Optimal damping systems for flexible footbridges

Pedestrian footbridges are characterized by a low inherent damping. Hence, to avoid comfort problems it is often necessary to add external damping to the structure. In this paper, two different damping strategies have been analyzed. The first one consists in placing point dampers between the ground and the bridge deck, close to the bridge ends. The second one in based on the idea of using non-structural elements to add damping to the structure; in the present case, a dissipative handrail has been hypothesized.

The damping system performances have been evaluated in terms of modal damping ratio within the frequency range interested by common vertical excitations. Point dampers create a non-diagonal modal damping matrix; thus, the system is non-classically damped and damping optimization problems arise. In the case of distributed dampers, as the dissipative handrail is, the modal damping matrix is approximately diagonal and a “classical damping” can be hypothesized; the modal damping increases as the damping coefficients increase.

The performed analyses show that the point dampers exhibit good performances on the first few modes. On the other hand, the dissipative handrail offers good performances on higher modes but is not able to damp the first ones. Subsequently, the best solution is recognized in the joint use of the two considered systems: point dampers at the bridge ends and dissipative handrail. The procedure to analytically evaluate the optimal parameters for both the systems is outlined and an application to a suspension footbridge is presented.