1. Professor, College of Civil Engineering, Hunan Univ., Changsha 410082, P.R. China.
2. Associate Professor, College of Civil Engineering and Architecture, Central South Univ., Changsha 410075, P.R. China.
3. Postdoctoral Research Associate, Dept. of Civil and Environmental Engineering, Univ. of Illinois at Urbana-Champaign, Urbana, IL 61801.
4. Nathan Newmark Professor, Dept. of Civil and Environmental Engineering, Univ. of Illinois at Urbana-Champaign, Urbana, IL 61801.