1. College of Civil Engineering, Tongji Univ., 1239 Siping Rd., Shanghai 200092, China.
2. Professor, College of Civil Engineering, Tongji Univ., 1239 Siping Rd., Shanghai 200092, China (corresponding author). E-mail: gqli@mail.tongji.edu.cn;
3. Professor, Dept. of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, U.K.