1. School of Civil Engineering, Southwest Jiaotong University, Chengdu 610031, China
2. Department of Civil and Environmental Engineering, Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208
3. College of Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0360
4. Department of Civil and Environmental Engineering, Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208 e-mail: