Darren New wrote:
> Actually, I've never seen anyone get the answer write to "why are 
> manholes round."  I almost hope they ask me that at my next job 
> interview, because the interviewer probably doesn't know either. ;-)

The snarky answer is "because manhole covers around round" is the 
punchline of a joke, but does not answer the serious question at hand.

A manhole is *circular* minimize the amount of extra material needed to 
make it impossible for the cover to fall into the manhole.

The manhole has a certain radius.  The manhole cover has a slightly 
higher radius.  In every direction, the diameter of the cover is larger 
than the diameter of the hole, taken in any direction.  This is 
accomplished if the cover's radius is even a millimeter larger, although 
to allow for wear and the thermal expansion of the hole or contraction 
of the cover, a larger difference--no more than a centimeter--is prudent.

With any other shape, the required amount to ensure non-falling-in 
behavior is larger.  A square, or a pentagon, or any n-gon, is larger 
from corner to opposing corner (or most opposing corner for n-gons with 
an odd number of vertices) than from side to opposing side (or corner to 
opposing side).  If the difference is larger than the extra amount added 
on to make the cover bigger than the hole, the lid falls in, and so the 
extra amount would need to be larger.

Minimizing the extra material is important because manhole covers are 
heavy; adding to the radius greatly increases the weight of the cover, 
because the weight is proportional to the area times the thickness. 
Area is proportional to the square of the radius.  As any programmer can 
tell you, when faced with an O(n^2) problem that is unavoidable, you 
make n as small as you can.

Also, a circular manhole fits in any direction.  A polygonal manhole 
cover has to be turned to fit, or else be picked up again (these things 
are heavy) or the corners will wear (or break) off after a bunch of 
near-misses.

Regards,
John

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John VanSickle <evi### [at] hotmailcom> wrote:
> A manhole is *circular* minimize the amount of extra material needed to 
> make it impossible for the cover to fall into the manhole.

  Are you sure it's a question of minimizing material and not simply because
it's the *simplest* possible solution for the problem (ie. avoid the cover
to ever fall into the manhole)?

-- 
                                                          - Warp

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"Darren New" <dne### [at] sanrrcom> wrote
> somebody wrote:

> >> Just hope that they don't ask you to write the answer...

> Yes. Blame the tequila.  (I knew someone would bring that up. :-)

> > Which, by the way, is of course "because the covers are round".

> Nope.

Wrong you are, but if you want a more verbose answer:

In "space", the natural shape for anything is a sphere (maximal symmetry for
anything finite). On the surface of the earth, gravity breaks symmetry
(vertical axis is different from the others) so spherical symmetry reduces
to cylindrical symmetry. The natural symmetry for *anything* on the surface,
barring any other constraints, is therefore cylindrical. Since there
typically is no good reason (ask not why is X, but why is not not X) to make
manhole covers non-cylindrical, most are cylindrical. Hence the holes.

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somebody wrote:
> typically is no good reason (ask not why is X, but why is not not X) to make
> manhole covers non-cylindrical, most are cylindrical. Hence the holes.

Errr, no. Indeed, if you actually look, most manholes on the *sidewalk* 
are not round. And even small "manholes" like for meters and such, as 
big as your spread hand, are round when they're in the street.

Chances are you won't just guess unless you've actually ever seen the 
outside of a manhole.

Given the hint that they're all round in the street and they're not 
round on the sidewalk... any more takers?

(As for the "so it won't fall in the hole" bit, people invented hinges 
long before they invented manholes.)

-- 
   Darren New / San Diego, CA, USA (PST)
     "That's pretty. Where's that?"
          "It's the Age of Channelwood."
     "We should go there on vacation some time."

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John VanSickle wrote:
> A manhole is *circular* minimize the amount of extra material needed to 
> make it impossible for the cover to fall into the manhole.

Are you guessing, or have you actually (say) worked someplace that 
designs manholes? :-)

> Minimizing the extra material is important because manhole covers are 
> heavy; 

Even six-inch wide manhole covers covering holes a foot deep are round.

