The Footbridge over the River Adige at Legnago in Italy Part 1: The Role of Modal Analysis in Design:

Weight reduction, achieved through the use of materials with high mechanical strength and, in terms of shape, by adopting visually light structures, gave rise to the design of constructions with low self-weight, which were therefore sensitive to the effects of live loads.This sensitivity is particularly acute in the case of footbridges, where pedestrian traffic translates into dynamic forces that can lead to resonance, since pacing frequencies would be similar to the natural frequencies of the structure itself. In general, the problem is one of serviceability.The footbridge over the River Adige at Legnago (Northern Italy), designed by some of the authors(1) of this paper, showed a marked sensitivity to repeated horizontal forces that tended to cause transversal and torsional vibration around its longitudinal axis.This paper examines how an analysis of Natural Frequencies (NF) and modal response revealed the excessive deformability of the piers to be the key reason for the structure to behave in this way. ...