1. Postdoctoral Researcher, Pacific Earthquake Engineering Research Center, Dept. of Civil and Environmental Engineering, Univ. of California, Berkeley, R126, Bldg. 454, 1301 South 46th St., Richmond, CA 94804 (corresponding author).
2. Byron L. and Elvira E. Nishkian Professor of Structural Engineering and Director of the Pacific Earthquake Engineering Research Center, Dept. of Civil and Environmental Engineering, Univ. of California, Berkeley, 777 Davis Hall, Berkeley, CA 94720.