1. Assistant Professor, Dept. of Civil Engineering, Univ. of Minnesota, Minneapolis, MN 55455 (corresponding author).
2. M.S. Student, École Polytechnique, 91128 Palaiseau Cedex, France; formerly, Undergraduate Research Assistant, Dept. of Civil Engineering, Univ. of Minnesota, Minneapolis, MN 55455.
3. Professor, Dept. of Civil Engineering, Univ. of Minnesota, Minneapolis, MN 55455.