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. As a result, a design solution was adopted that involved a more sound connection between the heads of the new and existing piers. In regard to the serviceability behaviour with respect to vibrations caused by human-induced dynamic loads, the NF analysis showed that the added links improved the behaviour of the structure. However, some natural frequencies remained close to the range of typical human pacing frequencies (walking, running or jumping). A dynamic analysis was therefore performed, the results of which are presented in the second part of the work - with the aim of ensuring that comfort conditions are verified. Finally, the experimental frequency determination enabled the design solution to be checked, demonstrating the excellent ability of the Finite Element Model used to predict dynamic structural behaviour. Key Words: Footbridge, aesthetic, structural concepts, lateral vibration, modal response, frequency, dynamic tests.
The Footbridge Over the River Adige at Legnago in Italy. Part 1: The Role of Modal Analysis in Design
BULLO, SANDRA;DI MARCO, ROBERTO;SIVIERO, ENZO
2007-01-01
Abstract
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. As a result, a design solution was adopted that involved a more sound connection between the heads of the new and existing piers. In regard to the serviceability behaviour with respect to vibrations caused by human-induced dynamic loads, the NF analysis showed that the added links improved the behaviour of the structure. However, some natural frequencies remained close to the range of typical human pacing frequencies (walking, running or jumping). A dynamic analysis was therefore performed, the results of which are presented in the second part of the work - with the aim of ensuring that comfort conditions are verified. Finally, the experimental frequency determination enabled the design solution to be checked, demonstrating the excellent ability of the Finite Element Model used to predict dynamic structural behaviour. Key Words: Footbridge, aesthetic, structural concepts, lateral vibration, modal response, frequency, dynamic tests.
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