SEISMIC MONITORING OF A BRIDGE - ASSESSING DYNAMIC CHARACTERISTICS FROM BOTH WEAK AND STRONG GROUND EXCITATIONS

An array of 24 strong-motion accelerometers produced records for the N ew-Lian River Bridge, a five-span continuous bridge, during 25 Februar y 1995 earthquake (weak motion) and 25 June 1995 earthquake (strong mo tion). This paper describes the application of linear discrete-time sy stem identification methodology to the array of strong-motion measurem ents, in order to assess seismic response characteristics of the bridg e. The structural system identification will concentrate not only on t he global identification but also on the local structural system ident ification. Results of this application show that: (1) weak and strong ground excitation will induce significant differences on the dynamic r esponse of the bridge; (2) linear models provide an excellent fit to t he measured motions of the bridge from the records of these two seismi c events; (3) the rigid-body rocking of the bridge pier during strong shaking is significant and cannot be ignored during identification; (4 ) the transverse motion at mid-span of the bridge is controlled by the quasi-static response from the boundary system and this phenomenon is quite significant during strong ground excitation. Also, systematic e stimates of modal damping ratio and equivalent assessments of pier sti ffness developed in the bridge during earthquake are discussed.