1. Researcher, Dept. of Civil Engineering, Univ. of Coimbra, 3030-788 Coimbra, Portugal (corresponding author).
2. Professor, Dept. of Civil Engineering, Univ. of Coimbra, 3030-788 Coimbra, Portugal.
3. Assistant Professor, Dept. of Civil Engineering, Univ. of Coimbra, 3030-788 Coimbra, Portugal.
4. Professor, Dept. of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139.