1. Department of Civil Engineering; Sharif University of Technology; P.O. Box 11155-9313; Tehran; Iran
2. Structural Engineering Department; Int. Institute of Earthquake Eng. and Seismology; P.O. Box 3913/19395; Tehran; Iran
3. Civil and Environmental Engineering Department, 5731E Boelter Hall; University of California; Los Angeles; CA; 90095; USA