1. Assistant Professor, Politecnico di Milano, Dipartimento di Ingegneria Strutturale, 20133 Milano, Italy (corresponding author).
2. Researcher, Chalmers Univ. of Technology, Dept, of Civil and Environmental Engineering, SE-412 96 Gothenburg, Sweden.
3. Associate Professor, Chalmers Univ. of Technology, Dept. of Civil and Environmental Engineering, SE-412 96 Gothenburg, Sweden.