1. Professor, Dept. of Civil Engineering, National Chung Hsing Univ., Taichung 402, Taiwan.
2. Graduate Student, Dept. of Civil Engineering, National Chung Hsing Univ., Taichung 402, Taiwan.
3. Associate Professor, Dept. of Civil Engineering, Texas A&M Univ., College Station, TX 77843-3136 (corresponding author).