1. Professor, Dept. of Civil Engineering, Univ. of Pisa, Via Gabba 22, 56126 Pisa, Italy (corresponding author). E-mail: s.pagliara@ing.unipi.it
2. PhD Student, Dept. of Civil Engineering, Univ. of Pisa, Via Gabba 22, 56126 Pisa, Italy. E-mail: p.chiavaccini@ing.unipi.it