Just curious... Do local state Dept of Transportation store as-built plans of bridges? For buildings, it'd be stored by county assessor (never had luck finding one though).
For local bridges in NYS, the government that owns the bridge is required to keep records and plans about the structure. They would most likely be kept at the office of the superintendent of highways, DPW commissioner, director of engineering, or some similar title. I would expect other states would have similar laws.
@Ac - NYSDOT is very good when it comes to maintaining records; I've even gotten soil boring records from 70+ years ago. However, there are some agencies that can't find plans for what they built 5 years ago.