1. Research Assistant, Dept. of Civil Engineering and Engineering Mechanics, Columbia Univ., 610 Seeley W. Mudd Building, 500 W. 120th St., Mail Code 4709, New York, NY 10027.
2. Associate Professor, Dept. of Civil Engineering and Engineering Mechanics, Columbia Univ., 610 Seeley W. Mudd Building, 500 W. 120th St., Mail Code 4709, New York, NY 10027 (corresponding author).