1. Consultant, 3055 23rd Ave., San Francisco, CA.
2. Assistant Professor, Dept. of Civil & Environmental Engineering, Michigan Technological Univ., 1400 Townsend Dr., Houghton, MI 49931 (corresponding author).
3. Dept. of Civil, Environmental and Architectural Engineering, Cooperative Institute for Research in Environmental Sciences, Univ. of Colorado, Boulder, CO 80309.