1. Assistant Professor, Civil Engineering Dept., Middle East Technical Univ., Building No. K6-111, 06531 Balgat Ankara, Turkey (corresponding author).
2. Willett Professor, Director of Mid-America Earthquake Center, Newmark Civil Engineering Laboratory, Civil and Environmental Engineering Dept., Univ. of Illinois at Urbana-Champaign, 205 Mathews Ave., Urbana, IL 61801-2352.