1. Dept. of Civil Engineering and Engineering Mechanics, Columbia Univ., 610 Seeley W. Mudd Building, 500 West 120th St., Mail Code 4709, New York 10027.
2. Dept. of Civil Engineering and Engineering Mechanics, Columbia Univ., 610 Seeley W. Mudd Building, 500 West 120th St., Mail Code 4709, New York 10027 (corresponding author).
3. Dept. of Civil and Environmental Engineering, Univ. of Illinois at Urbana-Champaign, Newmark Laboratory, 205 North Mathews Ave., IL 61801.