1. Professor, Dept. of Civil and Environmental Engineering, Nagaoka Univ. of Technology, Nagaoka 940-2188, Japan.
2. Professor, Dept. of Civil Engineering, Univ. of Tokyo, Tokyo 113-8656, Japan.
3. Professor, Dept. of Civil and Environmental Engineering, Saitama Univ., Saitama 338-8570, Japan.
4. Assistant Professor, Dept. of Civil and Environmental Engineering, Nagaoka Univ. of Technology, Nagaoka 940-2188, Japan.