1. Researcher, Dept. of Civil Engineering, Univ. of Tokyo, 7-3-1, Hongo, Bunkyoku, Tokyo 113-8656, Japan (corresponding author).
2. Project Lecturer, Institute of Industrial Science, Univ. of Tokyo. 4-6-1, Komaba, Meguroku, Tokyo 153-8505, Japan.
3. Associate Professor, Dept. of Civil Engineering, Univ.of Tokyo, 7-3-1, Hongo, Bunkyoku, Tokyo 113-8656, Japan.
4. Specially Appointed Professor, Institute of Advanced Science, Yokohama National Univ., 79-5, Tokiwadai, Hodogaya, Yokohama 240-8501, Japan.