1. Department of Civil and Environmental Engineering, Korea University, 5 Ga 1, An-Am Dong, Sung-Buk Gu, Seoul, 136-701, Korea
2. Livermore Software Technology Corporation, 7374 Las Positas Road, Livermore, CA 94550, USA
3. Department of Mechanical Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3111, USA