1. Professor, Dept. of Civil Engineering, King Saud Univ., P.O. Box 800, Riyadh 11421, Saudi Arabia.
2. Dept. of Civil Engineering, Helwan Univ., Cairo, Egypt; presently, Assistant Professor, Dept. of Civil Engineering, King Saud Univ., P.O. Box 800, Riyadh 11421, Saudi Arabia (corresponding author).