1. Senior Researcher, Dept. of Structural Engineering, Korea Institute of Machinery and Materials, Yusung, Daejeon, 305-600 Korea (work performed while a visiting scholar at the Univ. of Illinois).
2. Professor, Dept. of Aeronautical and Astronautical Engineering, Univ. of Illinois, Urbana, IL 61801.
3. Associate Professor, Dept. of Aeronautical and Astronautical Engineering, Univ. of Illinois, Urbana, IL 61801.