1. Postdoctoral Researcher, Dept. of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139.
2. Assistant Professor, Dept. of Civil and Environmental Engineering, Univ. of Massachusetts Dartmouth, Dartmouth, MA 02747.
3. George Macomber Professor, Dept. of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 (corresponding author).