Why are ancient buildings still standing 1500+ years later, but some buildings built like 30 years ago are falling apart?
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Because they’re not made of the same material?
Keep in mind not all buildings from long ago are still standing. It’s selection bias: we only see the ones that stand.
But a big mass of limestone like an Egyptian pyramid has a good chance to stay standing because it’s a stable shape. The ones that stand had shallow enough slopes on their sides that they can’t fall over like an obelisk can.
There were other pyramids with steeper sides that didn’t do as well.
Stone and earthen structures also tend not to burn down. A pyramid in Giza was the tallest building in the world for thousands of years. Then someone build a wooden church tower in England that surpassed it. Within 200 years, that tower burned and the pyramid was the tallest building again.
Selection and survivorship bias.
The vast majority of buildings built 1500+ years ago are no longer standing, and the vast majority of buildings built in the 1990’s are standing just fine, and of those that aren’t the vast majority were intentionally demolished rather than fell into decay.
Of the ancient buildings that still stand the vast majority were continuously occupied and maintained for that 1500+ year period.
Of the modern buildings that decayed the vast majority were abandoned and left to the mercy of the elements.
TL;DR: If you want a building to last a very long time you need to continually maintain it.
Survival bias. Only a tiny fraction of buildings from 1500 ago survive. And those that so, survive because they were either forgotten and hidden, or actually used and cared for.
The buildings that are still standing from a millennia ago were all built of stone. Even of stone buildings, only a very small fraction of a percent have survived.
Most small buildings today are built of wood and drywall, then surrounded by plastic. The only cement or steel they are likely to have is in the foundation. Those materials don’t stay together as long without maintenance (though they should last 30+ years if well-constructed).
Skyscrapers and giant factories can stand unattended for decades or longer because they’re made mostly out of cement and steel.
To add to other responses, there are very few buildings built 1500 years ago that are still standing, intact, without significant conservation efforts or having deteriorated significantly.
Probably the most famous “old” buildings are the three big pyramids at Giza. But if you [look at pictures of them](https://upload.wikimedia.org/wikipedia/commons/e/e3/Kheops-Pyramid.jpg) you can see they are falling apart, [the stones are chipped and worn](https://en.wikipedia.org/wiki/File:Cheops_pyramid_02.jpg), and [bits have fallen off](https://en.wikipedia.org/wiki/File:[email protected]). And that’s the obvious damage. If you take the pyramid of Khafre (the second biggest) [you can see how the top layer of the construction is almost all gone](https://en.wikipedia.org/wiki/File:Egypt.Giza.Sphinx.02.jpg) (and the same is true for the other pyramid, it has just *all* worn away, not just most of it).
Another famous ancient monument is [Stonehenge](https://en.wikipedia.org/wiki/File:Stonehenge2007_07_30.jpg), in southern England (parts of which predate the Giza complex). But Stonehenge has gone through various restorations; compare [this photo from 1877](https://en.wikipedia.org/wiki/File:Stonehenge_1877.JPG) and [this one from 2008](https://en.wikipedia.org/wiki/File:Stonehenge_on_27.01.08.jpg) from a similar angle.
Often older buildings are still standing because people have decided they are important enough to protect, preserve, and restore.
A few reasons. As another person here mentioned, there’s survivorship bias. There are only a handful of ancient buildings still standing, while the vast majority of ancient buildings stopped existing during antiquity. Most of the ones still standing were maintained due to regular use (i.e. The Pantheon became a church) or were built in desolate areas that became abandoned by the people that lived there. In most cases buildings were taken apart to reuse material elsewhere or to simply build something else on the land.
Another aspect is the actual engineering that went into it – or, probably more accurately, the engineering that *didn’t* go into it. Ancient architects didn’t have the mathematical engineering tools that we have now – instead they used rules of thumb and ratios. So things built with those techniques tend to be very sturdy and nigh indestructible. The tradeoff was that these were outrageously expensive and labor intensive. Modern engineering has allowed for lighter, cheaper, and more flexible construction. Basically nowadays we get more bang for our buck.
There’s an old joke in engineering: “It’s easy to make an indestructible bridge. It’s really, really hard to make a bridge that is just barely standing.”
All people above are right, yet I hove something to add. Most of ancient buildings, that we can see today, were made to stand a long time. There are usually temples or palaces or other important buildings that demonstrate power and knowledge of the civilization.
You shouldn’t compare them to common cheap houses made of shit and sticks. Compare them to some super expensive government’s complex made of good steel and best concrete. Such buildings don’t collapse in 30 years.
There’s a few things that are going on here.
First off, we only see the buildings that are still standing. This is survivorship bias. You don’t see thousands upon thousands of buildings that fell from that time.
Second, we’ve gotten better at engineering since then. There’s a phrase in engineering that goes “Anyone can build a bridge, but only an engineer can build a bridge that barely stands”. We could design a building to have the capacity to stand for 1500 years today, but the material and construction cost associated with it would be immense compared to what we’re used to paying for buildings now. Back then they didn’t have the same understandings in engineering, so stuff was typically either overbuilt or underbuilt, and not necessarily built in that just-right sweet spot.
Many people mention (correctly) survivorship bias, but one thing I’d like to point out as well is this:
I don’t know which building you have in mind, but most visible buildings that old are basically unusable skeletons or partial ruins. There’s a difference between keeping something usable and keeping something “recognisable”. Without adequate and continuous care no building survives that long in its usable format. And lots of modern buildings, if conditions are right (survivorship bias), can survive insanely long in “recognisable” form, but never in usable form, just like ancient ones.
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