1. McCormick School Professor and W.P. Murphy Professor of Civil Engineering and Materials Science, Northwestern Univ., 2145 Sheridan Rd., CEE/A135, Evanston, IL 60208.
2. Visiting Fellow, Northwestern Univ., 2145 Sheridan Rd., CEE/A135, Evanston, IL 60208; on leave from Ecole Polytechnique, Palaiseau, France.