1. Doctoral Candidate, Univ. of Illinois at Urbana-Champaign, Urbana, IL 61801.
2. Assistant Professor, Ulsan National Institute of Science and Technology, Ulsan 698-798, Korea.
3. Assistant Professor, Univ. of Tokyo, Tokyo 113-8656, Japan.
4. Nathan M. and Anne M. Newmark Endowed Chair in Civil Engineering, Univ. of Illinois at Urbana-Champaign, Urbana, IL 61801 (corresponding author).