1. Professor, School of Civil Engineering, Purdue Univ., West Lafayette, IN 47907 (corresponding author).
2. Professor, Dept. of Mineral Engineering, New Mexico Tech, Socorro, NM and Dept. of Civil Engineering, Tarbiat Modarres Univ., Tehran 87801, Iran.
3. Computational Physics Group, Lawrence Livermore National Laboratory, Livermore, CA 94551.
4. Computational Physics Group, Lawrence Livermore National Laboratory, Livermore, CA 94451.
5. Professor, Dept. of Civil Engineering, Univ. of Texas at Austin, Austin, TX 78712.