1. Assistant Professor, Dept. of Civil Engineering, National Cheng Kung Univ., No. 1 Ta-Hsueh Road, Tainan City 701, Taiwan.
2. Associate Professor, Dept. of Civil Engineering, Univ. of Texas at Austin, Austin, TX 78712 (corresponding author).
3. Professor, Dept. of Civil Engineering, Univ. of Texas, Austin, TX 78712.