1. Researcher, Central Research Institute of Electric Power Industry, Abiko, Chiba 270-1194, Japan.
2. Assistant Professor, Division of Engineering, Colorado School of Mines, Golden, CO 80401.
3. Nathan M. Newmark Professor of Civil Engineering, Dept. of Civil and Environmental Engineering, Univ. of Illinois at Urbana–Champaign, Urbana, IL 61801 (corresponding author).
4. Professor, Dept. of Civil Engineering, Washington Univ., St. Louis, MO 63130.