1. McCormick School Professor and W. P. Murphy Professor of Civil Engineering and Materials Science, Northwestern Univ., 2145 Sheridan Rd., Evanston, IL 60208 (corresponding author).
2. Professor, Dept. of Civil Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong, Yusong, Taejon 305-701, Korea.
3. Student, Dept. of Civil Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong, Yusong, Taejon 305-701, Korea.