1. Graduate Student, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, China.
2. Professor, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, China (corresponding author).
3. Professor, Dept. of Civil and Environmental Engineering, Univ. of Maryland, College Park, MD 20742.
4. Engineer, Glodon Company Limited, E-13, 10 Xibeiwang East Rd., Beijing 100084, China.
5. Graduate Student, Dept. of Civil and Environmental Engineering, Univ. of Maryland, College Park, MD 20742.