1. Professor, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, P.R. China (corresponding author).
2. Associate Professor, College of Architecture and Civil Engineering, Beijing Univ. of Technology, Beijing 100124, P.R. China.
3. Ph.D. Candidate, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, P.R. China.
4. Master, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, P.R. China.