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