1. Research Assistant, Dept. of Civil and Environmental Engineering, Univ. of Washington, Seattle, WA 98195-2700 (corresponding author).
2. Research Assistant, Dept. of Civil and Environmental Engineering, Univ. of Washington, Seattle, WA 98195-2700.
3. Professor, Dept. of Civil and Environmental Engineering, Univ. of Washington, Seattle, WA 98195-2700.