Experimental Study of Bending and Torsional Effects in Walking-Induced Infrastructure Vibrations: The Pasternak Footbridge

Slender, lightweight and modern footbridges are particularly susceptible to vibrations induced by pedestrian activity. While extensive research has focused on vertical and lateral forces produced by walking, torsional moments generated by eccentrically walking pedestrians remain largely overlooked. Traditional assessments typically neglect these torsional effects, which can be critical when eccentric pedestrian loading excites torsional modes, especially in footbridges with asymmetric geometries. To address this, the paper considers the coupling between bending and torsional effects in both the pedestrian action and structure reaction, including pedestrian forces and moments, as well as bending-induced deflections and torsion-induced rotations of the cross-sections. A simplified method is also presented, allowing standard bending-only analyses to be easily adapted to include torsional effects using analytically derived correction factors. For validation, several experimental tests are conducted on an asymmetric curved footbridge located in Modena, Italy, characterised by coupled bending-torsional vertical modes and hosting different pedestrian densities, pacing frequencies, and crowd distributions (both uniform and eccentric). Experimental and numerical analyses demonstrate that neglecting torsional effects oversimplifies the assessment, highlighting the importance of accounting for bending-torsion coupling for the serviceability of asymmetric footbridges under eccentric near-resonance loading.