1. Assistant Professor, Dept. of Civil Engineering, Yamaguchi Univ., Ube, Yamaguchi 755-8611, Japan.
2. Associate Professor, Dept. of Civil Engineering, Univ. of Auckland, Auckland 1142, New Zealand (corresponding author).
3. Professor, Dept. of Civil Engineering, Yamaguchi Univ., Ube, Yamaguchi 755-8611, Japan.