[1] JSCE, Dept. of Civil Eng. and Arch., Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan
[2] Grad. Sch. of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan
[3] JSCE, Dept. of Civil and Arch. Eng., Yatsushiro Coll. of Tech., 2627 Hirayama-Shinmachi, Yatsushiro 866-8501, Japan