1. Research Assistant, Dept. of Civil Engineering, Tohoku Univ., 6-6-06 Aoba Aramaki Aoba-ku, Sendai 980-8579, Japan (corresponding author).
2. Associate Professor, Dept. of Civil Engineering, Tohoku Univ., 6-6-06 Aoba Aramaki Aoba-ku, Sendai 980-8579, Japan.
3. Professor, Dept. of Civil Engineering, Tohoku Univ., 6-6-06 Aoba Aramaki Aoba-ku, Sendai 980-8579, Japan.