Why is Jing Geer round?

1. Save Materials
2. Convenient transportation
3. Easy installation
4. Appearance
5. Uniform Stress

Another answer:

"Why is the manhole cover circular ?"

--- The following is how Richard Van Mann answered this question during an interview with Microsoft.

Interviewer: Now let's ask you a question and look at your creative thinking abilities. Do not think too much. Use the common sense in your daily life to describe your thoughts. The question is, why is the manhole cover of the sewer circular?
Fan man: they are not all round. Some of them are square and some are indeed wide covers, but I have also seen square and long square.
Interviewer: But we only consider circular manhole covers. Why are they round covers?
Fan man: if we only consider the circle, they are actually round.
Interviewer: I mean, why is there a circular manhole cover? What is the special significance of designing a manhole cover into a circular shape?
Fan man: it has special significance. When the holes to be covered are circular, the lid is usually circular. It is the easiest way to create a circular hole with a circular lid.
Interviewer: can you think of the advantages of a circular manhole cover over a square manhole cover?
Van Man: Before answering this question, let's take a look at what is under the lid. The holes under the lid are circular, because the cylindrical shape is most able to withstand the pressure on the surrounding land. In addition, there must be enough space for a person to pass out of the sewer, and the cross section of a person who climbs down the ladder is basically circular, so the circular shape naturally becomes the shape of the entrance and exit of the sewer. The circular manhole cover only covers the circular holes.
Interviewer: Do you think there are security considerations? I mean, will a square manhole cover fall into it, causing personal injury?
Fan man: unlikely. Sometimes you can see a square lid on some square holes. This kind of lid is larger than the entrance, and there is a horizontal block around it. It is usually metallic and very heavy. We can imagine a square hole with a width of 2 feet and a horizontal block with a width of 1 to 1.5 inch. In order to let the manhole cover fall into, you need to raise one end, and then rotate 30 degrees, so that it is not blocked by the horizontal block, and then the manhole cover and the horizon into a 45 degree angle, at this time, the center of gravity of the transfer is enough to let the manhole cover fall down. Yes, there is a possibility of falling down the square manhole cover, but the possibility is very small. As long as the person in charge of the manhole cover is trained a little, he will not make such a mistake. From an engineering point of view, the shape of the manhole cover depends entirely on the shape of the hole to be covered.
Interviewer: (difficult) I want to talk to the management. (Left the room) 10 minutes later, the interviewer came back.
Interviewer: We recommend that you work in the sales promotion department immediately.