1. Professor, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, P.R. China (corresponding author).
2. Postdoctoral Fellow, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, P.R. China.
3. Associate Professor, College of Transportation and Civil Engineering, Fujian Agriculture and Forestry Univ., Fuzhou 350002, P.R. China.