90 |
|---|---|---|
| Governing body | American Concrete Institute | International FIB / European |
| Creep coefficient definition | φ(t, t₀) = ε_cr(t) / ε_el(t₀) | φ(t, t₀) = ε_cr(t) / ε_el(t₀) |
| Age at loading effect | Included | Included |
| Humidity effect | Partially included | Explicitly included |
| Member size effect | Through characteristic size | Through relative humidity and size |
| Concrete strength effect | Included | Included |
| Cement type effect | Limited | Explicitly included |
| Temperature effect | Not included | Included |
Methodology
The researchers employed finite element analysis software to compute the creep effects on 11 CFST arch bridges under both models. The analysis timeline spans from the stage when the bare steel tube arch is closed (i.e., the steel tube is erected and closed but concrete has not yet been poured) to 10 years after the bridge is completed and loaded. This is a particularly important time window because the initial stages of concrete hardening and the early loading of the arch ring are periods of maximum creep sensitivity.
The key parameters in the analysis include:
- Arch span, rise-span ratio, and cross-sectional dimensions
- Concrete grade (typically C40 to C60 for arch bridges)
- Steel tube grade (typically Q235, Q345, or Q420)
- Construction sequence and loading stages
- Environmental conditions (humidity, temperature)
Key Findings
The study finds that the differences between the results obtained from ACI 209R-92 and CEB-FIP MC90 are generally less than 5% for the 11 bridges analyzed. This is a practically significant finding because it suggests that, for CFST arch bridges, the choice between these two models does not lead to materially different design outcomes.
| Comparison Aspect | Result |
|---|---|
| Maximum difference in creep deflection | Less than 5% |
| Trend of creep development | Consistent between models |
| Time-dependent stress redistribution | Similar patterns |
| Sensitivity to concrete age at loading | Both models capture the effect |
The study recommends that the CEB-FIP MC90 model be adopted in the national specification for CFST arch bridges (the Chinese technical code for CFST arch bridges). The rationale is that CEB-FIP MC90 is more familiar to Chinese engineers and provides a more comprehensive treatment of environmental factors such as humidity and temperature, which are particularly relevant for bridges exposed to outdoor conditions.
Integration with Engineering Practice
From the perspective of steel pipe manufacturing and bridge engineering, the creep behavior of the concrete infill in CFST arch bridges has several practical implications:
- Steel tube design: The steel tube in a CFST arch bridge is not merely a formwork for concrete placement; it is a permanent structural component that shares the load with the concrete over the service life. Creep in the concrete leads to a time-dependent redistribution of stresses between the steel tube and the concrete. The steel tube may experience increasing compressive stresses over time as the concrete creeps under sustained load.
- Steel tube grade selection: Given that creep causes stress redistribution, the selection of steel tube grade must account for long-term stress levels. Higher-grade steel tubes (e.g., Q420 or Q460) provide greater margin against yielding under creep-induced stress increases, but also increase material cost.
- Welded tube considerations: For CFST arch bridges, the steel tubes are typically long-segment welded tubes (LSAW or UOE process) or spiral-welded tubes. The welding residual stresses in the tube, if not properly relieved, can interact with the creep-induced stress redistribution. Post-weld heat treatment or mechanical stress relief may be necessary for critical applications.
- Construction sequence: The timing of concrete placement relative to steel tube closure is critical. The ACI and CEB-FIP models both recognize that the age of concrete at the time of loading significantly affects creep magnitude. Placing concrete at a younger age and loading it earlier results in higher creep, which can be detrimental to the long-term performance of the arch.
Key Questions and Reflections
A notable limitation of this study is that it does not consider the interaction between steel tube corrosion and concrete creep over the long-term service life. In practice, CFST arch bridges in coastal or industrial environments may experience steel tube corrosion, which reduces the effective cross-sectional area of the steel tube and alters the load-sharing mechanism between steel and concrete. This corrosion-creep interaction could amplify the time-dependent deformations beyond what either model predicts independently.
Another reflection is that the study's conclusion that the difference between the two models is less than 5% is reassuring for practical design, but it does not address the accuracy of either model in absolute terms. Both models are empirical and may deviate from the actual creep behavior of concrete in CFST arch bridges, particularly under the complex stress states and confinement conditions present in these structures. Validation against long-term monitoring data from instrumented bridges would strengthen the conclusions.
Study Insights and Implications
The most important practical takeaway from this study is that for CFST arch bridge design, the choice between ACI 209R-92 and CEB-FIP MC90 does not materially affect the predicted creep effects, with differences remaining below 5%. This gives engineers confidence that either model can be used without significant risk of under- or over-design. The recommendation to adopt CEB-FIP MC90 in the national specification is well-founded, given its broader acceptance and more comprehensive treatment of environmental factors.
For steel pipe manufacturers and bridge engineers, the practical implication is that the long-term performance of CFST arch bridges is governed by the time-dependent behavior of the concrete infill, which in turn affects the stress state in the steel tube. Quality control of the concrete mix, placement, and curing is therefore as important as the quality of the steel tube itself. Engineers should ensure that concrete specifications include requirements for low-creep formulations, such as the use of supplementary cementitious materials (fly ash, slag) and optimized aggregate grading.
In summary, this study provides a valuable comparative assessment of two major creep models for CFST arch bridges, demonstrating their practical equivalence while recommending CEB-FIP MC90 for standardization, and highlighting the importance of long-term creep effects on the structural behavior of these composite members.