41 replies to this topic

[stanger828]

But hinges would either create an uneven driving surface if it was designed to have the cover flip 180 degrees, or the cover would be stick strait up perpendicular to the road. This would be a hazard since it could slam on someones head, or could make a driving hazard.

[Sharpie357]

Why would they want hinges on it anyway? it would be way to hard to lift.

[maishpotato]

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

[naveed]

Bingo excellent logic
Moreover in a unit dimension
a round has more are than a square

[Snowman]

A circle or condiam (continuous diameter curve) aren't the only shapes that aren't susceptible to falling through the hole: any triangle would be safe from this too. But climbing through a triangular hole would just be silly; and moving a triangular cover would be inefficient as well.

[ClattoVerataNicto]

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

[grottylittlewanker]

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.

[Topher]

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, 21 February 2008 - 11:04 PM.

[lowther]

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

There are many answers.

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.

[trojan2k8]

whats a manhole??