All of these methods can be applied to a bridge as most of them assume beam and plate type structures. In most vibration-based bridge health monitoring techniques, large numbers of sensors are installed on the structure to monitor the dynamic properties [7]. For example, many sensors have been mounted in a case study in Southern California for bridge health monitoring over an eight-year period, which included the occurrence of three earthquakes [9]. These approaches, in which sensors are installed directly on the bridge, are referred to here as direct methods and the on-site instrumentations may be costly, time-consuming, and even dangerous, depending on the location and type of bridge. The common practice is to mount the vibration sensors at different positions on the bridge and connect them to a data acquisition system. For the case where the ongoing traffic cannot be restricted for the purpose of installation, the task of equipment mounting may be risky. In addition, the implementation of SHM is not widespread for short and medium span bridges, which form the greatest proportion of bridges in service.