The American Association of State Highway and Transportation Officials recently published interim revisions to the fourth edition of its “Guide Specification for Seismic Isolation Design.”
[Above image by AASHTO]
First published in 2014, these interim revisions address major changes in the state of the art of seismic isolation design for highway bridges and reflects. Specifically, they reflect changes in the way “seismic hazard” is defined in the AASHTO LRFD Bridge Design Specifications, 9th Edition, and the AASHTO Guide Specifications for LRFD Seismic Bridge Design, 2nd Edition.
Those specifications include industry trends in the design and construction of isolators, as well as provisions in the design specifications that impact the design and testing of isolation bearings.
The 2023 interim revisions update the guide specifications to include a risk-targeted approach for the development of a design spectrum and the adoption of the 2018 United States Geological Survey’s National Seismic Hazard Model or NSHM.
The NSHM approach provides for a uniform risk throughout the country, AASHTO noted; differing from the uniform hazard approach used previously, which resulted in a non-uniform risk, due to the variance in the shape of hazard curves with location.