1. Applied Mechanics and Structure Safety Key Laboratory of Sichuan ProvinceSchool of Mechanics and EngineeringSouthwest Jiaotong University Chengdu Sichuan 610031 P. R. China
2. State Key Laboratory of Traction PowerSouthwest Jiaotong University Chengdu Sichuan 610031 P. R. China
3. Key Laboratory of Mechanics on Environment and Disaster in Western ChinaThe Ministry of Education of China Lanzhou Gansu 730000 P. R. China
4. Department of Mechanics and Engineering ScienceCollege of Civil Engineering and MechanicsLanzhou University Lanzhou Gansu 730000 P. R. China