1. Graduate Research Assistant, School of Civil and Environmental Engineering, Seoul 120-749, Korea.
2. Associate Professor, Dept. of Civil and Environmental Engineering, Yonsei Univ., Seoul 120-749, Korea (corresponding author).
3. Manager, Techbile, Inc., Daegu 701-701, Korea.
4. Assistant Professor, School of Architectural Engineering, Chung-Ang Univ., Seoul 156-756, Korea.