1. Ph.D. Student, Dept. of Civil and Environmental Engineering, Princeton Univ., E-201, E-Quad, Princeton, NJ 08544 (corresponding author). E-mail: zhihuaw@princeton.edu
2. Associate Professor, Nanyang Technological Univ., School of Civil and Environmental Engineering, BLK. N1, #01a-37, 50 Nanyang Ave., Singapore 639798, Singapore.