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