1. Professor, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, China (corresponding author).
2. Ph.D. Candidate, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, China.
3. Lecturer, Dept. of Civil Engineering, National Univ. of Defense Technology, Changsha, Hunan 410072, China.
4. Career Development Fellow, Institute for Infrastructure Engineering, Western Sydney Univ., Penrith, NSW 2751, Australia.