Do you mean 50 years without maintenance? I would be shocked if major bridges weren't designed to last indefinitely.
The average age of a bridge in the USA is 43 years. But we also have an epidemic of unmaintained bridges.
No bridge made today is designed to last indefinitely. Many different forces will degrade the bridge over time, even with maintenance. Steel stresses weaken it over time. Concrete weathers over time due to salt, chemicals, water, wind, and the steel reinforcements tend to corrode eventually.
Stone bridges may last for an exceptionally long time, but their weight and expense makes them only useful in limited applications, typically as small rail overpasses. Ones that were designed for horse and buggy end up slowly failing as heavier trucks and cars in traffic weaken them.
The same fate lies for timber bridges. When well maintained they can last for 75 years, but it's expensive and requires certain skills. They were also mostly designed before heavier cars and trucks, and for less traffic. Most famous covered bridges today are being closed to traffic due to increased wear.
FHWA provides up to $7 Billion in assistance in maintaining bridges. But it would take at least $125 Billion to begin dealing with the structurally deficient bridges in the USA, according to the ASCE Infrastructure Report Card[1].
Add to this the fact that many bridges in the US are privately owned, and it's the responsibility of the bridge owner to maintain them. But guess what's not profitable for a private enterprise? Maintaining bridges. Despite this, train derailments and property damage continue every year due to unmaintained bridge and train infrastructure.
The executive has proposed funding for repairs, but as usual, it's not enough[2]. So far they've gotten almost half that number of bridges repaired, and pledged another $300M,[3] which is still just a drop in the bucket of what's needed of the backlog; it doesn't address all the new maintenance that will be needed each year.
According to the American Road & Transportation Builders Association, there are 167 million crossings on 42,400 bridges rated in poor condition. [4]
[1] https://infrastructurereportcard.org/cat-item/bridges-infras... [2] https://www.nbcnews.com/news/us-news/biden-has-plan-fix-amer... [3] https://www.whitehouse.gov/briefing-room/statements-releases... [4] https://artbabridgereport.org/congressional/map
Is it impossible for these things to stop happening? No, not at all. But it's probably not going to stop, in this country, because we let private industry get away with whatever they want. If this were the UK the story would be very different.
I mainly made that comment because it still blows my mind the the City Bridge Foundation has been going for 740 years
Though to be clear designed to last 50 years isn't the same as saying it will only last exactly 50 years, or that theirs nothing you can do to extend a bridges lifespan if it's replacement is running a little late.