Application of French Guidelines in Design

This paper presents an example of the application of the French guidelines concerning the dynamic behaviour of footbridges, to a real footbridge on the river Seine near Paris, which spans are 50m – 100m – 50m long. The French methodology is applied from the beginning to the end, including the choices of the owner (traffic and level of comfort), the dynamic calculations (modes shapes, eigenfrequencies, accelerations due to dynamic loads), and the consequences to the comfort and the possibilities to improve the dynamic behaviour of the footbridge.

This footbridge presents two dangerous modes for the comfort, a lateral one that is sensitive to the lock-in effect (the natural frequency is 1.06 Hz) and a vertical one that is sensitive to the classical effects of the vertical dynamic force of the steps of the pedestrians. The lateral vibration problem is solved by connecting the footbridge to the concrete bridge next to it. The vertical vibration problem is solved by increasing the mass of the footbridge, or by adding Tuned Mass Damper.