1. Assistant Professor, Dept. of Civil Engineering, Univ. of British Columbia, 6250 Applied Science Lane, Vancouver BC, Canada V6T 1Z4 (corresponding author). E-mail: terje@civil.ubc.ca
2. Taisei Professor of Civil Engineering, Dept. of Civil and Environmental Engineering, Univ. of California at Berkeley, 721 Davis Hall, Berkeley, CA 94720-1710. E-mail: adk@ce.berkeley.edu