1. Assistant Professor, Dept. of Civil and Environmental Engineering, Sejong Univ., Seoul 143-747, Korea (corresponding author).
2. Nathan M. Newmark Professor, Dept. of Civil and Environmental Engineering, Univ. of Illinois at Urbana-Champaign, Urbana, IL 61801.
3. Professor, Dept. of Civil and Environmental Engineering, Korea Adv. Institute of Science and Technology, Daejeon 305-701, Korea.