1. Research Assistant, Univ. of Minho, C-TAC Research Unit, 4800 Guimarães, Portugal (corresponding author).
2. Associate Professor, Dept. of Civil Engineering, Univ. of Beira Interior, 6200 Covilhã, Portugal.
3. Full Professor, Dept. of Civil Engineering, Univ. of Minho, 4800 Guimarães, Portugal.