1. Associate Professor, Dept. of Civil Engineering, Federal Univ. of Rio Grande do Sul, Av. Osvaldo Aranha 99-3, Andar, 90035-190 Porto Alegre, Rio Grande do Sul, Brazil (corresponding author).
2. Research Assistant, Federal Univ. of Rio Grande do Sul, 90035-190 Porto Alegre, Rio Grande do Sul, Brazil.
3. Reader, Imperial College, Univ. of London, London, U.K.