1. Professor, Instituto de Ingenieria, Univ. Nacional Autonoma de Mexico, Apartado Postal 70-290, Coyocan CP 04510, Mexico City, México (corresponding author). E-mail: mrod@servidor.unam.mx
2. Dept. of Structural Engineering, Univ. of California, San Diego, CA 92093. E-mail: jrestrepo@ucsd.edu
3. School of Mines, National Univ. of Colombia at Medellín, Colombia. E-mail: jjblandon@unalmed.edu.co