1. Research Assistant Professor, Dept. Civil Engineering, Univ. of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan (corresponding author).
2. Professor, Dept. Civil Engineering, Univ. of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
3. Manager, Design Eng. Div., Metropolitan Expressway Co. Ltd., 1-4-1 Kasumigaseki, Chiyoda-ku, Tokyo 100-8930, Japan.