1. JSPS Postdoctoral Fellow, Dept. of Civil Engineering, Tokyo Institute of Technology, 2-12-1M1-9, Ookayama, Meguro-ku, Tokyo 152-8552, Japan; formerly, Ph.D. Student, Dept. of Civil Engineering, The Univ. of Tokyo (corresponding author).
2. Professor, Dept. of Civil Engineering, The Univ. of Tokyo, 7-3-1, Hongo, Bukyo-ku, Tokyo 113-8656, Japan.