1. McCormick Institute Professor and W. P. Murphy Professor of Civil Engineering and Materials Science, Dept. of Civil Engineering and Mechanical, Northwestern Univ., Evanston, IL 60208 (corresponding author).
2. Assistant Professor, Univ. of Pittsburgh, Pittsburgh, PA 15261; formerly, Assistant Research Professor, Dept. of Civil Engineering and Materials Sciences, Northwestern Univ., Evanston, IL 60208.