1. Ph.D. Candidate, College of Civil Engineering, Hunan Univ., Changsha 410082, P.R. China.
2. Professor, College of Civil Engineering, Hunan Univ., Changsha 410082, P.R. China (corresponding author).
3. Professor, Dept. of Civil Engineering, Nanjing Univ. of Aeronautics and Astronautics, Nanjing 210016, P.R. China.