1. Former Postdoctoral Researcher, 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).