Variation of Modal Parameters in Bridges Under Heavy Load Configurations: A Case Study

Abstract

Bridges are essential for connectivity, often enduring extreme load conditions that impact their dynamic behaviour. Understanding how modal parameters such as natural frequencies, damping ratios, and mode shapes change under heavy loads is crucial for Structural Health Monitoring (SHM). This paper presents a case study of the San Carlo Bridge, a newly built continuous-isolated steel-concrete composite bridge designed for hydraulic and seismic resilience in a flood-prone region of central Italy. To evaluate its dynamic behaviour, extensive vibration-based testing was conducted under both unloaded and controlled heavy-load conditions. Heavy loads were applied by strategically positioning fully loaded trucks in various configurations to produce extreme effects on the structure in terms of bending moment. Ambient vibration tests and operational modal analysis were performed to compare modal parameters across these scenarios. The findings reveal variations in modal parameters across different loading schemes, defining a range of dynamic responses that characterise the structure in safe but high-load states. These results provide experimental benchmarks for SHM, enabling the establishment of critical thresholds and informing predictive models for extreme events.