1. School of Civil EngineeringLanzhou Jiaotong University Lanzhou 730070 Gansu P.R. China
2. State Key Laboratory for Strength and Vibration of Mechanical StructuresXi'an Jiaotong University Xi'an 710049 Shaanxi P. R. China
3. School of ScienceLanzhou University of Technology Lanzhou 730050 Gansu P.R. China