1. Professor, Dept. of Civil and Environmental Engineering, Stanford Univ., Stanford, CA 94305-4020 (corresponding author).
2. Professor, Dept. of Civil and Environmental Engineering, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061-0105.
3. Retired, Advanced Research and Development, John Deere Construction Equipment Division, 33 Spring Creek Lane, Galena, IL 61036; formerly, Manager.