1. Research Engineer, AMEC Dynamic Structures Ltd., 1515 Kingsway Ave., Port Coquitlam, BC V3C 1S2, Canada. E-mail: ye.zhou@amec.com
2. Professor, Dept. of Civil Engineering, Univ. of British Columbia, 6250 Applied Science Lane, Vancouver, BC V6T 1Z4, Canada. E-mail: sigi@civil.ubc.ca