2014 年 2 巻 1 号 p. 77-93
The tsunami caused by the Great East Japan Earthquake hit the coast of Eastern Japan. The outflow conditions of 37 bridges were evaluated using β (the ratio of resistance to the tsunami force). β was found effective in judging the girder outflow. Moreover, two analytical approaches (video and numerical analyses) were conducted to evaluate the tsunami characteristics and outflow mechanisms of girders. In Shizugawa town, based on video analysis, the tsunami height and velocity variations were estimated. Besides, two mechanisms on the bridge girder at Shizugawa were analyzed (One, as the tsunami hit the girder but did not submerge it; the other, as the tsunami submerged the girder totally). It was noted that in both two mechanisms, the girder was strong enough to resist the tsunami. In Utatsu Bridge, by video and numerical analyses, the wave height was observed to rise slowly, which indicated that the wave shape was not a bore wave. Moreover, it was found that when the girders of Utatsu Bridge were flooded, due to effect of buoyancy, the resistance of the girders was insufficient to resist the tsunami force.