1. Professor, Dept. of Civil and Environmental Engineering, National Univ. of Singapore, 10 Kent Ridge Crescent, Singapore, Republic of Singapore 119260 (corresponding author).
2. Research Student, Dept. of Civil and Environmental Engineering, National Univ. of Singapore, 10 Kent Ridge Crescent, Singapore, Republic of Singapore 119260.
3. Lecturer, Dept. of Civil and Environmental Engineering, National Univ. of Singapore, 10 Kent Ridge Crescent, Singapore, Republic of Singapore 119260.