Steel Tube Cracking Accident Analysis of Concrete-Filled Steel Tube Arch Bridge

Literature Overview

This paper by Qin Rong, Xie Xiaoli, Peng Wenli, Li Xiumei from Guangxi University, and Zhang Jianmin from South China University of Technology, published in the China Civil Engineering Journal (Volume 34, Issue 3, 2001), presents a structural analysis of a steel tube cracking accident that occurred during the construction of a concrete-filled steel tube (CFST) arch bridge. The research was supported by the National Natural Science Foundation of China (Grant No. 19872020) and the Guangxi Natural Science Foundation (Guikezi 99120002). At the time of publication, China lacked dedicated design and construction codes for CFST arch bridges, making such accident analyses particularly valuable for the industry.

Accident Description and Structural Analysis

The accident involved the cracking of the steel tube in the arch ribs during construction, which posed a significant safety risk to the project. The authors employed finite element analysis (FEA) and spline function methods to investigate the structural behavior of the arch bridge under construction loads. The analysis focused on understanding the stress distribution within the steel tube, particularly during the stages of steel tube erection, temporary support application, and concrete filling.

Critical Construction Stages and Stress States

The finite element model revealed that during the concrete filling stage, the differential settlement between the steel tube and the surrounding concrete could induce significant longitudinal stresses in the tube wall. The spline function method was used to model the continuous beam behavior of the arch rib, accounting for the varying stiffness along the arch due to the progressive concrete filling.

Root Cause Analysis

The investigation identified several contributing factors to the cracking incident. First, the lack of standardized design procedures for CFST arch bridges led to inadequate consideration of construction stage stresses. Second, the interaction between the steel tube and the concrete during the filling process created complex stress states that were not adequately predicted by conventional elastic analysis. Third, the material properties of the steel tube, particularly the yield strength and strain-hardening characteristics, played a critical role in determining the cracking threshold.

The authors emphasized that the absence of specific codes for CFST arch bridges at that time meant that designers had to rely on general steel structure and concrete structure codes, which often did not adequately address the unique behavior of composite arch members. The steel tube in a CFST arch bridge serves not only as a structural element but also as a formwork during concrete placement, which introduces additional constraints on deformation and stress distribution.

Engineering Practice Recommendations

Based on the accident analysis, the authors proposed several recommendations for preventing similar failures in future CFST arch bridge projects:

1. Comprehensive construction stage analysis: The structural analysis must cover all construction stages, including steel tube erection, temporary support, concrete filling, and support removal, rather than focusing solely on the final service condition.
2. Material quality control: The steel tube material must be thoroughly inspected for chemical composition, mechanical properties, and surface quality. Particular attention should be paid to the welds in the steel tube fabrication, as weld defects can serve as crack initiation sites under high stress.
3. Progressive concrete filling: The concrete filling process should be designed to minimize differential settlement between adjacent sections of the arch rib. This may require staged filling with controlled intervals and monitoring of tube deformation.
4. Instrumentation and monitoring: Real-time monitoring of steel tube strain, displacement, and crack formation during construction is essential to detect abnormal stress states before catastrophic failure occurs.

Welding Quality Considerations

From a welding and fabrication perspective, the steel tube used in CFST arch bridges typically consists of welded steel pipe sections joined by longitudinal and circumferential welds. The longitudinal welds, whether produced by submerged arc welding (SAW) or electric resistance welding (ERW), must be thoroughly inspected for defects such as lack of fusion, porosity, and slag inclusion. Any such defects can significantly reduce the crack resistance of the tube wall under the complex stress states encountered during construction.

The circumferential welds joining the steel tube segments must be designed to achieve full penetration, as partial penetration welds can create stress concentrations that promote crack initiation. Post-weld heat treatment may be necessary for thick-walled tubes to relieve residual stresses that could interact with construction loads to initiate cracking. The heat-affected zone of these welds should be characterized by hardness testing and microstructural examination to ensure adequate toughness, particularly in the case of high-strength steels where the HAZ may be susceptible to embrittlement.

Study Insights

This accident analysis serves as a powerful reminder that the absence of codes does not mean the absence of risk. Engineers working on innovative structural systems must take extra care to conduct thorough analyses and implement robust quality control measures. The CFST arch bridge, while offering excellent strength-to-weight ratio and aesthetic appeal, presents unique challenges in construction sequencing, material interaction, and load transfer that must be carefully managed throughout the project lifecycle.