Why are manhole covers round?

If you go down a busy street in a big city, you'll see manholes. They're very common along the road.

Maintenance holes are openings that lead to an underground public utility such as a sewer or a drainage system. Engineers, laborers and inspectors use them to inspect, clean, repair and maintain utilities.

Why are manholes round? The answer is that the manholes are round. Josh Nelson, an associate professor in the Department of Design at San Jose State University, said that a host of practical, functional, engineering and economic decisions play into the decision to make a manhole cover.

According to Atlas Obscura, manholes date back to the 19th century when cities built underground sewage systems. Engineers needed to access the sewer systems to inspect the pipes during maintenance. The access points were called "manholes" and they became the portals through which engineers or inspectors could investigate the pipes.

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The manholes are covered with metal plates. The manhole covers in New York City are usually made from concrete or cast iron and weigh between 200 and 300 pounds. There is a lip around the edge of the open manhole. The covers protect unwary pedestrians from inadvertently falling into the sewer, but also protect the sewer from debris, such as trash or organic materials, which can potentially cause sewer and water system problems.

What causes pipes to burst?

The covers of the manholes are usually round. The shape of manholes has caused a lot of speculation, and the question of why manhole covers are round has become a cliché. Microsoft began to ask this question during job interviews in the 1990's.

Microsoft interviewers asked the question because it was a way for the interviewee to give a creative answer.

Nelson agreed. He said that the shape of many material objects is determined by a number of factors.

Nelson told Live Science that man holes are typically made from rotational tools. The best way to cover the manhole is a circular shape. Anything else wouldn't be as economical.

An answer that shows up consistently on internet searches is that a round manhole cover won't fall back through its opening if it's rotation. A square cover might fall through its opening if it rotates.

There are other reasons why manholes are round. Rolling a round cover along the ground can be used to transport it. The surface area of a round manhole cover is smaller than that of a square cover so it is more efficient to produce.

The original article was published on January 25, 2013).