1. Lecturer, Dept. of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, U.K. (corresponding author).
2. Full Professor, Dept. of Mechanics and Structures, School of Civil Engineering, Technical Univ. of Madrid, 28040 Madrid, Spain.
3. Full Professor, State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji Univ., Shanghai 200092, China.