[bomberjet]

Normal bridges are typically designed with a 75 year life span. If you're building something huge and complex, it should be designed to 100 years at least. Cost goes up of course. The bridges will likely last longer than that. But that's when you get into things beyond their service life that are falling down.

That's typical of today's construction. Bridge decks should last 25 years. It's a cost benefit thing. It'll be really expensive and early impossible to build a bridge deck that'll last 75 years. So build it cheaper and replace it twice during the bridge's life.

We're not building pyramids or roman aqueducts out of solid stone.