> Also, a circular manhole fits in any direction.  A polygonal manhole 
> cover has to be turned to fit, or else be picked up again (these things 
> are heavy) or the corners will wear (or break) off after a bunch of 
> near-misses.

Hinges are your friend. :-)

-- 
   Darren New / San Diego, CA, USA (PST)
     "That's pretty. Where's that?"
          "It's the Age of Channelwood."
     "We should go there on vacation some time."

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Darren New wrote:
> 
> Given the hint that they're all round in the street and they're not 
> round on the sidewalk... any more takers?

I can't actually agree on this, but it can depend on the country etc, so 
that doesn't matter. There's 2 reasons:
1) No corners -> no tire fatalities.
2) Harder pressure on ground surface (there can be tonnes of masses on 
one axle on the street) -> harder construction (cylindrical shape is 
better to handle pressure from outside).

-- 
Eero "Aero" Ahonen
    http://www.zbxt.net
       aer### [at] removethiszbxtnetinvalid

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On Sun, 08 Jun 2008 08:32:33 -0700, Darren New <dne### [at] sanrrcom>
wrote:

>Hinges are your friend. :-)

I'm not sure what you're thinking about but hinges are not your friend
when your talking about manholes. The manhole cover can be left up.
(Naughty, naughty) Also hinges would be a weak point and liable to
break. They would also involve lifting the heavy manhole cover upright
which would take more effort than lifting it up an inch or two and
sliding it away. 
-- 

Regards
     Stephen

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"Darren New" <dne### [at] sanrrcom> wrote in message 
news:484b0f3d$1@news.povray.org...
> Actually, I've never seen anyone get the answer write to "why are 
> manholes round."  I almost hope they ask me that at my next job 
> interview, because the interviewer probably doesn't know either. 
> ;-)
>
> -- 
>   Darren New / San Diego, CA, USA (PST)
>     "That's pretty. Where's that?"
>          "It's the Age of Channelwood."
>     "We should go there on vacation some time."

Round manholes fit whichever orientation you drop them.
Round manholes cover (usually) cyclindrical access spaces which are
better able to resist ground pressures and (again) permit putting 
thiings
into them without concern for orientation. (Round pegs/square 
holes:
square pegs/round holes).

Lots more reasons, but it is a field question as opposed to some
design or theoretical question: what works best and easiest is
what is done.
If you've ever worked with manhole covers you know it is enough 
work
without having to fit corners.

(also - odds are any sufficeintly sized circular manhole cover will 
fit
the odd hole and the odds are that covers with corners will not
work as a --- cover.)

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"Darren New" <dne### [at] sanrrcom> wrote in message
news:484bfade@news.povray.org...
> somebody wrote:

> > typically is no good reason (ask not why is X, but why is not not X) to
make
> > manhole covers non-cylindrical, most are cylindrical. Hence the holes.

> Errr, no. Indeed, if you actually look, most manholes on the *sidewalk*
> are not round. And even small "manholes" like for meters and such, as
> big as your spread hand, are round when they're in the street.
>
> Chances are you won't just guess unless you've actually ever seen the
> outside of a manhole.
>
> Given the hint that they're all round in the street and they're not
> round on the sidewalk... any more takers?

That's not a given by a long shot. Manholes here and all places I've seen
are round in the sidewalk too. The city isn't going to maintain two
different types of manholes unless there's a good reason (the sidewalk ones
might be thinner, I naturally did not inspect carefully). The large
rectangular ones you see on sidewalks on cities are not manholes but grills.
There are also rectangular storm drains (some are even formed into an S
shape between the road and the sidewalk for accessibility). However, neither
grills nor storm drains are meant for a "man" to go through a "hole", so are
not manholes, nor are the holes in the ground for meters that you mention.

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On Sun, 08 Jun 2008 08:32:33 -0700, Darren New wrote:

>> Also, a circular manhole fits in any direction.  A polygonal manhole
>> cover has to be turned to fit, or else be picked up again (these things
>> are heavy) or the corners will wear (or break) off after a bunch of
>> near-misses.
> 
> Hinges are your friend. :-)

Hinges introduce additional complexity and are subject to breakage under 
stress...

Jim

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