1. Research Assistant, Dept. of Civil and Environmental Engineering, Chung-Ang Univ., 84 Heukseok-Ro, Dongjak-Gu, Seoul 156-759, Republic of Korea.
2. Postdoctoral Researcher, Korea Expressway Corporation Research Institute, 208-96, Dongbu-daero 922 beon-gil, Dongtan-myeon, Hwaseong-si, Gyeonggi-do 445-810, Republic of Korea (corresponding author).
3. Professor, Dept. of Civil and Environmental Engineering, Chung-Ang Univ., 84 Heukseok-Ro, Dongjak-Gu, Seoul 156-759, Republic of Korea.