The survey result reported that Kr. Ritting Bridge suffered no damages although the tsunami was very high, while Lueng Ie Bridge slid approximately 3m although the tsunami was small. We conducted several field investigations around Northwest coast of Sumatra, and the surveys made the detailed numerical simulation around a structure possible. Our numerical simulation showed that the flow velocity at Lueng Ie Bridge was larger than that of Kr. Ritting Bridge. In order to understand whether the girder slide occurred or not, we defined a Safety Factor (S.F.). S.F. of Kr. Ritting Bridge was big enough, and that of Lueng Ie Bridge was small. Hence there was possibility that Lueng Ie Bridge was slide because the horizontal wave force exceeded the resistant force.
