Fun with Arches


Arches have lots of fun properties. For example, even though they look like curved beams, they carry load a completely different way. Beams carry load by bending, and this one cannot even support the weight
of two wood blocks. This curved beam also works by bending, and despite being curved like an arch, it still cannot support the weight of two blocks. However, if we support the ends of this curved structure so that it can carry compression, it can suddenly can support two blocks, with ease. If the curved shape is cut into pieces, no meaningful bending is transferred from one block to the next. Instead, the blocks can only transfer compression
to their neighbours. If we reconstitute the arch, only compression is transferred
from one block to the next, and yet the arch is still able to carry two
wood blocks. To further prove that no bending is transferred
from block to the next, we can put a roller between any pair of
adjacent blocks, and the arch still stands. Isn’t that cool! To better understand how an arch gets its strength, consider a piece of cloth. It cannot even support itself. However, if we allow it to take a curved shape like this, it can carry a substantial amount of load. And it does that because tension flows
along its curved shape. Arches work the same as this hammock, except that their shape is reflected upside-down, and they carry compression rather than tension. Another popular misconception about arches
is that they need a keystone. By a keystone, we mean a tapered stone at
the top of the arch. Our model arch was purposely made with an
even number of segments so that there is nothing in it that could
be construed to be a keystone. Here’s another fun thing: Did you know that some arches will collapse
if load is REMOVED? Just like a teeter-totter, arches require the forces acting on them to be suitably balanced. For example, this arch can carry heavy loads
if they are suitably balanced. However, if we remove one of the loads, the arch fails catastrophically. It’s a little tricky to explain why this happens. Our “Arches and Chains” video shows that for an arch to stand, its corresponding chain system, when reflected, must lie inside the profile of the arch. Here is the chain system corresponding to
the original arch. The chain and weights have been chosen so that their weights are proportional, respectively, to the weights of the arch itself and the containers of marbles. It is thus a true representation of the arch. As you can see, the arch passes this so-called
“chain test”. If we remove one of the side weights, the new chain shape does not lie inside the
arch profile, and the chain test fails. When we remove the corresponding load from the arch, it does indeed fall down. Can you picture how the chain will change shape
if the middle weight is removed? Will the arch stand or fall? Removing the middle weight produces a chain shape that falls outside of the arch profile, and the corresponding arch collapses. You might be curious how this arch, from our other video, can stand. It is easy to show that this arch passes the
chain test, and that is why it stands. If we add weights proportional to the
external load, the arch passes this modified chain test, and that is why it can still stand. And, here’s one last thing: Many people think that a semicircle
is a good shape for an arch, and sometimes it can be. However, thin semicircular arches, like this one, fail the chain test and fall down. To learn why they can still sometimes be used, you will have to watch our other video. Thanks for watching.

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Comments

  1. Arches do need a keystone, and your arch definitely has one. The word keystone simply means the stone at the summit of the archway. All you have done is replaced the keystone with two smaller ones, both of which are still keystones, only you have two of them. If you remove either of them, the arch will collapse.

  2. The Romans knew this centuries before Christ. In modern day Africa or Haiti they're still too stupid to have running water.

  3. This is like a cute home video but actually is professional somehow…..

    Love them and please keep 'em coming 🙂
    I'm learning lots

  4. A thin arch can sometimes be used because it is the material strength, not the shape, holding it up.

  5. Voice is dull, presentation is laborious, solution?

    Put the girl in a skimpy bikini, use inflection…problem solved, your welcome 😉😂

    P.S. since males are the predominant sex interested in engineering, the girl in a bikini will work but i jest about inflection, nothing will help your voice 🤣

  6. great video, I finally know what to do with my two wood blocks, six oranges, and three plastic containers of marbles!

  7. The title and the way the man Says "isn't that fun" reminds me of Sheldon Cooper's "fun with flags"

  8. interesting, thin semi circle arch does not work well. Now I'm questioning circular tunnel since isn't it like 2 semicircle?

  9. I thought intuitively removing the middle weight the arches would still stand.. But it fell.. Interesting..

  10. Reminds me of dark web porn where the old white guy cannot finish and ask the Asian prostitute to cook because he paid her hourly…LOL…

  11. The girl on the left is sooo interesting, idk why, but she look like smart nerd and some internet troll at the same time. Please don't lynch me in comments.

  12. Wow amazing, not a lot of videos on civil structures on youtube.
    Thank you very much for making such videos and keep up

  13. NASA….Nasa is a total hoax.
    Research NASA's founders.
    Citizens of the world cannot verify what NASA claims.
    Nasa can claim anything, and we just believe it.

  14. That is very strange way to answer the specific question: why arch failed? Because it didn't pass the chain test. Oh yeah, chain is a very good and efficient model, say, kind of analogue computer that allows to model arch quickly, but come on, that's A MODEL. Why an alligator can't fly? Cause it is green! Very logical: it is green indeed, so that's why it can't fly, theory confirmed. Instead of explaining, that compression force is vectorised and it's perpendicular portion directed outwards overgrows weight (force, to be exact its perpendicular component) directed inwards, then structure collapses due to inability to take any tension and structural failure. So to compensate that, you apply additional weight that overcomes perpendicular vector of compression.

  15. Thank you for the videos you produce. You’re doing a great service for the engineering students all over the world. God bless you all.

  16. Damn I feel ashamed haha, I'm doing a PhD in mechanics-related stuff and I had no idea about this chain rule. It's really beatiful too.

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