1. Professor, Dept. of Bridge Engineering, Southwest Jiaotong Univ., Chengdu 610031, PR China (corresponding author).
2. Ph.D. Candidate, Dept. of Bridge Engineering, Southwest Jiaotong Univ., Chengdu 610031, PR China.
3. Associate Professor, Dept. of Civil and Environmental Engineering, Colorado School of Mines, Golden, CO 80401.