1. School of Civil Engineering, Southwest Jiaotong Univ., Chengdu 610031, China.
2. Professor, School of Civil Engineering, Shijiazhuang Tiedao Univ., Shijiazhuang 050043, China.
3. Professor, School of Civil Engineering, Southwest Jiaotong Univ., Chengdu 610031, China.
4. Ph.D. and Scientific Staff, Dept. of Civil and Environmental Engineering Sciences, Ruhr-Universität Bochum, Universitätsstraße 150, D-44780 Bochum, Germany (corresponding author).