1. Assistant Professor, Dept. of Civil and Construction Engineering, National Taiwan Univ. of Science and Technology, Taipei, Taiwan 106 (corresponding author).
2. Postgraduate Student, Dept. of Civil Engineering, National Cheng Kung Univ., Tainan, Taiwan 701.
3. Postgraduate Student, College of Civil Engineering, Hunan Univ., Changsha, Hunan 410082, P.R. China.