1. Research Scholar, Division of Structures and Mechanics, School of Civil and Environmental Engineering, Nanyang Technological Univ., 50 Nanyang Ave., Singapore 639798. E-mail: sbhalla@pmail.ntu.edu.sg.
2. Professor, Division of Structures and Mechanics, School of Civil and Environmental Engineering, Nanyang Technological Univ., 50 Nanyang Ave., Singapore 639798 (corresponding author). E-mail: csohck@ntu.edu.sg.