1. Graduate Research Assistant, Dept. of Civil Engineering and Engineering Mechanics, Columbia Univ., 610 S.W. Mudd Bldg., 500 West 120th St., New York, NY 10027.
2. Assistant Professor, Dept. of Civil Engineering and Engineering Mechanics, Columbia Univ., 618 S.W. Mudd Bldg., 500 West 120th St., New York, NY 10027 (corresponding author).