When driving over the parkway bridge, and later in the truck the US9 Edison bridge, i had a thought, when is a bridge a new bridge? In the past i have brought this up, but let's have a discussion about it for bridges in general.
Is it a new bridge when it is of a radically different type/construction but only the piers are reused. Examples: sewickley bridge, Milton-Madison Bridge,
Is it the same bridge if the piers are reused and the construction remains almost the same: Edison Bridge (lowered the "peak" of the span but still a girder high-level span) Great Egg Harbor Bridge (no major changes to overall design, just using new girders in place of the 1970s ones)
Is it the same bridge only if a major original component remains in place: NJ 37's drawbridge is the only remaining bit of the 1950s bridge in use on the eastbound span. Everything leading up to it and away from it is brand new and wider as far as i know.
Theseus.
(By the way, it's a good topic. I just don't have an answer of my own.)