1. The Korea Transport Institute, 968-5 Daechi-Dong, Kangnam-Gu, Seoul 135-280, Korea
2. Department of Civil Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan
3. Department of Civil Engineering, Tokyo Institute of Technology, 2-12-1 Oookayama, Meguro-ku, Tokyo, Japan