1. Doctoral Student, Dept. of Civil and Environmental Engineering, Nagaoka Univ. of Technology, 1603-1 Kamitomioka Nagaoka, Niigata 940-2188, Japan.
2. Professor, Dept. of Civil and Environmental Engineering, Nagaoka Univ. of Technology, 1603-1 Kamitomioka Nagaoka, Niigata 940-2188, Japan (corresponding author).