1. Associate Professor, Dept. of Civil Engineering and Engineering Mechanics, Columbia Univ., 500 West 120th St., New York, NY 10027.
2. Graduate Research Assistant, Dept. of Civil Engineering and Engineering Mechanics, Columbia Univ., New York, NY 10027.
3. Associate Professor, Dept. of Civil and Environmental Engineering, Univ. of Delaware, Newark, DE 19716.
4. Professor, Dept. of Civil and Environmental Engineering, Univ. of Delaware, Newark, DE 19716.