1. Research Assistant Professor, Dept. of Civil and Environmental Engineering, Northwestern Univ., 600 Foster St., Evanston, IL 60208.
2. Assistant Professor, Dept. of Civil and Environmental Engineering, Univ. of Queensland, Brisbane, QLD 4072, Australia.
3. Professor, Dept. of Civil and Environmental Engineering, Northwestern Univ., 600 Foster St., Evanston, IL 60208 (corresponding author).