1. Associate Professor, Dept. of Civil and Environmental Engineering, Seoul National Univ., 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, South Korea (corresponding author).
2. Researcher, Korea Bridge Design Research Center, Seoul National Univ., 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, South Korea.
3. Graduate Student, Dept. of Civil and Environmental Engineering, Seoul National Univ., 1 Gwanak-ro, Gwanak-gu, Seoul 151-744. South Korea.