1. Professor, Dept. of Civil Engineering, Daegu Univ., Jillyang, Gyeongsan, Gyeongbuk, 712-714, South Korea (corresponding author).
2. Emeritus Professor, Dept. of Civil and Environmental Engineering, Univ. of Illinois at Urbana-Champaign, 205 N. Mathews Ave., Urbana, IL 61801.