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Topic: Medium: Manholes (Read 13659 times) |
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Mixster
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Manhole covers are built round so that they cannot be turned on the diagonal and fit down the hole they're covering. I'm guessing this happened once or twice and somebody got pissed after getting brained by one of these. 
-Mixster
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Nicodemus
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Mixster, I think you've named the correct answer. However, I once asked someone this question and they gave a different, though valid, answer:
Manhole covers are very heavy. If they're round, you can roll them from place to place. Creative! 
- Nicodemus
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greg
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Maybe they are round so they are more space-efficient? Doubtful, considering the price of steel, though.
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Ion Rush
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when tiling (not the roof type, the under ground water drainage type) was first invented, it used clay tiles that were half cylinders placed on eachother, and then lined up with other cylinders. This allowed water to seep in and flow away. Large sewer systems were expansions of this type of system. Also, when you want good water flow and desire to not have chuncks getting stuck, you would not want a square pipe (it would work fine if there was a large volume of water, but with little water, it would spread out and not flow well, flow slowly, zig zag across the floor, etc etc, a curved bottom collects the water, and even with just alittle bit flowing, you have a small deepish channel (like 3 inches wide 1 inch deep at deepest) rather than spread out (like 36 inches wide-the width of the square pipe- and less than a 10th of an inch deep)
so being that your horizontal subterranian pipe system is going to be round, why spend extra equipment and resources making special square pipes for going up into the street? Being that the shaft going upward is round, the most efficient shape for capping it would also be round
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kenny
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Re: Medium: Manholes
« Reply #4 on: Jul 30th, 2002, 12:02pm » |
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There are lots of answers to this, but I think the most common one is what Mixster said: so that the cover can't fall in the hole.
We all understand that a circle has the property that, if a circular manhole has a small lip inside it, and the cover is the same size as the hole above the lip, then it's impossible for the cover to fit all the way in the hole, past the lip.
A square, for example, does not have this property.
Question: Are there any other convex shapes, aside from circles, that have the same property, that the cover couldn't be rotated to fit past even an infinitesimal lip?
I think that's a more interesting puzzle.
-- Ken
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anshil
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An equilateral triangle ?
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Nicodemus
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Quote:
I don't think that works. Picture an equilateral triangle, one point (T) to 12 o'clock. Drop a line from T to the middle of the base edge. Let that be S (for short).
If we were to rotate the equilateral triangle 90 degrees out of plane around line S, the planar shadow of the triangle would be equivalent to S (plus some thickness). For any small thickness relative to the triangle, we can rotate S about 15 degrees around T, shift it away from T, and drop it through the triangle.
Sorry I don't have a way to diagram rather than describe this. 
I would contend that the circle is the only (planar) shape which fits the bill. The criteria (as far as I can tell) are that the shape have the same maximum width when measured at an angle; if there were two different angles where the maximum width were different, then the shape could be manipulated such that the lesser width orientation passes through the greater width orientation.
If we allow convex shapes then we introduce a whole category of possibilities (which make no sense for manholes), such as two tangentially connected circles or a donut (ring). But it seems that, even for convex shapes, they only work when built from circles.
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jkemp
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Manholes are round for the same reason that ceiling access panels are square. We don't want manholes falling down the hole, but we do want to be able to remove a ceiling access panel without breaking the ceiling.
There is one other shape other than a circle that will not fall through diagonally. Such a shape must have a constant diameter. I don't know the name for the shape, but I saw it pictured once as an "alternative" for the wheel: it is a kind of triangular shape but with rounded corners. Any line drawn through the centre has the same length, so its diameter is constant; it would not fall down its hole diagonally.
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Ryan
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An equilateral triangle works because there is no possible way to make a line traverse the inside of the triangle that is of greater length than the sides. No matter how you turn it, it will have a point that is equal to the length of one side.
There's another shape that works, although it is very unconventional. Take 3 equal size half circles and connect them diameter to diameter such that they enclose an equilateral triangle shaped space.
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jkemp
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No, an equilateral triangle (indeed, any triangle) can be turned and dropped down the hole. Not down the middle of the hole, obviously, but near one side.
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Ryan
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Ah, I see it now.
You're right.
