1. Ph.D. Candidate, School of Civil and Environmental Engineering, Yonsei Univ., 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Korea.
2. Professor, School of Civil and Environmental Engineering, Yonsei Univ., 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Korea (corresponding author).