1. Assistant Professor, Dept. PROCAM, Univ. of Camerino, Viale della Rimembranza, 63100 Ascoli Piceno, Italy. E-mail: alessandro.zona@unicam.it
2. Assistant Professor, Dept. of Civil and Environmental Engineering, Louisiana State Univ. at Baton Rouge, 3531 Patrick F. Taylor Hall, Nicholson Extension, Baton Rouge, LA 70803. E-mail: mbarbato@lsu.edu
3. Professor, Dept. of Structural Engineering, Univ. of California at San Diego, 9500 Gilman D., La Jolla, CA 92093-0085 (corresponding author). E-mail: jpconte@ucsd.edu