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

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.

Keywords

Footbridge aesthetic structural concepts lateral vibration modal response frequency dynamic tests

References

Italian National Code, D.M. 4 Maggio 1990 “Criteri generali e prescrizioni tecniche per la progettazione, l'esecuzione e il collaudo di ponti stradali” e relativa circolare del Ministero dei LL.PP. n. 34233, 25/02/1991.

Bachmann

, “Vibration Problems in Structures -Practical Guidelines”, 2nd Edition, Birkhäuser Verlag, Basel Berlin Boston, 1997.

European Committee for Standardization, Eurocode 1. General Actions Traffice loads on bridges. EN1991–2: 2003, Brussels, Belgium 2003.

Bachmann

, *““Lively” footbridges - a real challenge”, Proceedings of Footbridge 2002 - Design and Dynamic Behaviour of Footbridges, Paris, France, 2002.

CEB-FIP “Model Code 90”, Bulletin d'Information No 213/214, 1993.

British Standards Institution, BS5400: Part 2, Appendix C: “Vibration serviceability requirements for foot and cycle track bridges”, Highway Agency, London, UK, 1978.

British Standards Institution, BS 5400: Part 2, BD37/01, Appendix B, Steel, concrete and composite bridges. Specification for loads, Highway Agency, London, UK, 2001.

British Standards Institution, BSI 5400, DB 29/04, Design Manual for Road and Bridges: Design Criteria for Footbridges, Highway Agency, London, February, UK, 2004.

European Committee for Standardization, Eurocode 5 - “Design of Timber Structures-Bridges”, ENV1995–2:1997, Brussels, Belgium 2007.

10.

European Committee for Standardization, Eurocode, UNI EN 1990:2002, Annex A2.