The USDA NRCS (US Dept of Agriculture, Natural Resources Conservation Service) primarily works with farmers, and produces lots of guidance for more “diy” engineering. The first link is a state by state compilation of guidance documents, the second is for the nrcs National engineering manual and supplements.

https://efotg.sc.egov.usda.gov/

https://directives.sc.egov.usda.gov/directive/24

Another good source would be searching “xx state DOT standard bridge plans” Like this from caltrans: https://dot.ca.gov/programs/engineering-services/manuals/bridge-standard-details

You could also search terms like “bid” and “procurement” for your area/project of interest- most municipalities bid projects to the public including plans and specs (typically sites like QuestCDN allow you to register for free as a “bidder” and view project documents.)