Why are manhole covers round?



The in basket: As I told you in the last  Road Warrior, General Manager Larry Curles of the West Sound Utility District (the merged Karcher Creek Sewer District and Annapolis Water District) had a quasi-riddle for me when I called him to ask why sewer and water lines are laid in the street rather than on the shoulder.

He asked if I knew why manholes covers are round?

The out basket: Because the manholes are round, my wife, the Judybaker guessed.  But that just raises the question of why manholes are round. 

Larry said manhole covers are round because they can’t fall into the manhole. 

At first blush, you’d think square covers also wouldn’t fall in if they are slightly larger than the manhole.

But, Larry points out, the distance from corner to corner of a square is longer than the distance along a side. If the cover were oriented with the sides pointing at the corners, if easily could drop into the hole, injuring or killing someone below.

There probably are thousands of you who already knew that, but it was a new one to me.

“Some traditional practices have a lot of common sense behind them,” Larry noted. 


4 thoughts on “Why are manhole covers round?

  1. Re your Road Warrior feature of August 6, 2008:


    I imagine that what you mean when you say that “the distance from corner to corner of a square is shorter than the distance along a side,” is that the diagonal is shorter than the side.

    In fact, a diagonal is always LONGER than the sides of a square.

    The formula for finding the length of a diagonal of a square is “s” (the length of the side) times the square root of 2 (which is 1.41). So the diagonal of a square is always 1.41 times the length of a side.

    If both the hole and the cover are square and their sides are approximately the same size, the cover might well slip into the hole, since its sides will be shorter than the diagonal of the hole.

  2. Jack (and Bruce Loughridge a bit later) caught me in a stupid mistake in which I wrote the opposite of what I meant. Of course, the corner-to-corner measurement of a square is LONGER than that of the sides, something so obvious I suspect other readers who caught it wrote it off as the stupid mistake it was.
    I corrected the column online, so it now correctly reads “longer” not “shorter.”

  3. Travis,

    In talking with people today and after being in the teaching business, I can tell you that even a mistake so obvious goes unnoticed these days. You wouldn’t believe how many people don’t even know what the pythagorean theorem is let along know how to solve for the hypotenuse of a right triangle. I’d bet that more than half the people who read that column wouldn’t know.

    It just shows the sorry state of our math education these days. Math and science. Of course you can’t do well in science if you are poor in math.

  4. Bruce you’ll be happy to know…the Gates Foundation is making a difference…but not, apparently, welcome in Washington State.

    “Working with partners in 47 states and the District of Columbia, the Bill & Melinda Gates Foundation is supporting promising initiatives across America that are redefining the high school experience and working to prepare more students for college, career, and life:

    * KIPP (Knowledge is Power Program) graduated its first-ever high school class this year in Houston. Ninety-six percent of the graduating seniors at KIPP Houston High School are headed to college in the fall. There are currently 66 KIPP public schools nationwide; seven are high schools.

    * At University High School of Science and Engineering, in Hartford, Conn., 100 percent of the first senior class earned diplomas this year, and 80 percent of the graduates are headed to four-year colleges or universities. Students from the greater Hartford area are drawn to University High for its college-level coursework and college-going culture. It is a school model developed through the Early College High School Initiative (ECHSI), which allows traditionally underserved students to graduate with a high school diploma and one to two years worth of college credit. To date, ECHSI organizations have opened nearly 160 early colleges in 24 states and the District of Columbia. Ultimately, about 250 early college high schools will serve over 100,000 students each year.

    * In New York City, 93 of the new small schools opened since 2002 will graduate classes this year, sending thousands of graduates into the world better prepared for college, career, and life. At the Bronx Lab School, where students are challenged with a rigorous liberal arts college preparatory experience, approximately 90 percent of the school’s first class is on track to graduate. Additionally, nearly 85 percent of seniors passed the Math A and Global Studies Regents exams before the end of their sophomore year. Five Bronx Lab graduates are headed to college—at Middlebury, Depauw, Trinity, Brandeis, and Lafayette—on four-year scholarships from the Posse Foundation.

    * In Texas, all seniors at IDEA College Preparatory have been accepted to four-year colleges and universities, including Tufts, Baylor, Case Western Reserve, and Texas A&M. The class of 2008 is 94 percent Hispanic, and 71 percent of seniors will be the first in their families to enroll in college. Similarly, for the eighth year in a row, 100 percent of the graduating classes at five YES Prep campuses in Houston have been accepted to four-year colleges. IDEA and YES schools recently held college acceptance celebrations, where students signed their matriculation letters in front of family and friends.

    * The first group of DC Achiever Scholars—194 students in Washington, D.C., who were selected to receive college scholarships and application support—graduated from high school in our nation’s capital. The vast majority of these students are the first members of their families to go on to college.

    * At San Diego’s Met School, all 51 members of the first graduating senior class will attend college in the fall, including schools like Dartmouth, UC Berkeley, and Spelman. Opened in 2004, the Met is an alternative high school that offers small classes and a personalized curriculum centered around internships with local businesses. Students also are required to enroll in college classes. Part of The Big Picture Co., a nationwide network of alternative high schools, the Met serves a predominantly low-income and minority student population and has met state and federal targets for academic growth.

    * In Denver, all 79 seniors in the first graduating class at the Denver School of Science and Technology have been accepted into four-year colleges, and nearly half of them will be the first in their families to attend college. DSST is a public charter school serving a diverse student body with a math and science curriculum. It is part of a growing national network of science, technology, engineering, and math (STEM) schools that encourages problem solving, critical thinking, and creativity through a project-based, interdisciplinary curriculum.

    * At Withrow University High School in Cincinnati, Ohio, 98 percent of seniors graduated on time. Altogether the 171 students in the Class of 2008 have earned $2.5 million in scholarships and more than 80 percent of them have been accepted into their college of choice. This is the third graduating class since the high school was transformed from a traditional comprehensive high school into a college preparatory school in the fall of 2002.

    Unfortunately, these exemplary schools remain the exception rather than the rule in America. Too many students are still trapped in schools that don’t offer rigorous, high-quality learning experiences. African-American and Hispanic students are particularly at risk and graduate at a lower rate than average—55 and 58 percent, respectively. Leaving high school is a costly decision. Dropouts can expect to earn a million dollars less over a lifetime, compared to the average earnings of a college graduate, according to the College Board.

    “Improving our education system requires strong leadership from the White House to the schoolhouse and a commitment to solutions rather than soundbites,” said Roy Romer, former governor of Colorado and chair of Ed in ’08, a nonpartisan public awareness and advocacy effort focused on making education reform a top national priority. “We know what works. To be successful, every school needs a strong set of common standards, effective teachers in the classroom, and more time for students to learn.”

    …Thirty-three states have made a commitment to align standards, graduation requirements, assessments, and accountability policies with the demands of college and career by joining the American Diploma Project. … Only four states—Arizona, Delaware, Florida, and Utah—have taken all the necessary steps to update their data systems to track students, identify those at risk of dropping out, and accurately report graduation rates, according to the Data Quality Campaign. Similar data system improvements are underway in several other states.

    Nationally, the Bill & Melinda Gates Foundation and its partners are pursuing efforts designed to increase graduation and college-readiness rates. Since 2000, the foundation has invested more than $1.9 billion in more than 1,800 schools….”

    Why isn’t Washington State mentioned…?
    Sharon O’Hara

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