1. Visiting Student Researcher, Dept. of Civil and Environmental Engineering, Univ. of Michigan, 2350 Hayward St., Room 2340, G. G. Brown Bldg., Ann Arbor, MI 48109-2125.
2. Associate Professor, Dept. of Civil and Environmental Engineering, Univ. of Michigan, 2350 Hayward St., Room 2340, G. G. Brown Bldg., Ann Arbor, MI 48109-2125 (corresponding author).