1. Professor, Dept. of Civil and Environmental Engineering, Duke Univ., Hudson Hall, Research Dr., Box 90287, Durham, NC 27708-0287 (corresponding author).
2. Research Assistant, Dept. of Civil and Environmental Engineering, Duke Univ., Hudson Hall, Research Dr., Box 90287, Durham, NC 27708-0287.
3. Professor, Dept. of Civil and Environmental Engineering, Duke Univ., Hudson Hall, Research Dr., Box 90287, Durham, NC 27708-0287.