1. Graduate Student, Dept. of Civil and Environmental Engineering, Univ. of California, 5731 Boelter Hall, Los Angeles, CA 90095.
2. Associate Professor, Dept. of Civil and Environmental Engineering, Univ. of California, 5731 Boelter Hall, Los Angeles, CA 90095 (corresponding author).
3. Professor Emeritus, Dept. of Civil Engineering, Brigham Young Univ., Provo, UT 84602-4081.
4. Professor, National Chung-Hsing Univ., Taichung, Taiwan.