1. Research Assistant, Dept. of Civil and Environmental Engineering, Hanyang Univ. 1271, Sa-1 Dong, Ansan, Kyungki 425-791, Korea (corresponding author).
2. Professor, Dept. of Civil and Environmental Engineering, Hanyang Univ. 1271, Sa-1 Dong, Ansan, Kyungki 425-791, Korea.
3. Professor, Dept. of Civil Engineering, Hanyang Univ., Seoul, Korea.