New Seismic Retrofit Standards for Pre-1970s Bridges Published ​

The Federal Highway Administration has released updated guidance on seismic retrofit prioritization for pre-1970s highway bridges, which is a polite way of addressing the fact that thousands of American bridges were designed during an era when seismic engineering consisted largely of looking at the ground, shrugging, and adding 10% more rebar.

The new standards incorporate lessons from recent seismic events and updated probabilistic hazard models, including USGS's 2023 National Seismic Hazard Model. For California, this means several hundred bridges that were previously categorized as "adequate" have been gently reclassified to "we should probably get to those." The standards apply nationwide, but let's be honest — this is mostly a California and Pacific Northwest document with some polite nods to the New Madrid Seismic Zone.

What Changed ​

The key update is a revised vulnerability scoring system that now accounts for soil-structure interaction effects and liquefaction susceptibility in the prioritization matrix. Previously, a bridge could score well on structural condition but sit on soil that would turn to pudding in a M7.0 event, and the screening wouldn't catch it. The new methodology fixes this oversight, which is the kind of "well obviously" improvement that takes federal agencies approximately 15 years to implement.

Column retrofit requirements have also been updated to align with current Caltrans Seismic Design Criteria, because California's been doing this right for a while and the feds have finally caught up. Better late than never, FHWA.

California Impact ​

Caltrans has already identified 340 bridges in the state that may need re-evaluation under the new scoring criteria. Most are in the Bay Area and Los Angeles Basin, because of course they are. Preliminary estimates suggest the statewide retrofit backlog just grew by approximately $2.1 billion, which the state will fund with money it definitely has.

The full guidance document is available on the FHWA Bridge Preservation page.