1. Postdoctoral Research Associate, Northwestern Univ., Evanston, IL 60208.
2. Graduate Research Assistant, Doctoral Candidate, Northwestern Univ., Evanston, IL 60208.
3. McCormick Institute Professor, W.P. Murphy Professor of Civil Engineering and Materials Science, Northwestern Univ., 2145 Sheridan Rd., CEE/A135, Evanston, IL 60208 (corresponding author).