1. School of Civil Engineering, Central South University, Changsha 410075, P. R. China
2. National Engineering Laboratory for High Speed Railway Construction, Changsha 410075, P. R. China
3. L. B. Ryon Endowed Chair of Engineering, G. R. Brown School of Engineering, Rice University, 6100 Main Street, Houston, TX 77005, USA
4. Department of Civil Engineering, The Pennsylvania State University, Middletown, PA 17050, USA