1. Ph.D. Student, Dept. of Civil Engineering, Tsinghua Univ., Beijing 10084, PR China.
2. Professor, Dept. of Civil Engineering, Tsinghua Univ., Beijing 10084, PR China.
3. Research Associate, Dept. of Civil and Environmental Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK.
4. Professor, Dept. of Civil and Environmental Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK (corresponding author).
5. Structural Engineer, China Electric Power Research Institute, Beijing 100055, PR China.