1. Ph.D. Candidate, Dept. of Civil Engineering, National Taiwan Univ., 1, Sec. 4, Roosevelt Rd., Taipei City 106, Taiwan.
2. Professor, Dept. of Civil Engineering, National Taiwan Univ., 1, Sec. 4, Roosevelt Rd., Taipei City 106, Taiwan (corresponding author).
3. Professor, Dept. of Civil Engineering and Hydrotech Research Institute, National Taiwan Univ., 1, Sec. 4, Roosevelt Rd., Taipei City 106, Taiwan.