1. Professor, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, China (corresponding author).
2. Postdoctoral Fellow, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, China.
3. Senior Lecture, Dept. of Civil, Mining, and Environmental Engineering, Wollongong Univ., Northfields Ave., Wollongong, NSW 2522, Australia.