1. Postdoctoral research fellow, Dept. of Civil and Environmental Engineering, 407 McLaughlin Hall, Univ. of California, Berkeley, CA 94720-1712; formerly, Dept. of Civil Engineering, Yonsei Univ., Seoul, South Korea.
2. Professor, Dept. of Civil and Environmental Engineering, Yonsei Univ., Seoul 120-749, South Korea (corresponding author).
3. Assistant Professor, Dept. of Civil Engineering, Seoul National Univ. of Science and Technology, Seoul 139-743, South Korea.
4. Ph.D. student, Dept. of Civil and Environmental Engineering, Yonsei Univ., Seoul 120-749, South Korea.