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Yossarian
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Actually there is an infinite number of so-called equal width
shapes. An example is Releaux shapes.
So, the answer is : the circle is the most well-known
shape of equal width.
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anshil
Newbie
Posts: 41
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Re: Medium: Manholes
« Reply #12 on: Aug 5th, 2002, 9:40am » |
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But the circle is the only one having one central rotation centrum, or? I mean the 3 half circles figure, you can't hang it up in the middle and rotate it on a plane without the hangup point going up and down. (altough the distance top-plane is constant)
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Capt.Bushido
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Actually, I think I've heard an ENTIRELY different answer than
any given here, once before:
Why are manholes round?
Because manhole covers are round.
And vice versa, of course. 
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Jeremiah Smith
Full Member
Beep!
Posts: 172
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Re: Medium: Manholes
« Reply #14 on: Aug 5th, 2002, 1:17pm » |
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One of the reasons that circles are chosen over equilateral triangles is that you don't have to orient a circle in any particular way to get it in the hole.
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Jonathan_the_Red
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Posts: 102
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Re: Medium: Manholes
« Reply #15 on: Aug 5th, 2002, 1:37pm » |
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Yep. There are many good answers to the question of "why are manhole covers round"... perhaps the best general-purpose answer is "because that's the best shape."
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My arcade cabinet
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Yossarian
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> There are many good answers to the question of "why are
> manhole covers round"... perhaps the best general-purpose
> answer is "because that's the best shape."
If it's the best shape, then why not ALL manholes
are round ?
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Kozo Morimoto
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According to a civil engineer friend who actually designs the damn things, the dropping theory is pretty cool, but the main reason is probably to do with the same reason why portholes on ships are round - to distribute the load stress evenly and avoid stress points. When you have heavy loads (ie trucks and buses on roads driving over manholes constantly) having 'pointy' shapes would reduce the life of the manholes and increase maintenance requirements. If the dropping thing is correct, why don't they just attach the covers with cables/hinges or make them slide across rather than be able to completely remove them?
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jkemp
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I like the load-spreading answer. I still think the "not falling down the hole" answer is the best one, since it is a simple design with a simple reason. No need for hinges, cables or slides which would degrade over time and make the cover harder to open.
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LMD
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Well, technically, for any shape of manhole cover, you can make it impossible to fall through the manhole. Just make the cover hell of larger than the hole.
Round shape is just more effecient.
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KamBha
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Re: Medium: Manholes
« Reply #20 on: Oct 8th, 2002, 6:30pm » |
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Hi I am new to this board but this problem has me intrigued.
I had another answer (Sorry if it has already been mentioned). No, if I understand my manholes, to open a manhole you need a key that you stick into the cover then you lever open. Now, if you have a shape with corner then there will be a place where you cannot lever the manhole. (ie the corners)
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James Fingas
Uberpuzzler
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Re: Medium: Manholes
« Reply #21 on: Oct 9th, 2002, 6:46am » |
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I think they lever them open by sticking a pick-axe into one of those small square holes, then using the pick-axe as the handle. You could still do this on any shape of manhole cover.
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Doc, I'm addicted to advice! What should I do?
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The_Beast
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Re: Medium: Manholes
« Reply #22 on: Nov 23rd, 2002, 10:30am » |
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Perhaps it is because the human body is more round that square. Having a square manhole cover with lines tangent to the a circle inside the sqaure, would require more material. Multiply this wasted space by the millions of manholes, and you save $$$.
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jon_G
Junior Member
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Posts: 90
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Re: Medium: Manholes
« Reply #23 on: Dec 3rd, 2002, 5:25pm » |
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I had to use a crow bar to open one before. My first guess was structural integredy then how it would be easier to fit in being a human and all.
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Why climb the corporate latter, when you can own it.
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Samdog
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This really isn't an answer but rather just something that hasn't been brought up: manhole covers are not 2-dimensional. It seems for practical purposes they ought to be as thin as possible while maintaining structural integrity, but the fact is they must have some depth and could have a significant depth.
a 3ft square manhole cover that is 1.5ft deep could never fall through a 3ft square manhole (with the normal, required lip that prevents circular manholes from falling) because the smallest cross-sectional side of the cover would be 1.5ft by 3ft.
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