1. Assistant Professor, Dept. of Civil Engineering, Univ. of Roma Tre, Via V. Volterra 62, 00146 Rome, Italy.
2. Consulting Engineer, Loc. Lame 5, 05028 Penna in Teverina, Italy.
3. Associate Professor, Dept. of Structural and Geotechnical Engineering, Sapienza Univ. of Rome, Via Eudossiana 18, 00184 Rome, Italy (corresponding author).