1. Civil Engineer, U.S. Army Corps of Engineers, 190 5th St. East, Suite 401, St. Paul, MN 55101 (corresponding author).
2. Professor, Dipartimento di Ingegneria Strutturale, Politecnico di Milano, 20133 Milano, Italy.
3. MSES/Miles Kersten Professor, Dept. of Civil Engineering, Univ. of Minnesota, Minneapolis, MN 55455.