1. Assistant Professor, Dept. of Civil Engineering and Engineering Mechanics, Columbia Univ., Room 618, S.W. Mudd Building, 500 West 120th St., New York, NY 10027 (corresponding author).
2. Professor, Dept. of Civil and Environmental Engineering, Stanford Univ., Building 550, 416 Escondido Mall, Stanford, CA 94305-4020.
3. Graduate Student, College of Management of Technology, Ecole Polytechnique Fédérale de Lausanne, EPFL ODY 1.19, Station 5, CH-1015 Lausanne, Switzerland.