1. Ph.D. Student, Dept. of Mechanics and Engineering Science, School of Mathematics, Jilin Univ., Changchun 130012, P.R. China.
2. Associate Professor, Dept. of Building and Construction, City Univ. of Hong Kong, Tat Chee Ave., Kowloon, Hong Kong, P.R. China (corresponding author).
3. Professor, Dept. of Mechanics and Engineering Science, School of Mathematics, Jilin Univ., Changchun 130012, P.R. China.