1. Research Assistant, D.I.C.A.T.A., Univ. of Brescia, via Branze 43, 25123 Brescia, Italy (corresponding author).
2. Edmund K. Turner Professor and Head, Dept. of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139.
3. Senior Research Associate, Dept. of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139.
4. Professor, Univ. of Brescia, 25123 Brescia, Italy.