Implementation of the AASHTO LRFD Bridge Design Specifications for Substructure Design

Abstract

Load factor design (LFD) codes have been used in the US since the 1970’s, and in the highway industry since 1977, but their application was limited to the design of superstructure components. Introduction of the AASHTO LRFD Bridge Design Specifications (LRFD Specifications, 2002) in 1994 represents the first instance in the US where load and resistance factor design (LRFD) was codified for the geotechnical design of highway substructure features. While a few state departments of transportation (DOTs) proactively pursued implementation of the LRFD Specifications, the process has been slow. This implementation has been slowed, in part, by resistance to change, lack of applicable software, and inadequate staff training. More importantly, a number of problems have arisen that can be attributed to changes in the LRFD Specifications that resulted in noticeable differences with past design practice. This paper highlights the challenges that have been encountered and the measures taken to correct and implement the LRFD Specifications to gain the confidence of highway owners and designers. 1 LRFD IMPLEMENTATION The LRFD Specifications were first promulgated nearly 10 years ago. When initially published, the American Association of State Highway and Transportation Officials (AASHTO) envisioned full implementation by all DOTs by the year 2000. The leverage applied to drive implementation was the intention to sunset (i.e., no longer publish) the long-standing AASHTO Standard Specifications (Standard Specifications, 2002) that have provided national requirements for highway bridge superstructure and substructure design since the 1930s. However, this did not happen until 2002, and DOTs now have until 2007 to either fully-implement the LRFD Specifications or develop their own alternatives to them. As such, implementation of the LRFD Specifications remains transitional and spotty. Since the mid-1990’s, the Federal Highway Administration (FHWA) has supported LRFD implementation by developing and promoting two LRFD training courses (one for superstructure and the other for substructure design) intended for state DOT staff and their consultant designers. To date, nearly 40 superstructure and 50 substructure courses have been presented. In the process of teaching these courses, some knowledge was gained regarding the level of LRFD implementation. Based on empirical information learned from the substructure design course (Withiam, et al. 1998), reasons cited for delayed implementation include: Unwillingness to change from past practices that have worked well, especially when preferred design methods were not included in the LRFD Specifications Some aspects of the new code were inconsistent with or resulted in more conservative designs than past practice Lack of training Lack of design software Implementation of the LRFD Specifications for superstructure design is much further along than for substructure design. Using information compiled by LEAP Software, 22 state agencies have already implemented the code, 14 have set a date for implementation, 12 are reviewing but have yet to set a date