1. Ph.D. Candidate, Dept. of Civil and Environmental Engineering, Northwestern Univ., 2145 Sheridan Rd., A310, Evanston, IL 60208-3109.
2. Associate Professor, Dept. of Civil and Environmental Engineering, Northwestern Univ., 2145 Sheridan Rd., A332, Evanston, IL 60208-3109 (corresponding author).