Columns: Finding the Strongest Shape

Resource for Grades K-8

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Columns: Finding the Strongest Shape (Document)

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Background Essay

Columns are among the oldest and most effective building elements for resisting the pushing, or compression, that takes place in all kinds of structures. Paired with horizontal beams, these critical forms can carry the weight of an entire structure. Still, each column has its limits, and some types perform far better than others.

Columns can fail, or collapse, in two ways: by crushing and by buckling. Crushing results when the weight of a well-distributed load simply exceeds the compressive strength of the material from which the column is made. Because columns are usually constructed from dense materials such as stone, concrete, or steel, this type of failure is relatively uncommon.

Columns usually fail for reasons of instability, rather than lack of strength. Assuming that a column is attached to a solid foundation, stability is usually related to the column's dimensions. Shorter, thicker columns are far more stable than long, slender columns. Like short, thick horizontal beams, shorter, thicker columns are more resistant to bending than are slender columns, and a column that resists bending also resists buckling.

Shape also plays an important role in the stability of a column. The cylinder is one of the most widely used shapes in column construction. This is because its circular cross section places the column's material and its outer boundary at a consistent distance from the center, such that the column has no side that is weaker -- and thus more susceptible to buckling -- than any other.

Finally, proper placement of a load is critical to the stability of a column. The more closely a load is applied to the center of the top of a column, the more stable that column will be. As a load moves away from the center of a column, the side of column to which the load shifts takes on more and more of the load. Such an unbalanced load can cause a column to bend in the same way that an overloaded beam bends, and may lead to buckling.

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Discussion Questions

Both figures with straight sides -- the square and the triangle -- collapsed. Do you think we could make a column with straight sides that would work?

How important do you think the size of the circle is? What would happen with bigger and smaller circles?

How would you test a column to compare its strength in compression (a force pushing down on the vertical axis of the column) and in tension (a force pushing or pulling horizontally on the column)?

What are the advantages of columns for use in buildings?

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Transcript

(humming)

VOICE: (gasps) Ooh!

PABLO: Tom G. of Revere, Massachusetts, sent us this challenge. Make four shapes of paper—a tent, a triangle, a box and a cylinder—then test them to see which one can hold the most books.

ALISA: What do you think will happen?

PABLO: Well, I think the tent will not support not even one book, because it's, like, there's not much support. There's just this thing right here, and they'll just go...

ALISA: There's nothing, like, underneath to help it stay up.

PABLO: This might, but... I don't know.

ALISA: I think these three might work good.

PABLO: Maybe.

ALISA: Let's test this one first, because these all look like they have enough support, so maybe they'll work. Okay, do it gently. I'm not even putting it on. Whoa... that was a flop.

PABLO: No, I don't think...

ALISA: Let's try it another way.

PABLO: How about the long way?

ALISA: Okay. Kerplunk. Okay. (imitating plane dive-bombing)

PABLO: Guess that one didn't work because, um... it was just... it was, like, this was no support—the support was right there.

ALISA: In the fold.

PABLO: All right, let's test...

ALISA: Triangle?

PABLO: Triangle. This one looks like it can support it.

ALISA: Yeah. Um... nope. I have to say that didn't work.

PABLO: Uh... Why do you think it didn't work?

ALISA: Maybe because, like, where the folds are, there's the most strength, because it can hold my finger up, but right here, there's no strength so that the book—when I put pressure here, it collapsed.

PABLO: Oh.

ALISA: So... Okay, this one now?

PABLO: Yep.

ALISA: This is almost the same thing as the triangle, so do you think it will work? Because it's got the folds, and then this part...

PABLO: It will probably do the same thing as the triangle, but let's just see.

ALISA: A girl's touch is always more careful.

PABLO: Yep. (gasps)

ALISA: No! You have to face it—our square...

PABLO: Oh, but it almost did.

ALISA: I know, it, like, stayed for, like, a "ding!" and then it dropped. You're our only hope.

PABLO: Come on, Mr. Cylinder.

ALISA: Okay, so, you have to place the book right even, right?

PABLO: Yeah.

PABLO AND ALISA: Yeah!

ALISA: Whoa, watch out!

PABLO: Okay.

ALISA: Okay. Try the next one.

PABLO: No! But at least it held one.

ALISA: Yeah.

PABLO: Well, first—why do you think it did hold?

ALISA: Because, remem... Well, I think because, like, you know how I said that the folds were, like, stronger and the sides were, like, bleah? Well, it's all the same. All the weight is even on this...

PABLO: Oh!

ALISA: So it, like, took it evenly. Hip, hip, hooray! Yay!