Condition assessment of existing bridge structures has become a critical issue due to the deterioration of structural performance and the requirements of structural safety.Structural health monitoring has received significant attention in past decades in order to assess the safety and reliability of bridges during their service life (Brownjohn and Xia 2000).Structural damage identification techniques based on vibration measurements show great promise because they allow for quick and global damage detection at a relatively low cost after a severe loading event.These techniques rely on the fact that any change in stiffness caused by damage in a structure leads to a change in modal parameters of the structure such as frequencies and mode shapes (Doebling et al.1996).Although successful applications of these techniques have been developed to some extent,vibration based local damage assessment of bridges remains a challenge in practice.