1. Lecturer, School of Civil Engineering, Beijing Jiaotong Univ., Beijing 100044, P.R. China (corresponding author).
2. Professor, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, P.R. China.
3. Professor, School of Civil and Environmental Engineering, Univ. of New South Wales, Sydney, NSW 2052, Australia.
4. Ph.D. Candidate, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, P.R. China.