1. Ph.D. Student, Research Assistant, Dept. of Civil Engineering and Architecture, Instituto Superior Técnico, Technical Univ. of Lisbon, Ave. Rovisco Pais 1049-001 Portugal. E-mail: jcorreia@civil.ist.utl.pt
2. Full Professor, Dept. of Civil Engineering and Architecture, Instituto Superior Técnico, Technical Univ. of Lisbon, Ave. Rovisco Pais 1049-001 Portugal (corresponding author). E-mail: fbranco@civil.ist.utl.pt