Dynamic responses of bridge substructures subjected to high-speed trains

The substructures of high-speed rail (HSR) structures are typically rigid, as they are designed to meet stringent longitudinal displacement limits specified in most HSR design standards. For site conditions such as deep valleys and soft soils, tall columns or low foundation stiffness can lead to a flexible substructure system. Flexible substructures are preferred for bridges in seismic areas because their lower horizontal frequencies yield smaller seismic forces. This cost-effective design strategy is recognised in the California HSR design criteria manual, which specifies larger relative longitudinal displacement limits. In this work, the influences of substructure flexibility on relative displacements were investigated through dynamic analyses of 36 different numerical models representing a 20-span viaduct supported by substructures with first horizontal frequencies of 0.566-3.706 Hz. Four key parameters were investigated: column height, span length, bearing plan layout and ratio of depth to span length. Train models from BS EN 1991-2:2003 were used as dynamic moving loads. The results of this study show that the chances of dynamic responses larger than design limits do exist as the stiffness of substructures departs from being rigid, where resonance effects on substructures are the main contributor to response amplification.