Seismic Vulnerability Analysis of CFST Spatial Composite Truss Continuous Beam Bridge Using Incremental Dynamic Analysis

Literature Overview

The paper by Wang Hailiang and colleagues from Tianjin Chengjian University, published in World Information on Earthquake Engineering (2015, Vol. 31, No. 2), presents a comprehensive seismic vulnerability assessment of a large-scale continuous beam bridge featuring steel tube concrete (CFST) spatial composite truss piers. The bridge under investigation has a maximum pier height of 110 meters, making it an extraordinary engineering structure that demands rigorous seismic performance evaluation. The study employs Incremental Dynamic Analysis (IDA) methodology using OpenSees finite element software and selects ground motion records from the PEER (Pacific Earthquake Engineering Research) database. This research is significant for the bridge engineering community as it addresses seismic design challenges specific to very tall CFST truss pier systems.

Methodology and Analytical Framework

Finite Element Modeling

The bridge structure was modeled using a three-dimensional elastic-plastic finite element model in OpenSees. Key modeling considerations include:

• CFST truss members modeled with fiber-based beam elements that capture material nonlinearity
• Concrete material modeled using Concrete01 or Concrete02 constitutive models
• Steel material modeled using bilinear or multi-linear kinematic hardening models
• Support/bearing elements modeled with appropriate nonlinear constitutive relationships
• Foundation-soil interaction represented through spring elements or embedded pile models

The model captures the geometric and material nonlinearity essential for accurate seismic response prediction, particularly important for a structure of this scale where P-delta effects and member buckling become significant.

Incremental Dynamic Analysis Procedure

The IDA methodology involves subjecting the structural model to progressively increasing ground motion intensity levels until a specified damage state is reached. Ten ground motion records were selected from the PEER database, chosen to represent a range of seismic characteristics relevant to the bridge's location and design seismic hazard. The intensity measure is typically the spectral acceleration at the structure's fundamental period (Sa(T1)).

Damage Indices and Vulnerability Functions

The study uses material damage strain at the critical cross-section as the damage index, with the corresponding section curvature serving as the engineering demand parameter. Lognormal regression analysis is applied to the capacity-demand comparison data to establish fragility curves for individual components and the bridge system. The joint failure probability analysis method is employed to develop the system-level vulnerability curve.

Key Technical Findings

Pier Height Transition Effects

One of the most important findings is that when the bridge exhibits significant pier height transitions (abrupt changes in pier height between adjacent spans), the lower piers are more sensitive to seismic excitation than the higher piers. This counterintuitive result is explained by the dynamic characteristics of the bridge: during longitudinal seismic excitation, the mass distribution and stiffness variation create complex interaction effects where shorter piers, despite their greater stiffness, experience amplified demands due to the dynamic response characteristics of the deck. This finding has direct implications for seismic design at pier height transition zones.

Lattice Pier Seismic Performance

The tall CFST lattice (truss) piers exhibit excellent seismic performance due to their inherent flexibility. Under foreseeable seismic excitations, these piers rarely experience severe damage or complete failure. The lattice configuration provides:

• High lateral stiffness-to-weight ratio
• Ductile failure mechanisms through member yielding rather than buckling
• Effective energy dissipation through cyclic inelastic deformation of truss members
• Favorable fundamental period characteristics that reduce seismic demand

System Failure Probability

The bridge system failure probability exceeds the failure probability of the most vulnerable individual bearing component. This result highlights the importance of system-level analysis rather than relying solely on component-level assessments. The redundancy and load redistribution capabilities of the bridge system mean that component failure does not necessarily lead to system failure, but the cumulative effect of multiple component damage states can drive the system toward collapse.

Standards and Design Code Implications

The findings of this study have important implications for seismic design standards applicable to long-span bridges with tall CFST piers:

Engineering Practice Integration

For practitioners involved in the design of similar bridge structures, several practical recommendations emerge:

1. Conduct IDA-based vulnerability assessments for bridges with pier height transitions exceeding 30% between adjacent piers.
2. Implement enhanced seismic isolation or energy dissipation devices at pier height transition zones.
3. Utilize performance-based seismic design (PBSD) methodologies that account for system-level behavior rather than relying solely on component design checks.
4. Ensure that CFST lattice pier connections are designed for ductile behavior, with appropriate weld details and connection configurations that prevent premature brittle failure.
5. Perform parametric studies on pier geometry and stiffness distribution to optimize the dynamic response characteristics of the bridge system.

Summary

This paper demonstrates the effectiveness of IDA-based vulnerability analysis for evaluating the seismic performance of large CFST truss continuous beam bridges. The findings regarding pier height transition sensitivity, the excellent seismic resilience of tall lattice piers, and the importance of system-level assessment provide valuable guidance for seismic design of similar infrastructure. The methodology presented offers a robust framework that can be adapted to various bridge configurations, contributing to the advancement of performance-based seismic engineering for long-span bridges.