|MadSci Network: Engineering|
Hi Grace, Great question! Tubes and folds help make things stronger by changing the way that the load or the stress is applied to the material. If you take a normal sheet of paper, it's got quite a bit of tensile (pulling) strength. Try this: Take a fresh sheet of paper, grab two opposite edges about in the middle of the sheet and start pulling straight out. It should take a lot of force to make it tear. However, paper has virtually no compressive (squeezing) or flexural (bending) strength because the paper bends. In engineering, this failure mechanism is called buckling, and it's the failure mechanism that structural engineers and architects worry the most about. So for paper, as well as any material, by curving or folding the material in a particular direction, the compressive strength can be increased in that direction by preventing buckling. Imagine that you're one of the millions of tiny fibers in the paper. You're strong if someone pulls on you, but you bend really easily. If all of the fibers in the sheet point the same way you do, then all of you are easily bent. However, if you wrap the paper into a tube, then when a force is applied that would cause you to bend, some of the other fibers are rotated so that they are getting a tensile force, and they can support that easily. So the paper tube can take forces in directions (Compression, Bending) that would cause a piece of paper to buckle. There's a bunch of equations I could use here, but suffice it to say that larger diameter tubes are typically stronger than small diameter tubes, and thicker tube walls are typically stronger than thin walls. For any structure that you're going to build, you have to consider the weight, the cost, and the strength of the materials you're going to use. That's why hollow tubes and I-beams are always used in construction. And not just when we build bridges, your bones are hollow for this same reason. More strength for less material. Here's one last thing to try. Take an empty, undamaged can of soda outside. Stand it on the ground. Carefully put one of your feet on top, and slowly transfer all of your weight onto it until you're standing on one foot on the can. It'll help if you have something to hold onto with your hands. Have a friend or family member take a ruler (Or a pencil, or a stick, but definitely do NOT use their finger!) and tap the side of the can. Once they hit the side hard enough to dent it, the can should quickly crush under your weight. This illustrates how strong the can is when the tube is solid, and how much weaker it becomes once it buckles. The aluminum is like your paper, and is strong in tension, but not very strong under bending. Next, grab another can, pre-dent the side, then try and stand on it. It might not hold your weight at all. Keep asking questions! Jeff Yap Mad Scientist Related links: American Architectural Foundation M.N. Gogate's Glimpses of Structural Engineering Free software to predict buckling Bicycles and thin walled aluminum
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