1. McCormick School Professor and W.P. Murphy Professor of Civil Engineering and Materials Science, Northwestern Univ., 2145 Sheridan Rd., Tech A135, Evanston, IL 60208.
2. Ramón y Cajal Fellow, ETSECCPB-ETCG, Univ. Politecnica de Catalunya, Jordi Girona 1-3, Ed.D2 D.305, Barcelona 08034, Spain.formerly, Visiting Scholar, Northwestern Univ., 2145 Sheridan Rd., Evanston, IL 60208.