Stability criterion for lateral vibration of footbridges

Annoying human-induced lateral vibrations introduces a serviceability limit state that must be considered in the design of footbridges and it is often conquered by increasing the structural damping. To quantify the amount of damping needed in each frequency there is a need for a decisive factor which can predict whether the bridge is going to be unstable or not. Employing human-induced lateral force model introduced by Nakamura, a discussion is then presented on an energy balancing method in which the energy entered into the system as a result of pedestrians walking on the bridge, is compared with the energy dissipated through inherent structural damping. From the results a serviceability criterion is proposed. The pedestrian Scruton number is introduced as an instability criterion and is assessed for some footbridges and compared with criteria proposed by others. It is shown that the bridge instability can be well predicted by the proposed criterion