1. Ph.D. Student, Dept. of Civil and Environmental Engineering, Carnegie Mellon Univ., Pittsburgh, PA 15213.
2. Research Assistant, Dept. of Civil and Environmental Engineering, Carnegie Mellon Univ., Pittsburgh, PA 15213.
3. Assistant Professor, Civil Engineering, The Pennsylvania State Univ., University Park, PA 16802.
4. Duquesne Light Company University Professor of Engineering, Dept. of Civil and Environmental Engineering, Carnegie Mellon Univ., Pittsburgh, PA 15213 (corresponding author).