1. Ph.D. Candidate, Dept. of Mechanical Engineering, Univ. of California, Berkeley, CA 94720-1740.
2. Professor, Dept. of Civil and Environmental Engineering, Univ. of California, Berkeley, CA 94720-1710 (corresponding author).
3. Professor, Dept. of Mechanical Engineering, Univ. of California, Berkeley, CA 94720-1740.