1. Research Assistant, Civil Engineering Dept., Polytechnic Institute of Coimbra, Rua Pedro Nunes, 3030-199 Coimbra, Portugal.
2. Professor, Civil Engineering Dept., Univ. of Coimbra, Polo II, Pinhal de Marrocos, 3030-290 Coimbra, Portugal (corresponding author).
3. Professor, Steel and Timber Structures, Faculty of Civil Engineering, Delft Univ. of Technology, P.O. Box 5048, 2600 GA Delft, The Netherlands.