Seismic Response Analysis of CFST Spatial Composite Truss Continuous Beam Bridge on River Valley Sites

Literature Overview

This paper by Wang Hailiang, Wang Shiguang, Liu Zhongxian, and Huang Lei, published in World Information on Earthquake Engineering in 2017, investigates the seismic response of a concrete-filled steel tube (CFST) spatial composite truss continuous beam bridge located on a river valley site. The study uses the Ganhaizi Grand Bridge on the Beijing-Kunming Expressway as the engineering case study. The research was supported by the National Natural Science Foundation (51678390), the Tianjin Natural Science Foundation Key Project (12JCZDJC28900), and the Tianjin Science and Technology Project (KJXH2012-19).

Core Technical Methodology

The research employs a sophisticated computational framework that couples finite element method (FEM) with the indirect boundary integral equation method (IBIEM) to simulate the soil-structure interaction (SSI) effects on the bridge seismic response. The earthquake motion is input through a multi-point excitation approach using the improved Newmark method (improved LMM), which accounts for wave propagation effects across the bridge length.

Computational Framework

Key Seismic Response Findings

The study systematically compared the seismic responses under three conditions: uniform excitation, traveling wave effect (multi-point excitation), and local site effect (river valley topography). The results revealed several important patterns:

1. Pier height sensitivity: Low piers are more sensitive to seismic excitation than high piers, with the most pronounced effect occurring in the region of abrupt pier height change. This counterintuitive finding is attributed to the dynamic amplification caused by the impedance mismatch between the stiff low pier and the flexible high pier.
2. Traveling wave effect on high piers: Compared to uniform excitation, the traveling wave effect increases the pier base axial force, mid-span bottom chord member axial force, and diagonal web member axial force, but decreases other measured values.
3. Traveling wave effect on low piers: For low piers and their corresponding beam spans, the traveling wave effect reduces the pier base bending moment, pier top displacement, and mid-span displacement.
4. Local site effect: The river valley topography significantly amplifies the displacement and internal forces of most piers and beam spans compared to both uniform excitation and traveling wave effects alone. The amplification is most pronounced at the low pier and beam span in the abrupt pier height change region.

Engineering Practice Implications

The findings have direct implications for the seismic design of long-span bridges on river valley sites, which are common in mountainous and hilly regions of China. The design code provisions for seismic action typically assume uniform ground motion across the bridge length, which may not be conservative for all structural components. The study demonstrates that:

• The local site effect must be explicitly considered in the seismic design of bridges on river valley sites, particularly for the low pier and adjacent beam spans in the abrupt height change region.
• The traveling wave effect can be either beneficial or detrimental depending on the specific structural component and its location along the bridge.
• A comprehensive seismic assessment should include both SSI effects and multi-point excitation to capture the full range of seismic response characteristics.

From a structural engineering perspective, the CFST spatial composite truss system offers advantages in terms of lightweight construction and high stiffness-to-weight ratio. However, the complex force transmission mechanism of the truss system means that seismic forces are distributed differently than in conventional beam-girder bridges. The axial forces in the truss members are particularly sensitive to the seismic input characteristics, and the design must ensure adequate strength and ductility of all critical members.

Study Insights and Reflections

This research represents a significant advancement in the seismic analysis methodology for complex bridge structures. The coupling of FEM with IBIEM provides a rigorous approach to soil-structure interaction that is more accurate than simplified methods such as the substructure method or the equivalent spring method. However, the computational cost of this approach is substantial, and its application to routine design may be limited.

The engineering case study approach is commendable, as it grounds the research in a real-world application. The Ganhaizi Grand Bridge is a critical infrastructure project on a major expressway corridor, and the seismic performance of such bridges is of paramount importance for public safety. The findings can directly inform the seismic retrofitting or design optimization of similar bridges.

One area that warrants further investigation is the effect of soil liquefaction on the seismic response of CFST bridge piers founded on river valley sites. River valley soils are often composed of loose, water-saturated granular materials that are susceptible to liquefaction during strong earthquakes. The combined effect of liquefaction-induced ground settlement and lateral spreading with the traveling wave effect could produce severe structural demands that are not captured by the current analysis framework.

The research underscores the importance of site-specific seismic analysis for critical bridge infrastructure. Engineers should not rely solely on uniform excitation assumptions when designing bridges on topographically complex sites, as the local site effect can significantly alter the seismic demand on structural components.