1. Assistant Professor, Dept. of Mechanics, Faculty of Civil Engineering, Czech Technical Univ. in Prague, Thákurova 7, 16629 Praha 6, Czech Republic; and Visiting Scholar, Northwestern Univ., 2145 Sheridan Rd., Evanston, IL 60208.
2. Graduate Research Assistant, Northwestern Univ., 2145 Sheridan Rd., Evanston, IL 60208.
3. McCormick Institute Professor and W. P. Murphy Professor of Civil and Mechanical Engineering and Materials Science, Northwestern Univ., 2145 Sheridan Rd., CEE/A135, Evanston, IL 60208 (corresponding author).