1. Assistant Professor, Dept. of Civil and Environmental Engineering, National Univ. of Singapore, Engineering Dr. 2, E1A-07-03, Singapore 117576 (corresponding author).
2. Professor, Dept. of Architecture and Building Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8550, Japan.
3. Professor, Dept. of Engineering, Univ. of Cambridge, Schofield Centre, Madingley Rd., High Cross, Cambridge CB3 0EL, U.K.