1. Professor, Dept. of Civil Engineering, Beihang Univ., Beijing 100191 (corresponding author).
2. Ph.D. Student, Dept. of Civil Engineering, Beihang Univ., Beijing 100191.
3. Assistant Professor, Dept. of Civil and Environmental Engineering, Univ. of Windsor, Windsor, Ontario, Canada, N9B 3P4.
4. Professor, Dept. of Civil Engineering, Beihang Univ., Beijing 100191.