1. Research Professor, Facultad de Ingeniería y Ciencias Aplicadas, Universidad de los Andes, Monseñor Álvaro del Portillo, Las Condes 12455, Santiago, Chile; presently, Graduate Student, Dept. of Structural Engineering, Univ. of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0085.
2. Graduate Student, Dept. of Structural Engineering, Univ. of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0085.
3. Professor, Dept. of Structural Engineering, Univ. of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0085 (corresponding author).