1. Research Associate, Laboratorio de Estructuras, Univ. Nacional de Tucumán, Av. Roca 1800, (4000) San Miguel de Tucumán, Tucumán, Argentina.
2. Assistant Professor, Dept. of Civil and Environmental Engineering, Univ. of Southern California, Los Angeles, CA 90089-2531 (corresponding author).
3. Linbeck Professor, Dept. of Civil Engineering and Geo. Sci., Univ. of Notre Dame, Notre Dame, IN 46556-0767.