1. Associate Professor, Dept. of Structural Engineering, Politecnico di Milano, Piazza L. da Vinci, 32-20133 Milan, Italy (corresponding author).
2. Professor, Dept. of Structural and Geotechnical Engineering, Univ. of Rome “La Sapienza,” Via Eudossiana 18-00184 Rome, Italy.
3. Professor, Dept. of Civil, Environmental, and Architectural Engineering, Univ. of Colorado, CB 428, Boulder, CO 80309-0428.
4. Professor, Dept. of Structural Engineering, Politecnico di Milano, Piazza L. da Vinci, 32-20133 Milan, Italy.