Why are ancient buildings still standing 1500+ years later, but some buildings built like 30 years ago are falling apart?
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Not all 1500+ year old buildings are standing, in fact vast majority of them have collapsed ages ago. Only the rare ones that were built a certain way and cared for well enough are left.
One thing most remaining ancient structures have in common is that they are built to only withstand compressive stress. A pyramid is already a pile of rocks, what is supposed to happen to it? It can’t exactly tip over.
The problem with compressive structures is that they are very inefficient in terms of space and cost. For example the very old ones that predate invention of proper arches had to resort to this sort of thing: [https://www.magzter.com/stories/Education/Ancient-Egypt/The-Monumental-Vaults-in-Pyramids-A-Major-Technical-Challenge](https://www.magzter.com/stories/Education/Ancient-Egypt/The-Monumental-Vaults-in-Pyramids-A-Major-Technical-Challenge) They did not know how to make a flat ceiling in such a place so that it wouldn’t collapse. They didn’t even know how to make arches (ancient aliens and forgotten advanced civilizations my arse).
Something more romanesque would heavily use arches, making for much lighter structures with much more of the volume being actually usable [https://en.wikipedia.org/wiki/Romanesque_art#/media/File:Leon_(San_Isidoro,_pante%C3%B3n).jpg](https://en.wikipedia.org/wiki/romanesque_art#/media/file:leon_(san_isidoro,_pante%c3%b3n).jpg)
In contrast, modern modern buildings are basically all usable volume and the amount of material used is next to nothing. [https://www.designingbuildings.co.uk/wiki/Concrete_frame](https://www.designingbuildings.co.uk/wiki/concrete_frame) The cost of it though is that the floor panels are all under tension, the rebar in the concrete is what’s holding it together, without that composite structure it would be impossible to build like that. It also means that such structures will not last forever, a building is experiencing dynamic loading from all sorts of sources and that means the rebar will fatigue and the concrete will eventually fail. There will be no thousand year old skyscrapers.
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