1. Graduate Research Assistant, Dept. of Civil Engineering, Univ. of Nebraska at Lincoln, 225 PKI, 1110 S. 67th St., Omaha, NE 68182-0178.
2. Professor, Dept. of Civil Engineering, Univ. of Nebraska at Lincoln, 203F PKI, 1110 S. 67th St., Omaha, NE 68182-0178 (corresponding author).
3. U.S. Army Research Laboratory, Weapons and Materials Research Directorate, Aberdeen Proving Ground, MD 21005-5069.