Direction effect of vertical ground motion on seismic damages of friction-based isolated RC piers

Abstract The major focus in earthquake analysis is typically on the excitation caused by two in-plane horizontal components. However, the direction of vertical ground motion (VGM) is a critical aspect of an earthquake’s inherent nature that has received less attention in recent research. Accordingly’, the present study investigates the effects of VGM direction on the responses of a multi-span continuous concrete bridge pier seismically isolated with a friction pendulum system (FPS). The friction-based isolator is chosen because vertical force influences the coefficient of friction through variations in the normal force. To precisely assess the impact of vertical ground motion, a coefficient is proposed based on the ratio of the peak ground acceleration (PGA) of the vertical component in the upward and downward directions. The incremental dynamic analysis approach for the vertical component of an earthquake (IDA-V) is introduced and applied using a set of ground motion records. These records are categorized by their predominant vertical acceleration direction to emphasize the vertical impacts of inertia forces on the overall bridge responses. Fragility curves for substructure components are then developed. The results indicate that when the predominant direction of VGM is upward, it can cause compression-based damage in structural components, such as column compression failure and increased displacement of the friction pendulum systems (FPS). Conversely, when the predominant direction of VGM is downward, it can lead to tension-based damage, including tensile forces along the columns and a higher probability of isolator uplift. In general, incorporating the effects of VGM in the analysis increases the probability of failure across all structural components and damage states. Additionally, accounting for the upward or downward direction of VGM results in distinct types of damage.