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Comment by hprotagonist | Hacker News Reader

If i'm not mistaken, this is by design for Gothic buildings. I forget what the proper term is, but Gothic architecture exploits dynamic tension really thoroughly. The whole building is fighting against itself to destroy itself, and so remains balanced and static.

Take a component out, and the result is unstable.

Appropirately for a cathedral, the sermons practically write themselves.

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Now I'm curious how, hundreds of years ago, engineers and architects determined what designs were viable.

Did they have quantitative data about materials' tensile strength, shear strength, etc.? If yes, what kind of math did they have for applying those data? If no, how did they safely design first-of-a-kind structures?

Usually very old buildings (at least the ones that have survived) are extremely overbuilt, with solid walls many times thicker than what we would use today.

“safety factor: even more rock, please!”

This is at least partially survivor bias. Sometimes they miscalculated and buildings fell down.

Even the ones still standing often aren't because of fantastic original engineering but because they are the ones people have cared to carefully maintain and to perform renovations on to compensate for original engineering flaws.

And sometimes simply by sheer mistake; the leaning tower of Pisa was supposed to be supported by the Corinthian columns and limestone internal blocks, but it turns out that its structure is actually supported significantly by the marble façade.

An example is the Hagia Sophia which had a lot of reinforcements added after the original design was found to be unstable. The building you see today is not how it was originally designed to look, not just because of the minarets.

So did we.

As I understand it, they tried to to make sure pretty much everything was under compression, and tested using scale models (which work well in that case).

Cut some pieces of wood and test out smaller scale models

Wood and rock have such dissimilar structural properties that scale models out of wood don't say anything useful about full sized buildings out of rock.

The article has a good diagram of the roof that provides the counterbalancing force to be discharged through the walls and buttresses.

On an unrelated note, a sibling post, though also summarizing the article, seems to be correct. How did it get killed within a minute of posting?

>On an unrelated note, a sibling post, though also summarizing the article, seems to be correct. How did it get killed within a minute of posting?

Yeah. I noticed that too. I assume that the algorithm weights the time between down-votes and it got a couple real quick or something. It wasn't like it was even an opinionated comment so that's still odd.

I recently heard it called as being in a state of equilibrium. Not sure if that's the one you were thinking of, but it was in the context of ancient construction techniques and describes the same thing.

https://en.wikipedia.org/wiki/Flying_buttress perhaps

That's part of it, but the buttresses are all intact, the wooden roof is the only structural component that did not survive the fire.

If I am pushing against you pushing the other way, and one of us stops, the other falls forward. That's what happens with a buttress without the roof and internal arches pushing the other way. Not enough to push it instantly over, but give it time or a strong wind...

Some of the vaulting did not survive (notably, the section that the spire crashed through, and I think one other). The vaulting is what pushes the walls out while the buttresses push them in. (To my understanding, the roof, by comparison was a lot less structural—knowing that fires happen all the time, the 13th century engineers designed a roof that would burn away without destroying the underlying stonework.)

Essentially, the flying buttresses are designed to hold the weight of the roof pushing outside.

Without that weight, there isn't much to prevent the walls from caving in.

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