1. Professor, Dept. of Civil and Environmental Engineering, Sungkyunkwan Univ., 300 Chun-Chun Dong, Jang-An Gu, Suwon 440-746, Korea.
2. Graduate Student, Dept. of Civil and Environmental Engineering, Sungkyunkwan Univ., 300 Chun-Chun Dong, Jang-An Gu, Suwon 440-746, Korea.