1. Prof., Dept. of Constr. Engrg., Nat. Taiwan Univ. of Sci. and Technol., P.O. Box 90-130, Taipei, Taiwan. E-mail: sjchen@mail.ntust.edu.tw (corresponding author).
2. Grad. Student, Dept. of Constr. Engrg., Nat. Taiwan Univ. of Sci. and Technol., P.O. Box 90-130, Taipei, Taiwan.