1. Assistant Professor, Dept. of Civil Engineering, Nihon Univ., College of Science and Technology, Kanda-Surugadai 1-8, Chiyoda-ku, Tokyo 101-8308, Japan (corresponding author).
2. Professor, Dept. of Civil Engineering, Nihon Univ., College of Science and Technology, Kanda-Surugadai 1-8, Chiyoda-ku, Tokyo 101-8308, Japan.