1. Ph.D. Student, School of Civil Engineering, Chongqing Univ., Chongqing 400045, P.R. China.
2. Professor, School of Civil Engineering, Chongqing Univ., Chongqing 400045, P.R. China (corresponding author).
3. Ph.D. Student, Dept. of Engineering, Univ. of Cambridge, Cambridge CB2 1PZ, U.K.