1. Grad. Student, Dept. of Civ. Engrg., Seoul Nat. Univ., San 56-1, Shillim-dong, Kwanak-gu, Seoul 151-742, Korea.
2. Full-time Lect., Dept. of Civ. Engrg., Dong-A Univ., Hadan-dong, Saha-gu, Busan 604-714, Korea.
3. Asst. Prof., Dept. of Civ. Engrg., Seoul Nat. Univ., San 56-1, Shillim-dong, Kwanak-gu, Seoul 151-742, Korea; corresponding author. E-mail: chslee@plaza.snu.ac.kr
4. Prof., Dept. of Civ. Engrg., Seoul Nat. Univ., San 56-1, Shillim-dong, Kwanak-gu, Seoul 151-742, Korea.