1. Ph.D. Student, Dept. of Structural Engineering and Geotechnics, Sapienza Univ. of Rome, 00197 Rome, Italy.
2. Associate Professor, Dept. of Civil Engineering, Univ. of Minho, Campus de Azurém, 4810-058 Guimarães, Portugal (corresponding author).
3. Full Professor, Dept. of Structural Engineering and Geotechnics, Sapienza Univ. of Rome, 00197 Rome, Italy.