1. Assistant Professor, Dept. of Civil Engineering, Kyungpook National Univ., 1370 Sangyeok-dong, Buk-gu, Daegu 702-701, Korea (corresponding author).
2. Professor, Dept. of Civil Engineering, Kyungpook National Univ., 1370 Sangyeok-dong, Buk-gu, Daegu 702-701, Korea.