1. Graduate Research Assistant, Dept. of Civil and Environmental Eng., Univ. of Illinois at Urbana-Champaign, 205 N. Matthews, Urbana, IL 61801.
2. Associate Professor, Dept. of Civil and Environmental Eng., Univ. of Illinois at Urbana-Champaign, 205 N. Matthews, Urbana, IL 61801 (corresponding author).