Comparison of AASHTO LRFD and ASD specifications for structural design of cantilever abutments and retaining walls

Abstract

The American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specification became the mandatory design specifications for United States highway structures in October 2007. This has introduced a number of changes in mandatory engineering design and analysis procedures. The differences between the allowable stress design (ASD) standard specifications and the LRFD specification are evaluated in this paper, as well as how they apply to earth retaining structure design. The effect of the new design standards on structural proportioning and cost were examined through several design examples. The difference between the LRFD specifications and the ASD standard specifications was insignificant for cantilever abutments and retaining walls 4.6 m and taller. However, an increase in footing length of about 30% and an increase in cost of approximately 10% of the cost of the retaining structures can result from designing shorter abutments and retaining walls using the LRFD specifications. While in some designs, these cost impacts to a structure can be significant, the AASHTO LRFD specifications provides a safe and reliable design method of paramount importance to engineers and the public.