1. Ph.D. Candidate, Dept. of Civil Engineering and Engineering Mechanics, Columbia Univ., New York, NY 10027.
2. Assistant Professor, Dept. of Civil and Environmental Engineering, Univ. of Perugia, via G. Duranti, 93 06125 Perugia, Italy.
3. Professor, Dept. of Civil Engineering and Engineering Mechanics, Columbia Univ., New York, NY 10027 (corresponding author).