1. Assistant Professor, Dept. of Safety Engineering, Incheon National Univ., 12-1 Songdo-dong, Yeonsu-Gu, Incheon 406-772, Korea.
2. Associate Professor, Dept. of Civil and Environmental Engineering, Univ. of Illinois at Urbana Champaign, 205 N. Mathews Ave., MC-250, Urbana, IL 61801 (corresponding author).