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