Vibration serviceability assessment and vibration mitigation of through-truss footbridges in Iran under human-induced excitation

This study evaluates the vibration serviceability of widely implemented steel through-truss footbridges in Iran, with a detailed analysis of three representative case studies. Numerical models are developed to assess natural frequencies and maximum accelerations induced by pedestrian excitations. Free-vibration tests conducted on two existing footbridges provide their experimental natural frequencies in the vertical direction, which are used to calibrate the corresponding numerical models. For the third footbridge, which was under construction, a theoretical approach accounting for uncertainties in structural modal properties is incorporated into its model. The vibration serviceability assessment reveals that all lateral natural frequencies lie outside the critical range for pedestrian-induced lateral vibrations, indicating no lateral serviceability concerns. However, several vertical and longitudinal modes fall within the critical range. Subsequent acceleration analysis shows that under two dense pedestrian traffic conditions, vertical accelerations in all footbridges exceed the unacceptable threshold of 2.50 m/s 2 , and longitudinal accelerations in two of the footbridges exceed the unacceptable threshold of 0.80 m/s 2 . These issues are addressed by implementing a tuned mass damper (TMD) and by modifying the stiffness of the bridge piers to reduce the contribution of pier bending deformation to the modal displacements, respectively. An optimally designed TMD for the most critical traffic class achieves an 83% reduction in maximum vertical acceleration, ensuring adequate structural performance for this traffic class. To maintain robust performance under varying pedestrian masses associated with different traffic classes, a frequency-detuned TMD design is subsequently proposed. This approach yields response reductions ranging from 61 to 86% across all examined traffic conditions.