1. R&D Center, Samsung Engineering Co. Ltd., 415-10 Woncheon-Dong, Youngtong-Gu, Suwon, Gyeonggi-Do 443-823, Korea
2. Department of Civil and Environmental Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 133-791, Korea, Tel. ; Fax: