Why are man hole covers round?
The answer is obvious AFTER you know the answer.
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Replies
They can't fall down the hole no matter how you turn 'em.
The can't-fall-through explanation is a nice one, but there are other shapes which have that property.
Correct Father John and Jamie. How many shapes could the man hole covers be and not drop down and kill someone? Please contribute your shape and let the "fun" thread continue a little longer.
Round, elliptical, triangular, heptagonal, and any other shape that approaches circular.
"I cut this piece four times and it's still too short."
Not true, any other shape other than a circle can fall through the hole that it covers. You can turn an ellipse, triangle, heptagon, etc. in some way that it can fall through.
You can get technical and say that it depends on the size of the lip that holds up the manhole. The wider the lip, the smaller the inner hole is. If you make the inner hole small enough by making the lip wider, then nothing will fit through the hole, regardless of its shape.
A reuleaux polygon will not fall through its own hole.J.P.http://www.jpkfinefurniture.com
Edited 12/5/2005 7:32 pm ET by JP
http://www.maa.org/mathland/mathland_10_21.html
I stand corrected
Cool Thanks for the link. I once made "shields" (for my kids dressup bin) that were shaped like the rounded triangle out of luan - then I scored the back and bent them (and attached them) to a curved form. Came out quite cool... (Painted them gold, attached straps on the back, painted each kids initial really big on the shield) We still have the dress-up bins when the little cousins come over, we get knights in the basement :-)Mark
Measure it with a Micrometer,
Mark it with Chalk,
Cut it with an Ax.
Visit my woodworking blog Dust Maker
Dang! I had to look THAT one up! and I just LOVE Math...http://mathworld.wolfram.com/ReuleauxPolygon.htmlI think a Star Of David will not.. Thinks it's five sided?Onto a math study to prove it...
I don't know why that popped up in my head. But I remember a similar question in one of my math classes from high school.Sometimes it is really weird, the things you remember.Maybe has something to do with the weird lights coming from my computer?J.P.http://www.jpkfinefurniture.com
I was definitely waaaayyyyy off with the elipse, but I should have said equilateral triangle- I'm not sure it would go through. Again, when it approaches a circle, it's hard to get it through because of any lip. However, if it's getting close to a circle, what's the point of making it harder? I'm sure they tried a lot of different options, though.
"I cut this piece four times and it's still too short."
An equalateral triangle will also work. there is no orientation where it could pass through if there is a lip, no matter what the size of the lip. Also any odd number shape with equal sides will have the same properties for the same reason.
Mike
Some can fall in. Sad but true. Older covers can have a large hole on the side for the hook to pull the cover.(Usually only found on storm systems or combined storm and sanitary) If the hole is large enough for the lip of the frame to fit the lid can fall in. Trust me....going in a manhole is bad enough. Getting a lid out can wreck your day.
DAMN! Chicago is full of them things! Cars fall into them all the time!
Because they cover round pipes. Pipes are round because it is the strongest cross-sectional shape.
Another reason is because the workers get big round butts sitting around all day drinking coffee instead of working!
Actually, I think pipes are round because they have the largest area for their dimensions. Any other shape will take up more space because of the corners.A=Πr² "Area equals pi r squared" "No, pie are round, cake are square!""I cut this piece four times and it's still too short."
Edited 12/5/2005 8:57 pm by highfigh
Actually, most manhole covers don't go directly over a pipe connection...there's typically an underground concrete vault, and pipes or conduits run into or through the vault.If you ever need to remove a manhole cover, get out your pickaxe and stick it in the hole in the middle of the cover. Lift a little and give it a tug, and it slides right out on the ground. Also, an interesting thing I've noticed is that most public works trucks (around here at least) have an old rear view mirror off a pickup tossed in the back - on sunny days, the workers use them to reflect sunlight down the manhole, it works a lot better than a flashlight.
I don't think that you're correct on the strongest cross section on the pipes. We use pipe for lighting battens in theater because they are easy to clamp to, but we would by much better structurally with an I beam for example. But pipes do make good columns that are equally strong in any direction if they are pushed on.Datachanel
Doing things the hard way
An I-beam doesn't include a volume, so it's not suitable for pipes.
If you compare square, round and elliptical (or even triangular) pipes, the one that provides the most resistance to compressive forces will be the round pipe. It's the same reason why the Romans used arches to span columns--you can span a much larger gap with an arch than you can with a straight piece of stone. Square pipes would collapse along the sides at much lower compressive forces than would collapse a round pipe.Recommending the use of "Hide Signatures" option under "My Preferences" since 2005
Thank you, true, they are the best shape for transporting a liquid, I guess I just got fixated on "the strongest cross-sectional shape" statement. I am a true believer in the power of the curve, without it we wouldn't have all those gorgeous cathedrals.Datachanel
Doing things the hard way
The arch is used because it has support on the sides that resist compression forces. Same reason a buried metal pipe (culvert) doesn't easily collapse underground. The fill keeps the sides from expanding outward (using a load perpendicular to the surface as an example). A cylinder is, however, an inherently strong shape when a force is acted on it along it's length. Until it gets too long relative to its diameter and the radius of gyration issue kicks in (think of bending a straw- if it's really short, it won't gyrate much before bending but a long one does).
"I cut this piece four times and it's still too short."
The radius of gyration is constant for a given cross section. You're thinking of KL/r (where "r" is the radius of gyration).
Exactly. Except to note that a hollow cylinder is stronger than a solid cylinder of the same mass, when a bending force is applied to the length of the cylinder. It's one reason why our bones have a spongy interior, instead of being solid bone througout.
To think those biophysics courses came in handy when discussing sewers...Recommending the use of "Hide Signatures" option under "My Preferences" since 2005
The surface strength of materials is stronger than the internal strengths. That's why a hollow tube is stronger, there are two surfaces - one inside and one outside.
OK, I didn't mention hollow, but that's what I was referring to.
"I cut this piece four times and it's still too short."
It's been a while since the "Strength in Materials", "Statics", and "Dynamics" classes, but I sure seem to remember I-beams being the most efficient cross-sectional load carrying shape.....due to the high moment of inertia.
Steve
Doesn't an I-beam have the greatest support in just the one direction? Where a cylinder would have the same strength, albeit not as much, in any orientation.
We're talking pipes here, Steve, not load-bearing members. We got started talking about man-hole covers...Recommending the use of "Hide Signatures" option under "My Preferences" since 2005
And the reason why manhole covers are usually round is more to do with the original Irish immigrant iron and steel workers and a length of string moved continuously at a fixed distance from a center pivot point.
By the way,using a mirror to deflect sunlight is THE WAY to check if your newly drilled borehole is straight (well it was in Africa)Philip Marcou
using a mirror to deflect sunlight is THE WAY to check if your newly 'LADY' walkin' by is gettin' your attention...
last time I checked you can't crawl through an I beam no matter the size.
Derek
Edited 12/12/2005 9:29 am by derekb
Concerning pipes and their round cross section--I don't think it's as much that they resist bending and so forth (unless they are acting as tunnels or conduit or culverts where collapsing from dirt pressing on them is the problem) as much as it is that they best resist internal pressure.
you can't crawl through an I beam no matter the size.
Not true.. When little me and my brother did that! Crall along the 'I' beams UNDER the R/R bridge to cross the river.. MOM told us to NEVER walk across a R/R bridge or we may get killed!
If only mom's only knew... Just like mom's telling their 18 year old sons that they should drink any beer or drive their cars to fast. Yep, never did that stuff either.
Derek
Another reason is because the workers get big round butts sitting around all day drinking coffee instead of working!I thought that was a 'flat' butt?
Well, I wouldn't know--really! Round cross section, at least. Or so they say.Recommending the use of "Hide Signatures" option under "My Preferences" since 2005
I think it's because they won't fall in, and because they give the greatest area for the least amount of material.
check out this BBC link for a story about manhole cover thefts.
http://news.bbc.co.uk/2/hi/asia-pacific/3440451.stm
Datachanel
Doing things the hard way
youre all wrong.they are round because if you cut a sewer workeer in half, his cross section is round and perhapaps a bit bulbous
Wicked Decent Woodworks
(oldest woodworking shop in NH)
Rochester NH
" If the women dont find you handsome, they should at least find you handy........yessa!"
There round for two reasons, they wont fall in/through. Yea neither will a lot of other designs but the second one that none of the others have, they fit back in anywhere, or think of a clock, at any time!
Doug
You nailed it Doug. Round covers can't drop back into the manhole and they fit back in without worrying about orientation. That's a very important consideration when you're closing up a hole in the middle of a busy street. Coning off a lane just makes things a little safer. Lots of morons still want to zip by at high speed.
Another poster suggested using a pick to pop a lid and I suppose it would work but I'll stick with my hook. Those covers are HEAVY and are often stuck tightly by years of gunk. I've had to beat covers with a sledge hammer just to break them loose so they could be lifted.
Dave
I knew a guy in college that lifted weights, but didn't have the money to buy any so he was going to go out at night and pop a few man hole covers off and use those!!!
Damn fool, I asked him about the poor soul that would drive by later and hit the hole. He said he never thought of that! Duh. Hopefully he never did it.
Doug
But do you notice when they paint the center line through the cover and then have to lift the cover. It never matches the rest of the centerline but veers of to one side or the other.
Doug, my thoughts exactly. There's nothing to line up... just pop 'em in any old way. 'Nother thing, Since there's no fulcrum , traffic won't 'upend' a circular one as quick as a square or multi sided one.
One man with one tool can pop them open and easilly pop 'em back. They weigh several hundred pounds . Wear steel tipped shoes .Steinmetz.
All good points but you have forgotten one. Manhole covers are round so that one operator can move it or replace it. Think of the wheel, all one needs is a little exertion to get the heavy thing (usually around 60-80 lbs.) on its side and then its just a matter of rolling it to where it needs to go.
Ahh, but no one mentioned that regardless of the shape it should have three small nearly equidistant legs (little bumps) on the bottom of the lid around the periphery. This will keep the man hole cover from going KA-LAAAANG when driving across in the middle of the night and disturbing our sleep. With three little protrusions it will always contact the rim into which it fits like a three legged stool never teeters from a leg out of dimension or the floor out of level.
PatrickG
"Ahh, but no one mentioned that regardless of the shape it should have three small nearly equidistant legs (little bumps) on the bottom of the lid around the periphery. This will keep the man hole cover from going KA-LAAAANG when driving across in the middle of the night and disturbing our sleep. With three little protrusions it will always contact the rim into which it fits like a three legged stool never teeters from a leg out of dimension or the floor out of level."A few years ago, a city crew opened up the manhole in the street in front of my house for inspection, and when they put the cover back a little bit of gravel or something got caught underneath which allowed it to rock back and forth. I got to listen to cars driving over that @#$% thing all night. Man, was that annoying.I figured it would take days for the city to respond if I called and complained, so the next morning I went out there and fixed it myself.
Reuleaux polygons, I never heard of those before. Very cool.
However, manhole covers may still need some improvement in design. Here in Raleigh, during a terrific rainstorm last June, a manhole cover floated out of its hole due to flooding in the sewers, right in front of my florist shop (wwking is a hobby for me).
A car came along with three teenagers, and when the wheel struck the hole, it stopped RIGHT NOW, deploying the airbag and thumpin' them kids good.
OK, son, hand me the plywood stretcher... Bill Bryan
Bill, I once saw a speeding taxi cab strike TWO raised manhole covers in rapid succesion on a street in New YorK that was under construction and hadn't been paved yet. the covers were in line and spaced about
eight ft apart
When he hit the first one, the cab was airborne only to precisely hit the second one and broke off both front wheeks
Steinmetz
I didn't know they put manhole covers in the sidewalks in NY.
"I cut this piece four times and it's still too short."
Has seen some covers with bolts here in Chicago.. To bad the workers never put the bolts back!
It's Obvious.
Their the same shape and size as the workers that go down them. View Image
It would seem to me that a round shape would also be the cheapest to manufacture and keep true. Cast it, spin it on the lathe to true the shape, and you are all set.
Good thread
Andy
"It seemed like a good idea at the time"
What is wrong with you people?!
Manhole covers are round because manholes are round.
Duh.
I thought they made the holes that size so they would fit the covers. Guess I had it backwards. ;)Andy"It seemed like a good idea at the time"
i got a question, why did kamikazee pilots wear helmets?
Kenney66
Probably for the earphones for radio communications, or for the sound attenuation during the majority of the flight to the target.
Chip -
Actually, most manholes are box shaped with a round hole in the top. I've been in some that were as large as small rooms and some even have multiple levels. I've been told that some manholes in San Francisco are 2-3 stories deep.
I'm starting to rethink my opposition to putting smilies in posts that are obviously not meant to be taken seriously. :-)
LOL
How come Madison Square Garden is round?
Life may be short but it can be very wide!
How's come ya can't park in Central Park? Or for that matter, park on a 'Parkway'?.....Drive on a driveway?
Don't remember when, but maybe in preparation for some visiting dignitary, all the manholes in Manhattan were fitted with black rubber 'gaskets'
Steinmetz
Edited 12/25/2005 3:37 am ET by Steinmetz
Still waiting for the punchline there, Don.
Cheers,
eddie
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