1. Professor, College of Civil Engineering, Hunan Univ., Changsha, Hunan 410082, China (corresponding author).
2. Research Assistant, College of Civil Engineering, Hunan Univ., Changsha, Hunan 410082, China.
3. Edwin B. and Norma S. McNeil Distinguished Professor, Dept. of Civil and Environmental Engineering, Louisiana State Univ., Baton Rouge, LA 70803.