1. Professor, Dept. of Civil Engineering, Federal Univ. of Rio Grande do Sul, Porto Alegre 90035-190, Brazil (corresponding author).
2. Research Assistant, Dept. of Civil Engineering, Federal Univ. of Rio Grande do Sul, Porto Alegre 90035-190, Brazil.
3. Professor, Dept. of Civil Engineering, Federal Univ. of Rio Grande do Sul, Porto Alegre 90035-190, Brazil.