1. Geotechnical Engineer, Halcrow Group Ltd., Elms House, 43 Brook Green, London W6 73F, United Kingdom.
2. Professor, Dept. of Structural Engineering, Politecnico di Milano, Piazza Leonardo da Vinci, 32 20133 Milano, Italy (corresponding author).
3. Professor, Dept. of Structural Engineering, Politecnico di Milano, Piazza Leonardo da Vinci, 32 20133 Milano, Italy.