Seismic Vulnerability Analysis of CFST Truss Continuous Curved Bridge Based on Incremental Dynamic Analysis

Literature Overview

The paper by Wang Hailiang, Zhang Duo, and Liu Zhongxian, published in the Journal of Disaster Prevention and Mitigation Engineering in 2017, presents an incremental dynamic analysis (IDA) of a continuous curved bridge with CFST space truss piers. The research was conducted at Tianjin Chengjian University and supported by the National Natural Science Foundation of China (Project 51278327). The study focuses on a bridge with a maximum pier height of 50 meters, employing a three-dimensional finite element model built in OpenSees and utilizing 10 ground motion records from the PEER earthquake database.

Core Technical Methodology

The IDA methodology involves subjecting the structural model to a suite of ground motions with incrementally increasing intensity, recording the structural response at each intensity level, and deriving fragility curves that express the probability of exceeding damage states as a function of ground motion intensity.

The study establishes fragility curves for pier columns and bearings, and then combines them using joint failure probability analysis to form bridge system fragility curves. Multiple comparison models are developed to analyze the influence of pier-to-span ratio and curve radius on bridge vulnerability.

Key Findings and Engineering Implications

The study reveals several important findings with direct relevance to bridge engineering practice:

• When CFST truss piers have similar heights, the pier adjacent to a reinforced concrete pier exhibits larger vibration response, suggesting dynamic interaction effects between different pier types.
• The extensive use of rubber bearings between piers and girders creates elastic connections and hinge-like behavior, effectively reducing the probability of complete bridge failure.
• Increasing pier-to-span ratio leads to increased probability of complete bridge system damage.
• Increasing curve radius generally increases damage probability, although the arch structure inherent in curved bridges provides some resistance to longitudinal seismic forces.

From a welding and fabrication standpoint, CFST space truss piers present unique challenges:

• The truss members are typically fabricated by welding steel tubes together at various angles, requiring precise alignment and high-quality welds. The quality of these welds directly affects the seismic performance of the truss.
• CFST truss members involve welding steel tubes to concrete-filled sections, which requires careful control of the welding sequence to prevent distortion of the concrete-filled members.
• The dynamic interaction between CFST truss piers and adjacent RC piers suggests that the welding quality at the interfaces between different structural systems is critical. Any weakness in these connections could lead to disproportionate damage during seismic events.

Key Questions and Reflections

The study raises an important question about the representativeness of the selected ground motion records. The PEER database contains records from various seismic events with different characteristics, and the selection of 10 records may not fully capture the variability of ground motions that the bridge could experience. The fragility curves derived from this limited set may have significant uncertainty, particularly for rare, high-intensity events.

Another reflection concerns the applicability of the IDA methodology to curved bridges. The curved geometry introduces complex dynamic behavior, including out-of-plane responses and torsional coupling, which may not be fully captured by the simplified fragility analysis. The study acknowledges that the arch structure provides some resistance to longitudinal seismic forces, but the interaction between in-plane and out-of-plane responses in a curved bridge under seismic loading is a complex topic that warrants further investigation.

Study Insights and Implications

This research provides a valuable application of IDA methodology to a complex bridge structure with CFST truss piers, demonstrating the potential of performance-based earthquake engineering for bridge assessment. The findings on the influence of pier-to-span ratio and curve radius offer practical guidance for bridge design optimization. The use of rubber bearings as a seismic isolation measure is validated as an effective strategy for reducing complete failure probability. However, the study's reliance on a limited set of ground motions and simplified damage indicators represents limitations that should be addressed in future work. The integration of welding quality considerations into seismic vulnerability assessment is an important area for further development, as the actual seismic performance of CFST truss bridges is highly dependent on the fabrication quality of the welded connections.