1. State Key Lab. of Struct. Anal. of Engrg., Dept. of Engrg. Mech., Dalian Univ. of Technol., Dalian 116023, P. R. China.
2. State Key Lab. of Struct. Anal. of Engrg., Dept. of Engrg. Mech., Dalian Univ. of Technol., Dalian 116023, P. R. China; corresponding author. E-mail: haitian@ dlut.edu.cn