1. Research Fellow, Dept. of Structural Engineering, Milan Univ. of Technology (Politecnico), P.za L. da Vinci, 32, 20133 Milan, Italy (corresponding author).
2. Professor of Theory and Design of R.C. and P.C. Structures, Dept. of Structural Engineering, Milan Univ. of Technology (Politecnico), P.za L. da Vinci, 32, 20133 Milan, Italy.