1. Postdoctoral Research Associate, Dept. of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; formerly, Graduate Research Assistant, Dept. of Civil and Environmental Engineering, Northwestern Univ., 2145 Sheridan Rd., CEE/A135, Evanston, IL 60208.
2. McCormick Institute Professor and W. P. Murphy Professor of Civil and Mechanical Engineering and Materials Science, Northwestern Univ., 2145 Sheridan Rd., CEE/A135, Evanston, IL 60208 (corresponding author).