The state of practice of the seismic design of bridges is continually evolving, and the AASHTO Guide Specifications for LRFD Seismic Bridge Design were developed to incorporate improvements in the practice that have emerged since publication of ATC 6, Seismic Design Guidelines for Highway Bridges, the basis of the current AASHTO seismic design provisions. While small improvements have been incorporated into the AASHTO seismic design procedures in the intervening years since ATC 6 was published in 1981, the publication of the First Edition of these Guide Specifications in 2009 and related changes to the current AASHTO LRFD Bridge Design Specifications represented a major overhaul of the AASHTO procedures.

The development of the First Edition of these Guide Specifications was performed in accordance with the recommendations of the NCHRP 20-07/Task 193 Task 6 Report. The Task 6 effort combined and supplemented existing completed efforts (i.e., AASHTO Standard Specifications Division I-A, NCHRP 12-49 guidelines, SCDOT specifications, Caltrans Seismic Design Criteria, NYCDOT Seismic Intensity Maps (1998), and ATC-32) into a single document that could be used at a national level to design bridges for seismic effects. Based on the Task 6 effort and that of a number of reviewers, including representatives from state departments of transportation, the Federal Highway Administration, consulting engineers, and academic researchers, these Guide Specifications were developed.

These Guide Specifications were subsequently updated in 2011 as a Second Edition to make revisions based on observations from use of the First Edition of the Guide Specifications. The Second Edition included the addition of design flowcharts.

This Third Edition of the Guide Specifications involves the following changes:

• Adopts a risk-targeted approach for development of a design spectrum.
• Updates the 2002 AASHTO Seismic Ground Motion Maps to 2018 United States Geological Survey (USGS) National Seismic Hazard Model (NSHM), including the direct use of site class based on the time-averaged shear wave velocity in the upper 100 feet of geologic profile.
• Adds additional site classifications to provide a smoother transition between site classes.
• Provides a webtool for accessing the updated Seismic Design Ground Motion Database for 22 periods of ground motion response between 0 s and 10 s at 5 percent damping.
• Identifies additional procedures and makes modifications to account for the risk-targeted ground motions being used in these updated Guide Specifications, including the addition of Appendix C with background on the risk-targeting procedure.