1. Ph.D. Candidate, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, China.
2. Professor, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, China (corresponding author).
3. Associate Professor, Dept. of Civil Engineering, Monash Univ., Melbourne, VIC 3800, Australia.
4. Professor, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, China.