ABSTRACT
A bridge may suffer some stiffness losses or property changes after a certain service period. Influence lines provide a suitable comparison of bridge response under the same load-a moving unit force-between the current and original states of the bridge. In this study, the stress responses of a healthy bridge to a unit vertical force are taken as baseline SILs. When a bridge is subject to several severe local damages, SILs may exhibit apparent changes that can be detected through a comparison with baseline SILs. Therefore, the change of SILs can be regarded as a damage index:
Ω Φ Φ( )x ( )x − ( )xBL (5) where Φ(x) and ΦBL(x) are the newly obtained and baseline SILs, respectively, both of which are functions of the abscissa x of the unit force in the longitudinal direction. If the structure does not suffer any damage or the damage location is far from the output location of stress response, the SIL change Ω(x) should be minimal and negligible. Otherwise, the magnitude of Ω(x) may increase evidently when a unit force travels on the bridge.
