Seismic Vulnerability Analysis of CFST Spatial Composite Truss Continuous Beam Bridges via Incremental Dynamic Analysis

Literature Overview and Research Context

The paper by Wang Hailiang, Zhang Duo, Wang Jian, and Liu Zhongxian (2015, World Information on Earthquake Engineering, Vol. 31, No. 2), supported by the National Natural Science Foundation of China (Grant No. 51278327), presents a comprehensive seismic vulnerability assessment of a continuous beam bridge featuring steel tube concrete (CFST) spatial composite truss piers. The engineering background is a bridge with a maximum pier height of 110 metres, representing the upper range of CFST lattice pier applications in bridge engineering. The authors employed OpenSees software to develop an elastoplastic three-dimensional finite element dynamic analysis model and selected 10 ground motion records from the PEER database for incremental dynamic analysis (IDA).

Core Methodology and Technical Approach

The IDA methodology involves progressively scaling ground motion intensity and tracking the structural response at each intensity level. The damage indicator used in this study is the section curvature corresponding to material damage strain at the most unfavourable cross-section of typical piers. Logarithmic function regression analysis of capacity-demand comparisons was used to compute failure probabilities at different damage states for various components.

The analysis was conducted for longitudinal seismic action, which is often the governing direction for long-span bridges. The vulnerability curves were established at three levels: individual pier columns, bridge bearings, and the overall bridge system. The system vulnerability was derived using joint failure probability analysis methods.

Key Findings and Technical Interpretation

Three principal findings emerge from this study that are of considerable importance to bridge engineers:

1. Pier height transition zones are critical: When the bridge features significant changes in pier height along its length, the shorter piers are more sensitive to seismic excitation than the taller piers. This counterintuitive result is explained by the dynamic interaction between piers of different stiffness—the shorter, stiffer piers attract higher inertial forces during seismic events.
2. CFST lattice piers exhibit excellent seismic performance: The tall CFST lattice piers, despite their height, demonstrated good flexibility and resistance to seismic damage. Under foreseeable seismic actions, these piers showed minimal severe damage or complete failure, indicating that the spatial truss configuration provides effective energy dissipation through its inherent geometric redundancy.
3. System vulnerability exceeds component vulnerability: The bridge system failure probability is greater than the failure probability of the most vulnerable individual bearing. This highlights the importance of system-level vulnerability assessment rather than relying solely on component-level analysis.

Engineering Practice and Design Implications

From a practical standpoint, the findings of this research carry several important implications for the design and assessment of CFST lattice pier bridges:

• The transition zones between piers of different heights require special attention in seismic design. Engineers should consider the dynamic interaction effects and may need to implement additional damping or isolation measures at these locations.
• The CFST lattice pier configuration is well-suited for tall pier applications in seismically active regions, as the spatial truss geometry provides both vertical load capacity and lateral flexibility.
• System-level vulnerability assessment should be a standard part of the seismic evaluation process for bridges with CFST lattice piers, as component-level analysis alone may underestimate the risk of bridge unseating or global failure.
• The bearing system deserves particular attention in the design phase, as the bearing vulnerability curve indicates that bearings may be the weakest link in the seismic performance chain.

Study Insights and Reflections

Having spent decades working with steel pipe fabrication and welding processes, I appreciate the engineering elegance of using CFST lattice configurations for tall bridge piers. The combination of steel tube confinement of concrete and the spatial truss arrangement creates a structural system that leverages the compressive strength of concrete, the tensile capacity of steel, and the geometric redundancy of the truss configuration. The 110-metre pier height is remarkable and demonstrates that CFST technology can be scaled to very tall applications.

The use of IDA as an assessment methodology is particularly valuable because it provides a probabilistic framework that accounts for ground motion variability. Unlike deterministic analysis, which uses a single ground motion record, IDA captures the range of possible structural responses across a suite of earthquakes. This probabilistic approach is better aligned with performance-based seismic design philosophy.

One limitation I would note is that the study focuses on longitudinal seismic action. Transverse and oblique seismic actions may produce different vulnerability patterns, particularly for bridges with asymmetric pier arrangements. Additionally, the long-term degradation of CFST lattice piers due to corrosion of the steel tubes or concrete deterioration is not addressed, which is a concern for bridges with long service lives in aggressive environments.

Reference Value and Outlook

This paper establishes a rigorous methodology for seismic vulnerability assessment of CFST lattice pier bridges and provides valuable design guidance. The findings confirm that CFST spatial composite truss piers are a robust solution for tall bridge piers in seismic regions. Future research should extend to transverse and multi-component seismic actions, incorporate long-term material degradation effects, and develop practical design guidelines based on the vulnerability analysis framework presented.