1. Civil Engineering Laboratory, Central Research Institute of Electric Power Industry, 1642, Abiko 270-1194, Chiba, Japan e-mail:
2. Graduate School of Engineering Science, Osaka University, 1-3, Machikanayama-cho, Toyonaka 560-8531, Osaka, Japan e-mail:
3. Tokyo Electric Power Company Holdings, Inc., 4-1 Egasaki-cho, Tsurumiki-ku, Yokohama 230-8510, Kanagawa, Japan e-mail: