1. Professor, Key Laboratory of Structural Engineering and Vibration of China Education Ministry, Dept. of Civil Engineering, Tsinghua Univ., Beijing, China 100084.
2. Research Assistant, Key Laboratory of Structural Engineering and Vibration of China Education Ministry, Dept. of Civil Engineering, Tsinghua Univ., Beijing, China 100084 (corresponding author).
3. Professor, Dept. of Civil and Environmental Engineering, Louisiana State Univ., Baton Rouge, LA 70803.