round manholes

cecil please read some engineering and update the answer you gave about round manholes…strenght of materials says that if any other shape than oval or circle will expose great stress forces by the edges ,the airplane windows is oval and not square or triangle because of stress force theory,stress will route around an oval or circular shape resulting to no cracks.
thanks


Link to column: http://www.straightdope.com/columns/read/220/why-are-manhole-covers-round

Uhhh…what?

Airplane windows are rectangular with rounded corners. But portholes are round. I guess that’s because of the high pressure at sea level.

I have an oval or circular shape and I seem to be a stress magnet.

What did Cecil say about them, anyway?

No doubt round manholes are superior for stress issues as well, at least in some locations–but anyone who’s ever had to lift a cover out and put it back is glad that a circular cover can’t fall in. Any other shape could.

Is this a dirty thread?

Ever been in the stacks of an old library?

That in fact pretty much is what he said, except that ones shaped like a Reuleaux triangle won’t either (which explains the occasional triangular access cover I’ve seen I guess).

While it is certainly true that circles (curves) are better resistant to stress loading and cracking, you have provided no evidence that that played any role in the decision to make man hole covers round.

The fact of the matter is that fracture and stress is not really relevant to the issue, because manholes don’t see the high pressure situations that airplane windows do.

The simplest answer is that fance reauleaux triangles and whatnot are funky shaped, and thus more complicated and therefore expensive than circles. Squares or rectangles would be simple, but have the annoying ability to fall through the opening.

Unless you want to put hinges on one side and mount the cover to the rim. Then it can’t fall in or get moved out of alignment. But then you have hinges in the roadway.

Also, round covers can be maneuvered short distances by rolling on edge.

Welcome to the Straight Dope Message Boards, demertis, we’re glad you found us. When one starts a thread, it is helpful to other posters to provide a link to the column under discussion. There’s lots of 'em in the Archive, and the link saves search time and helps keep us all on the same page. I’ve edited the link in for you, you’ll know for next time, no prob. And, as I say, welcome!