1. Research Assistant, Dept. of Civil Engineering, Univ. of Minnesota, MN 55455-0116 (corresponding author).
2. James L. Record Professor and Head, Dept. of Civil Engineering, Univ. of Minnesota, MN 55455-0116.
3. Assistant Professor, Dept. of Structural Engineering, Politecnico di Milano, Piazza L. Da Vinci, Milan 20133, Italy.