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# Round vs. Square

## 42 posts in this topic

Posted · Report post

Snowman..the triangular shaped ones aren't manholes...

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Posted · Report post

But what about a semi-circle? A shape that will not fall through the shape of the hole...

ummm, if you turn it with the flat edge verticle, it will fit through a lot of places in the hole.

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Posted (edited) · Report post

You can drop a manhole cover through a manhole if they are both equilateral triangles and the cover is less than about 1.155 (sqrt(4/3)) times the size of the hole. That's adding about a sixth of the length as lip, which is fairly substantial. This can happen because the "height" (distance from one vertex to the center of the opposite side) is smaller than the side (specifically sqrt(3/4) of the side) -- so there is room.

You really do need a constant diameter shape or you can line up the maximum diameter of the hole with the minimum diameter of the cover and drop it in. Of course, you could use a large lip, but that is wasteful.

Edited by Topher
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Posted · Report post

We can thank Microsoft for the popularity of this question. This is a standard interview question now.

Shape to not fall in: this is an OK answer, but really, all they need to do is put flanges or a rim on the inside of the hole and any shape would be fine.

Shape for "rolling": this is a better answer, but then again, the cover must be transported to its destination. Regardless of the form of transport, it has to be removed and put on the ground. To remove it from the vehicle is going to require either: a person lifting it (all the problems associated with moving it into place) or a mechanical system to unload it, which could then reasonably be designed to unload it directly into place.

Maximum area for entry/exit: I kind of like this one, since it is undeniable and applies to the hole itself as well.

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Posted · Report post

whats a manhole??

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Posted · Report post

a round cover can never fall in.

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Posted · Report post

You guys!!! Any hole, whether round, square, triangular or whatever will allow the piece cut out of it to fall through, UNLESS there is a flange added at the bottom of the hole to keep the cover from falling in. Can't see where math in any form is usable here.

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Posted · Report post

Aside from the obvious 2 reasons listed safety and move ability you have to consider a few other reasons...

Weight distribution over a round hole is more constant where as anything with a corner would have pressure points and be overall weaker.

Materials, it is the most efficient use of materials for the shape and purpose compared to other shapes. Larger area with less materials.

Putting back and removing it, a circle doesn't care how you put it in, you would have to line up corners with other shapes, and if you were under a manhole you can push it up from any side vs. a cornered geometric shape having to make sure you "hinged" it on a flat side.

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Posted · Report post

Any hole, whether round, square, triangular or whatever will allow the piece cut out of it to fall through, UNLESS there is a flange added at the bottom of the hole to keep the cover from falling in. Can't see where math in any form is usable here.

That's true, but even with the flange, shapes other than constant-width-diameter curves will still be able to fall through the hole. You have to assume that the maximum diameter of the cover includes the difference due to the flange, and the maximum diameter of the hole does not. Let's say it's a difference of 1 inch, and you have a square manhole cover 24 inches in width (and thus a hole 22 inches width). That still leaves you with a diagonal opening of over 31 inches, so flange or not, the square cover can easily fall through the hole. The "math" angle of the discussion is quite relevant.

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Posted · Report post

can we do the bit about the whole truth, or the more you take away from it the bigger it gets, or pi, or can we say the gist of this is 'covered'

no more holes pls

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Posted · Report post

The obvious answer is that manhole covers should be round because manholes are round.

I agree.

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Posted · Report post

Materials, it is the most efficient use of materials for the shape and purpose compared to other shapes. Larger area with less materials.

This really depends on what is going up and down the hole. If you're transporting boxes that are cube shaped, then of course it is most efficient to use a hole square in cross section. If you're moving spheres, then obviously the circular hole is the way to go. If you're moving something that is well approximated by a line of diameter as it passes through the hole - panes of glass, for instance, slid through vertically - then the most efficient shape is a Reuleaux Triangle . This shape has a constant diameter, so the pane of glass can be rotated (and the cover won't fall through) with the best ratio of diameter to area. This allows the least amount of material removal in digging the hole for a given size of glass.

For the other reasons mentioned, though, as well as the fact that a wide variety of people and loads probably need to go down manholes, a circle is the best option.

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Posted · Report post

if they were square they would fall down into the hole, also, so they can roll them since they are heavy.
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Posted · Report post

dudes ... you guys are like having a smarticles fight... freaky.PINK!

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Posted · Report post

I thought becauz men weren't square

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Posted · Report post

The obvious answer is that manhole covers should be round because manholes are round.

that was my first thought.."so that they fit" lol

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Posted · Report post

Veracity, the veracity of your statement is suspect:

"There is not a single shape (besides perfectly round) that could not be positioned in a way that would cause it to fall in the hole." , You couldn't be more wrong!

heard of "constant diameter curves" ?

A circle is just one of many such curves:

and This

That said, you are bang on target with your second statement about the ease of rolling

R

Reuleaux triangle FTW!!!

Thank you! Like Veracity, I also enjoy learning new things.

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