1. Ph.D. Student, College of Civil Engineering, Hunan Univ., Changsha 410082, China.
2. Professor, College of Civil Engineering, Hunan Univ., Changsha 410082, China (corresponding author).
3. Professor, Dept. of Civil and Environmental Engineering, Univ. of California, Davis, CA 95616; Distinguished Visiting Professor, Nanjing Tech Univ., Nanjing 211816, China.