1. Graduate Student, Dept. of Structural Engineering, Technical Univ. (Politecnico) of Milan, Milan 20133, Italy.
2. McCormick School Professor and W. P. Murphy Professor of Civil Engineering and Materials Science, Northwestern Univ., Evanston, IL 60208.
3. Professor of Structural Engineering, Dept. of Structural Engineering, Technical Univ. (Politecnico) of Milan, Milan 20133, Italy.