| A cable-stayed bridge in a state of cantilever under construction has quite different seismic characteris-tics from those of a completed system．This paper describes strong motions recorded at the Tokachi Ohashi Bridge（center span 251m）while under construction during earthquakes such as the Hokkaido Toho-oki Earthquake，which occurred off the east coast of Hokkaido on October 4 1994（M=8.1）．Earthquake-induced behavior of the bridge is also reported．［＊］Vibration modes derived from strong motion records and analytical modes were almost identical．In a mode associated with rocking of the foundation，the frequency was found to be extremely variable depending on the values of the spring constant of the ground；hence its evaluation is important．A response analysis of the construction system revealed that the damping ratio of the construction system was smaller than the damping ratio used for determining elasticity at concrete structures in design of the completed system． |
