1. Ph.D. Student, Dept. of Structural, Geotechnical and Building Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy.
2. Graduate Student, Dept. of Structural, Geotechnical and Building Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy.
3. Visiting Professor, Dept. of Structural and Environmental Engineering, Univ. of California, Berkeley, Davis Hall, Berkeley, CA 94720-1710 (corresponding author).