1. School of Civil Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu, P.R. China
2. State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Jiaotong University, Xi’an, Shaanxi, P.R. China
3. School of Science, Lanzhou University of Technology, Lanzhou, Gansu, P.R. China