1. Ph.D. Student, Federal Univ. of Rio Grande do Sul, Av. Osvaldo Aranha, 99, Porto Alegre, RS, 90035-190, Brazil.
2. Associated Professor, Dept. of Civil Engineering, Federal Univ. of Rio Grande do Sul, Av. Osvaldo Aranha, 99, Porto Alegre, RS, 90035-190, Brazil.