The Footbridge Over the River Adige at Legnago in Italy. Part 2: The Assessment of Human-Induced Vibrations

Pedestrian bridges are usually characterised by moderate dead loads and high deformability; such conditions make them particularly sensitive to dynamic loads, which can induce significant vibrations.
The footbridge over River Adige in Legnago (Northern Italy) is a triangulated space-lattice truss made using steel tubes; after its construction, the structure proved to be particularly sensitive to horizontal actions, that can produce an oscillation involving lateral translation and the deck torsion on its longitudinal axis.
The effectiveness of the stiffening work - carried out in order to reduce such movements - has been assessed through analyses that were able to reveal the structure’s response to dynamic loads, such as those induced by pedestrian traffic.
In particular, the study was broken down into two successive analyses, following the current procedures for vibration control in similar structures. A modal analysis for the evaluation of natural frequencies and mode shapes (with experimental tests for validation) - constituting the first part of the work [Pubblicazione n. 4] - enabled us to verify whether the structure’s natural frequencies are included within the typical frequencies range of human-induced dynamic load; and a dynamic analysis - whose outcome is illustrated below - aimed to assess the entity of the vibrations and maximum accelerations due to the transit of one or more pedestrians, with view to verify the emergence of uncomfortable conditions for the users.

Key Words: Footbridge, structural concepts, vibration, human-induced load, crowd, comfort criteria.