1. Assistant Professor, Dept. of Civil and Environmental Engineering, Waseda Univ., Shinjuku-ku, Tokyo 169-8555, Japan.
2. Professor, Dept. of Civil and Environmental Engineering, Waseda Univ., Shinjuku-ku, Tokyo 169-8555, Japan (corresponding author).
3. Associate Professor, Dept. of Civil and Environmental Engineering, Maebashi Institute of Technology, Maebashi-shi Gunma 371-0816, Japan; formerly, Senior Researcher, Railway Technical Research Institute of Japan,Tokyo 185-8540, Japan.