1. Associate Professor, Dept. of Civil and Environmental Engineering, Univ. of Southern California, Los Angles, CA 90089; Cheung Kong Scholar, Hunan Univ. E-mail: yanxiao@usc.edu
2. Research Assistant, Hunan Univ., China.
3. Graduate Research Assistant, Dept. of Civil and Environmental Engineering, Univ. of Southern California, Los Angles, CA 90089.