1. Ph.D. Student, Dept. of Civil Engineering, Shanghai Jiaotong Univ., Shanghai 200240, P. R. China (corresponding author).
2. Professor, Dept. of Civil Engineering, Shanghai Jiaotong Univ., Shanghai 200240, P. R. China.
3. Graduate Student, Dept. of Civil Engineering, Shanghai Jiaotong Univ., Shanghai 200240, P. R. China.