1. Dept. of Structural Engineering, Technical University of Milan, I-20133 Milan, Italy
2. Centro di Ricerca, Sviluppo e Studi Superiori in Sardegna (CRS4), Cagliari, Italy
3. Dept. of Mathematics, Technical University of Milan, Italy and Centro di Ricerca, Sviluppo e Studi Superiori in Sardegna (CRS4), Cagliari, Italy
4. Dept. of Mathematics, Technical University of Milan, I-20133, Milan, Italy