1. Professor, Dept. of Civil Engineering, National Taiwan Univ., Taipei, Taiwan 10617 (corresponding author).
2. Graduate Student, Cornell Univ., Ithaca, NY 14850; Formerly, Graduate Student, Dept. of Civil Engineering, National Taiwan Univ.
3. Graduate Student, Univ. of Illinois, Urbana-Champaign, IL 61801; Formerly, Graduate Student, Dept. of Civil Engineering, National Taiwan Univ.
4. Distinguished Professor, Dept. of Civil Engineering, National Taiwan Univ.