1. Professor, Dept. of Civil Engineering, Yonsei Univ., Seoul 120-749, Korea (corresponding author).
2. Graduate Research Assistant, Dept. of Civil Engineering, Yonsei Univ., Seoul 120-749, Korea.
3. Professor, Dept. of Civil Engineering, Yonsei Univ., Seoul 120-749, Korea.
4. Professor, Dept. of Civil Engineering, Univ. of Tokyo, Tokyo 113, Japan.