Seismic Vulnerability Analysis of Semi-Through Steel Tubular Concrete Arch Bridges

Literature Overview and Scope

The paper by Zhuo Weidong, Yan Quanzhe, Wu Meirong, and Gu Yin, published in the Journal of the Railway Society of China in 2019 (Vol. 41, No. 5, pp. 126-132), presents a systematic approach to establishing seismic fragility curves for semi-through steel tubular concrete (CFST) arch bridges using a theoretical analytical method. The study is supported by the National Natural Science Foundation of China (Grant No. 51578157) and draws upon statistical data from more than one hundred completed CFST arch bridges in China. The research focuses on three-span thrust-free semi-through CFST tied-arch bridges as the representative sample, combining linear elastic static analysis, time-history analysis, and incremental dynamic analysis (IDA) to define performance levels and construct fragility curves. This work is particularly significant for steel pipe and welding engineers because the structural integrity of the arch ribs, suspenders, and tie rods depends heavily on the quality of steel tube manufacturing, welding procedures, and connection details, all of which influence the seismic performance of the entire bridge system.

Core Technical Content and Methodology

The methodology employed in this study follows a structured progression from statistical survey to probabilistic assessment. The authors first conducted a comprehensive survey of over one hundred completed semi-through CFST arch bridges in China, summarizing the typical design parameter ranges and structural characteristics. Based on this statistical foundation, a representative three-span thrust-free CFST tied-arch bridge was selected as the typical sample. Linear elastic static analysis and time-history analysis were then performed to identify the vulnerable components under seismic loading combinations. Performance levels were defined for each component based on the identified damage mechanisms, and the incremental dynamic analysis method was subsequently used to construct theoretical seismic fragility curves for the typical bridge sample.

The key finding is that for the three-span thrust-free system, the pier caps and their pot-type bearings are the primary vulnerable components, while the arch ribs, suspenders, tie rods, and foundations generally do not fail under seismic action. The study also reveals that compared to Site Class I conditions, the damage probability is relatively higher under Site Class II conditions, and the influence of site conditions becomes more pronounced as the seismic intensity increases. This observation has direct implications for the design of seismic isolation and energy dissipation systems at the pier cap level.

Vulnerable Component Identification

The identification of vulnerable components is the cornerstone of any fragility analysis. In this study, the pier caps and pot-type bearings emerge as the critical weak links in the three-span thrust-free CFST tied-arch bridge system. From a steel pipe and welding engineering perspective, this finding is both reassuring and instructive. It suggests that the arch ribs, which are typically fabricated from large-diameter seamless steel tubes with welded connection details, possess sufficient seismic reserve capacity when properly designed. However, the vulnerability of the pier caps and bearings indicates that attention must be directed toward the connection interfaces between the arch ribs and the substructure, where welding quality, joint geometry, and material compatibility play decisive roles.

Interpretation from a Steel Pipe and Welding Perspective

For steel pipe and welding engineers, the implications of this study extend beyond the structural analysis itself. The arch ribs of CFST arch bridges are typically fabricated from large-diameter seamless steel tubes, often in the range of 600 mm to 1200 mm in diameter, with wall thicknesses of 16 mm to 40 mm, conforming to standards such as GB/T 8162 or ASTM A530. The steel grade is commonly Q345 or Q390, with yield strengths of 345 MPa and 390 MPa respectively. The welding of these arch ribs involves longitudinal and circumferential welds that must withstand not only the dead load and traffic load but also the dynamic seismic forces.

The finding that the arch ribs generally do not fail under seismic action is encouraging from a welding quality assurance standpoint. It suggests that the current design and fabrication practices for CFST arch bridge arch ribs provide adequate seismic margin. However, this does not diminish the importance of maintaining rigorous welding quality control. The connection nodes between arch ribs, where multiple steel tubes intersect and are welded together, represent potential weak points. These nodes typically involve multi-layer, multi-pass welding with complex joint geometries, and the heat-affected zone (HAZ) properties are critical to the overall seismic performance.

Welding Considerations for CFST Arch Bridge Connections

The connection details in CFST arch bridges present several welding challenges that must be addressed to ensure seismic resilience. The primary challenges include:

1. Multi-directional stress states: The connection nodes experience complex multi-axial stress states under seismic loading, which can lead to premature failure if the weld HAZ properties are not properly controlled.
2. Thermal distortion: The large heat input required for welding thick-walled steel tubes can cause significant thermal distortion, affecting the geometric accuracy of the arch rib alignment.
3. Material compatibility: The welding consumables must be compatible with the parent steel grade to ensure adequate toughness and ductility in the weld metal and HAZ.
4. Residual stress management: Welding residual stresses in the connection nodes can interact adversely with seismic stresses, potentially reducing the effective seismic capacity.

The use of low-hydrogen welding consumables, such as E7018 or equivalent, with proper preheating and interpass temperature control, is essential for ensuring weld quality in these critical connections. Post-weld heat treatment (PWHT) may be required for thick sections to relieve residual stresses and improve the toughness of the HAZ.

Site Condition Effects and Engineering Implications

One of the most practically significant findings of this study is the pronounced effect of site conditions on the seismic damage probability. The comparison between Site Class I and Site Class II conditions reveals that Site Class II leads to higher damage probabilities, with the effect becoming more pronounced at higher seismic intensities. This finding has direct implications for the selection of steel pipe grades, welding procedures, and quality control requirements based on the site classification.

For bridges located on Site Class II or softer soil conditions, the following engineering measures are recommended:

• Enhanced welding quality control: Implement 100% ultrasonic testing (UT) and phased array ultrasonic testing (PAUT) for all critical welds in the arch ribs and connection nodes, rather than relying on random sampling.
• Higher-grade steel materials: Consider using Q390 or higher-grade steel for the arch ribs to provide additional margin against the amplified seismic demands.
• Improved connection details: Design connection nodes with generous fillet weld sizes and full-penetration groove welds to ensure adequate ductility under seismic loading.
• Seismic isolation and energy dissipation: Incorporate seismic isolation bearings or energy dissipation devices at the pier caps, which are identified as the primary vulnerable components.

The interaction between site conditions and structural response is a reminder that steel pipe and welding engineers cannot operate in isolation from geotechnical considerations. The seismic demands imposed on the structure are fundamentally influenced by the soil-structure interaction, and the welding and fabrication practices must be calibrated accordingly.

Key Questions and Reflections

Several questions arise from this study that warrant further investigation. First, the study focuses on the three-span thrust-free system, but what about other structural configurations such as two-span or multi-span systems? The vulnerability pattern may differ significantly. Second, the study defines performance levels based on linear elastic analysis, but the actual seismic response of CFST arch bridges is inherently nonlinear, particularly in the connection nodes where plastic deformation may occur. Third, the study does not address the effect of welding quality degradation on the seismic performance, which is a practical concern for existing bridges.

From a personal perspective, having worked extensively with steel pipe fabrication and welding quality control, I find it encouraging that the arch ribs are not identified as the primary vulnerable components. This suggests that the current design philosophy, which emphasizes robust steel tube fabrication and high-quality welding, provides adequate seismic reserve. However, the vulnerability of the pier caps and bearings underscores the need for a holistic approach to seismic design, where the weakest link in the structural chain determines the overall performance.

Study Insights and Implications for Practice

The most valuable insight from this study is the demonstration that a theoretical analytical method, combining statistical survey, linear elastic analysis, and incremental dynamic analysis, can effectively construct seismic fragility curves for CFST arch bridges. This approach provides a practical alternative to the more resource-intensive shake table testing and full-scale seismic testing, making it accessible for routine engineering practice. For steel pipe and welding engineers, this means that the seismic performance of CFST arch bridges can be assessed without the need for extensive experimental programs, provided that accurate structural models and appropriate material properties are used.

The study also highlights the importance of considering site conditions in the seismic design of CFST arch bridges. For projects located on Site Class II or softer soil conditions, the welding and fabrication requirements should be enhanced to compensate for the amplified seismic demands. This includes the use of higher-grade steel materials, more rigorous welding quality control, and improved connection details. The study serves as a valuable reference for steel pipe and welding engineers involved in the fabrication and construction of CFST arch bridges, providing a clear understanding of the seismic vulnerability patterns and the corresponding engineering measures.

In summary, this study makes a significant contribution to the seismic assessment of semi-through CFST arch bridges by establishing fragility curves through a theoretical analytical method. The identification of pier caps and bearings as the primary vulnerable components, combined with the quantification of site condition effects, provides actionable guidance for steel pipe and welding engineers in ensuring the seismic resilience of these important transportation infrastructure structures.