1. McCormick School Professor and W. P. Murphy Professor of Civil Engineering and Materials Science, Northwestern Univ., Evanston, IL 60208.
2. Research Associate, Dept. of Structural Engineering, Technical Univ. (Politecnico) of Milan, Milan 20133, Italy.
3. Professor, Dept. of Structural Engineering, Technical Univ. (Politecnico) of Milan, Milan 20133, Italy; formerly, Visiting Fellow, Northwestern Univ., Evanston, IL 60208.