1. Graduate Research Assistant, Dept. of Civil, Environmental and Infrastructure Engineering, Univ. of Washington, P.O. Box 352700, Seattle, WA 98195-2700.
2. Professor, Dept. of Civil, Environmental and Infrastructure Engineering, Univ. of Washington, P.O. Box 352700, Seattle, WA 98195-2700 (corresponding author).
3. Professor, Dept. of Applied Mathematics, Univ. of Washington, P.O. Box 353925, Seattle, WA 98195-3925.