Traffic Safety Analysis of Lightweight CFST Truss-Girder Lattice Pier Bridge Under Seismic Action

Literature Overview

The research by Huang Yufan, Wu Qingxiong, Yuan Huihui, and Chen Baochun from Fuzhou University, published in Journal of Highway and Transportation Research in 2018, investigates the traffic safety of a lightweight bridge system consisting of steel tube concrete (STC) composite truss girders and lattice piers under seismic excitation. The study uses the Ganhaizi Grand Bridge as a case study and is funded by multiple National Natural Science Foundation projects. This work addresses a critical and relatively unexplored area: the interaction between seismic-induced bridge deformations and vehicle stability during or after an earthquake.

Core Technical Content

The researchers developed a vehicle-bridge coupled vibration equation that accounts for seismic action and integrated the computational method into a dual nonlinear finite element program based on beam elements (NL_Beam3D). This approach enables the coupled calculation of vehicle-bridge system interaction under earthquake loading. The vehicle overturning indicator and bridge lateral deformation are considered in the traffic safety analysis.

A remarkable finding is the filtering effect of flexible tall piers on seismic waves. When seismic waves propagate to the bridge, their intensity is significantly attenuated due to the flexibility of the lattice piers. This natural filtering mechanism provides an inherent safety benefit that is not typically accounted for in conventional design procedures.

Key Technical Parameters and Findings

The study reveals that the design code limits for pier top displacement may be relatively conservative. Under 1.6 times the E1 (frequent earthquake) ground peak acceleration, the pier top lateral displacement exceeds the limit specified in the Highway Steel Tube Concrete Bridge Design and Construction Guide, yet the vehicle overturning indicator remains below the threshold. This suggests that the code limit of 1/300 for pier top displacement ratio may be overly restrictive for this type of lightweight bridge system.

Engineering Practice Implications

For the steel pipe and welding aspects of this bridge type, several considerations arise. The lattice pier construction involves numerous welded connections between steel tube members, and the quality of these welds directly affects the pier's flexibility and filtering characteristics. The truss girder system also requires high-quality welded joints at the nodes. Given the importance of the filtering effect for traffic safety, maintaining the designed flexibility of the piers is essential, which means avoiding over-stiffening through excessive weld reinforcement or incorrect member sizing.

The steel tubes used in the lattice piers and truss girders should be selected with appropriate yield strength and ductility characteristics. The welding processes should be designed to maintain the toughness of the weld metal, particularly in regions where cyclic loading occurs during earthquakes. Post-weld heat treatment may be necessary to relieve residual stresses and improve the fatigue performance of critical welded joints.

Study Insights and Reflections

This research demonstrates the value of vehicle-bridge coupled analysis in evaluating traffic safety under seismic conditions, an area that has received insufficient attention in traditional bridge engineering. The finding that flexible tall piers provide a natural filtering effect is particularly encouraging, as it suggests that lightweight bridge designs can inherently offer better seismic performance than conventional rigid systems. The observation that code limits may be conservative provides valuable insight for code revision discussions. However, the study is limited to a specific bridge type and seismic condition, and further research across different bridge configurations and seismic intensities is warranted to generalize these findings.