1. Ph.D. Student, Dept. of Civil and Environmental Engineering, Univ. of Illinois at Urbana-Champaign, Urbana, IL 61801.
2. Associate Professor, Dept. of Civil and Environmental Engineering, Seoul National Univ., Seoul 151-742, Republic of Korea (corresponding author).
3. Associate Professor, Dept. of Civil and Environmental Engineering, Univ. of Illinois at Urbana-Champaign, Urbana, IL 61801.