Shapes Lab Forces Lab | Materials Lab | Loads Lab | Shapes Lab About This Lab These labs simplify the real-life conditions that affect structures (shapes), in order to illustrate key concepts. In the real world, many variables affect the strength and stability of any give shape. The choice of materials, joints, load distribution, and size and thickness of a structure all affect its ability to resist loads. For example, a triangle made of paper would collapse much sooner than an arch made of steel -- an effect that was not demonstrated in this lab. The shape comparisons in this lab depend upon the following conditions: each shape is of equivalent thickness, the joints are hinged, and the live load is applied downward to the structure at a single point at its top and center. Intro/Instructions The shape of a structure affects how strong it is. Rectangles, arches, and triangles are the most common shapes used to build big structures. One Elephant on Rectangle The weight pushes down on the rectangle and causes the top side to bend. One Elephant on Arch The weight presses down on the arch and is spread outward along the curve to the ground below. One Elephant on Triangle The weight causes the top two sides to squeeze together and the bottom side to pull apart. Three Elephant on Rectangle The weight caused the top side to bend too much, so it failed! Three Elephant on Arch The arch likes to be pushed and squeezed. The weight pushes this arch into a stable, tightly squeezed shape. Three Elephant on Triangle Unlike the rectangle, the sides of the triangle did not bend under the tremendous weight. This is why the triangle is still standing. Six Elephant on Rectangle The weight caused the top side to bend too much, so it failed! Six Elephant on Arch The arch likes to be pushed and squeezed, but not this much! When an arch is pushed too hard, the sides spread apart and collapse. Six Elephant on Triangle The triangle is still standing because the pulling force in the bottom side is balancing the pushing forces in the upper sides. Nine Elephant on Rectangle The weight caused the top side to bend too much, so it failed! Nine Elephant on Arch The arch likes to be pushed and squeezed, but not this much! When an arch is pushed too hard, the sides spread apart and collapse. Nine Elephant on Triangle Even triangles have their limits! All this weight made the third side stretch so much that it snapped in half. Push Rectangle What happens when you push the side of a rectangle? The rectangle is a wobbly, unstable shape. When you push the side, it flops into a slanted parallelogram. This happens without any of the rectangle's sides changing length. Push Braced Rectangle Now when you push the side, the diagonal brace gets squeezed, preventing the rectangle from flopping over. Push Arch What happens when you push down on an arch that is not supported on both sides? The force of the finger pushes the sides of the arch outward. Push Braced Arch As the arch tries to spread outward, external supports, called buttresses, push back on the sides of the arch and prevent it from spreading apart. Push Triangle What happens when you push the side of a triangle? The outer edge squeezes together, and the inner edge pulls apart. When one side experiences these two forces at the same time, it bends. The weakest part of the triangle is its side! Push Braced Triangle When you poke the top of the triangle, the two sides squeeze together and the bottom side pulls apart. The triangle doesn't bend because each side experiences only one force at a time. When used properly, triangles are the most stable and rigid shapes used in construction today. Flash version of this lab

 This Web site was produced for PBS Online by WGBH. Web site © 2000-2001 WGBH Educational Foundation.