1. Assistant Professor, Dept. of Civil Engineering, Johns Hopkins Univ., 3400 N. Charles St., Baltimore, MD 21218 (corresponding author).
2. Nathan M. and Anne M. Newmark Endowed Chair of Civil Engineering, Dept. Civil and Environmental Engineering, Univ. of Illinois at Urbana-Champaign, 205 N. Mathews Ave., Urbana, IL 61801.
3. William J. and Elaine F. Hall Endowed Professor of Civil Engineering, Dept. Civil and Environmental Engineering, Univ. of Illinois at Urbana-Champaign, 205 N. Mathews Ave., Urbana, IL 61801.