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

Overview of the Study

This paper by Wang Hailiang, Zhang Duo, and Liu Zhongxian (2017), published in the Journal of Disaster Prevention and Mitigation Engineering, presents a comprehensive seismic vulnerability analysis of a continuous curved bridge with a maximum pier height of 50 meters using concrete-filled steel tube (CFST) spatial composite truss piers. The research was supported by the National Natural Science Foundation of China (Grant No. 51278327). The study employs Incremental Dynamic Analysis (IDA) using OpenSees to evaluate the seismic performance of the bridge system, providing valuable insights into the behavior of CFST truss structures under earthquake loading.

Methodology and Analytical Framework

The study utilizes the Incremental Dynamic Analysis method, which involves subjecting the structural model to a series of ground motion records with progressively increasing intensity levels. The ground motion records were selected from the PEER (Pacific Earthquake Engineering Research) database, ensuring a representative selection of seismic input that captures the variability of earthquake loading.

The damage indicator selected for this study is the material damage strain at the most unfavorable section of the typical pier, which is converted to section curvature for vulnerability assessment. This approach provides a direct measure of the structural damage accumulated during earthquake loading, allowing for the establishment of vulnerability curves that relate the probability of damage to the intensity of seismic excitation.

Key Technical Findings and Vulnerability Analysis

The study establishes vulnerability curves for both pier components and bearing components, as well as system-level vulnerability curves based on joint failure probability analysis. The results reveal several important findings regarding the seismic behavior of the CFST truss bridge.

The first key finding is that when CFST truss piers have similar heights, the first truss pier adjacent to a reinforced concrete pier exhibits larger dynamic response compared to other truss piers. This observation is attributed to the stiffness discontinuity at the interface between the CFST truss piers and the reinforced concrete pier, which creates a concentration of dynamic forces at the transition region. Engineers should pay particular attention to the design and detailing of pier-to-pier connections in mixed structural systems to mitigate this effect.

The second key finding relates to the effectiveness of rubber bearings in reducing the probability of complete bridge failure. The bridge employs a large number of rubber bearings between the piers and the deck, creating elastic connections and hinge-like behavior that effectively isolates the bridge deck from the seismic forces transmitted through the piers. This design strategy significantly reduces the probability of complete bridge failure, demonstrating the value of seismic isolation measures in bridge design.

The third key finding concerns the effect of pier span ratio on system vulnerability. As the pier span ratio increases, the probability of complete bridge failure also increases. This is attributed to the increased flexibility of the bridge system with larger spans, which leads to greater deformations and higher demands on the structural components during earthquake loading.

The fourth key finding addresses the influence of curve radius on seismic vulnerability. As the curve radius increases, the damage probability gradually increases. This may seem counterintuitive, as larger curve radii typically result in more uniform load distribution. However, the study notes that the arch-like structure inherent in curved bridges provides some resistance to longitudinal seismic forces, and the interaction between the curved geometry and the seismic loading creates a complex response that requires careful analysis.

Integration with Bridge Engineering Practice

The findings of this study have direct implications for the seismic design of CFST truss bridges. The use of Incremental Dynamic Analysis provides a more comprehensive and realistic assessment of seismic performance compared to traditional single-event analysis methods. Engineers should consider incorporating IDA into their design and assessment workflows for critical bridge structures, particularly those with complex geometries such as curved or skewed configurations.

The study also highlights the importance of considering the interaction between different structural components in the seismic design of bridge systems. The vulnerability of the bridge system is not simply the sum of the vulnerabilities of individual components; rather, it depends on the joint failure probability of all critical components. This systemic perspective is essential for developing reliable seismic design strategies that ensure the overall performance of the bridge system under earthquake loading.

Practical recommendations derived from this study include:

• Use of rubber bearings or other seismic isolation devices to reduce the probability of complete bridge failure
• Careful design of pier-to-pier connections in mixed structural systems to minimize stiffness discontinuities
• Consideration of pier span ratio in the optimization of bridge layout to balance seismic performance with economic constraints
• Incorporation of curve geometry effects in seismic analysis and design to account for the complex response of curved bridges

Key Questions and Reflections

The study raises several important questions that warrant further investigation. First, the study uses only 10 ground motion records for the IDA, which may not fully capture the variability of seismic input. Future studies should consider using larger databases of ground motion records to provide more robust vulnerability assessments. Second, the study focuses on material damage strain as the damage indicator, but other indicators such as inter-story drift, plastic hinge formation, or bearing displacement may provide complementary insights into the seismic performance of the bridge.

Additionally, the study does not address the effect of soil-structure interaction on the seismic vulnerability of the CFST truss bridge. For bridges with tall piers, the flexibility of the foundation and the soil-pile interaction can significantly influence the dynamic response. Future research should incorporate soil-structure interaction effects to provide a more comprehensive assessment of seismic vulnerability.

Summary and Implications

This study provides a comprehensive seismic vulnerability analysis of a CFST truss continuous curved bridge using Incremental Dynamic Analysis, contributing valuable insights into the seismic behavior of complex bridge structures. The identification of key factors influencing vulnerability, including pier span ratio, curve radius, and the effectiveness of rubber bearings, provides engineers with practical guidance for improving the seismic resilience of CFST truss bridges. The systemic approach to vulnerability assessment, based on joint failure probability analysis, offers a more realistic evaluation of bridge performance than component-level analysis alone. These findings should inform the development of seismic design guidelines for CFST truss bridges and contribute to the advancement of performance-based earthquake engineering for bridge structures.