1. Graduate Research Assistant, School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0355; current address: Institute A for Mechanics, Univ. of Stuttgart, D-70569 Stuttgart, Germany.
2. Professor, School of Civil and Environmental Engineering, and G. W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0355 (corresponding author).
3. Professor, G. W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405.