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. Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, China; Associate Lecturer, School of Civil Engineering, Univ. of Sydney, Sydney, NSW 2006, Australia.
4. Professor, School of Civil Engineering, Shenyang Jianzhu Univ., Shenyang 110168, China.