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As a European this argument is a bit strange to me (since we have so many structures that are hundreds of years old and are still in use today). Don't people sometimes just want something that works and keeps on working? A monument of engineering to be proud of? Something to show to visitors? And on the economic side of things I'm not at all convinced that rebuilding every century would be cheaper for something like a bridge, considering the cost of closing it down.

Want? Yes, but want to pay for? I like it that I can see a 600+ year old church from my living room, too, and that affected the price of my apartment, but I don't think I would be willing to pay much to have a new building that will be considered iconic in 500 years time in my view.

Most modern bridges (and buildings, for that matter) get designed for 75, maybe 100 years of life. I don't think that is different in Europe. Older ones that still stand typically are sturdier, partly due to the use of larger safety margins by engineers who (according to today's knowledge) didn't know much about materials science, partly due to natural selection (bad designs collapsed or were taken down decades ago)

For every centuries old bridge, scores have been demolished because they couldn't handle increased traffic or just became too expensive to operate. And that happens in Europe, too, even in old-stuff loving Great Britain (http://en.wikipedia.org/wiki/London_Bridge)

Looking at bridges, I think the only ones that will stand for centuries are stone and masonry ones, and those rarely are built anymore. It is way easier to get large spans using reinforced concrete or steel. Both contain metal that rusts. Preventing that is expensive; fully preventing it probably prohibitively so. Even for landmarks bridges such as the Firth of Forth the Brits do not aim for eternal life (http://en.wikipedia.org/wiki/Forth_Bridge#Maintenance: "Network Rail has estimated the life of the bridge to be in excess of 100 years. However, this is dependant [sic] upon NR’s inspection and refurbishment works programme for the bridge being carried out year on year")

If masonry is able to withstand the weather an order of magnitude longer than steel - wouldn't it make sense to reinvent masonry building techniques for building stuff that's expensive to close down? I get that labor cost is prohibitive for this kind of stuff - maybe we will see a renaissance of masonry once we have flexible enough robots to handle it?

I'm not an expert on this, but I wouldn't know of "stuff that is expensive to close down" relative to the extra costs of a masonry bridge.

Let's consider the Golden Gate Bridge as an extreme example (extreme because lots of traffic across it has no reasonable alternative routes). If we needed to replace it, we could build a replacement bridge next to it, connect roads, and then open them over a weekend with little disruption.

If we chose to shove the new bridge in place of the old one in a week or so, that would have to disrupt traffic, but I doubt it would be more expensive than building _and_paying_for_ 20+ piers instead of the two we have _now_ (looking at http://en.wikipedia.org/wiki/List_of_longest_masonry_arch_br..., we cannot build masonry spans over 100m. The arch bridge is the best we can hope for when using stone because stone isn't strong in tension).

Also, such a disruption, announced years in advance, need not be that much of a disruption. People will take a few days of, stay with friends or family, maybe a temporary camp will be set up, etc)

On the other hand, that Wikipedia page mentions that many 50m+ stone arches have been built in China since 1950.

The Golden Gate Bridge is actually a lot less critical than you might imagine. For a really critical bridge, consider the neighbouring San Francisco/Oakland Bay Bridge, which carries about twice the daily traffic. And in fact, the eastern span of the Bay Bridge was replaced last year.

The reason many stone and masonry structures are still standing is two-fold.

One: most of them are not impressive from a structural point of view. Two: most of them have been retrofitted so many times that they're hardly the "original" structure.

Older bridge structures in the US are like this too -- especially old viaducts and masonry bridges built for trains.

They were overbuilt because they didn't have the tools to estimate and calculate loads well, and didn't have alot of choice in materials. Essentially the choices were wood trusses or masonry arches.

I agree with you about lifespan -- look at New York's Tappan Zee bridge as an example -- it's a major traffic corridor, with a bridge with a lousy 50 year lifespan. Replacement will cost something like $10 billion!

It's something deep in American culture I guess. Thomas Jefferson was worried 200 years ago about all the buildings we were making in wood. He predicted the nation would have no architectural heritage, a tabula rasa after 100 years when the wooden structures have to be replaced. Many of the buildings Jefferson designed and built back then then are still going strong, but I guess that sentiment never caught on here.

As an American, I'm extremely curious. What does your nationality have to do with that comment?

I'm a bit confused about your comment.

1. I didn't even mention my nationality.

2. Right in my post I explained, why growing up in Europe might change perspective, since we're surrounded by old structures still in use.

3. To extend on this point: I do believe that the thinking here is a bit different when it comes to longevity of buildings. Hence why most homes here are made out of bricks or concrete with heavy foundations and a cellar, while the typical American home seems to be built out of wood and doesn't have a cellar. The abundance of land is probably heavily factoring into this, so more people can afford to buy land and build a house on it[1].

[1] E.g. Switzerland, my home country, has a home ownership of only 34% vs. 67% in the USA.

The occurrence of cellars is dependent more on the geography in the US. For example in Florida where the water table is very high, it makes little sense to dig a cellar since it would constantly have to be pumped out. However in the midwest and north east cellars are quite common.

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