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Bridge design and destruction! (part 1)

This is a two part video that introduces the different types of bridges. The bridge types will be introduced in historical order from simple (beam and arch) in the first video to more complex (truss and suspension) in the second video. This will show how bridges evolved as our designs and available materials expanded. The design of each bridge will be shown schematically along with the load distribution. The advantages and limitations of each type will be described. Video of actual bridges around Boston of the different types will be shown with the load distributions overlaid on freeze frames. Other iconic bridges will also be shown as still images with load distributions. Technical concepts covered will be tension, compression, bending, span, force balances, material selection and beam/cable design. License: Creative Commons BY-NC-SA More information at http://k12videos.mit.edu/terms-conditions. Created by MIT+K12.

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Video transcript

When was the last time you had to do this, or this, to get across a stream, river, fjord or lava pit? Probably never, because we have these things called bridges. Bridges are awesome. But what are all these different types of bridges that you see around? Why don't they all look the same? Let's find out. Engineering. The most basic type of bridge is a beam bridge. It has the most simple design, with a beam, also called the bridge deck, that is supported at both ends. Let's drop into our diagram mode to see how the forces act upon this bridge. When a load is applied to these bridges, they bend and experience two types of forces. The top of the beam is pushed together, which is called compression. The bottom of the bridge is pulled apart, which is called tension. Here, we are using lasers to build model bridges out of balsa wood and plywood. Always wear purple gloves while handling balsa wood. Safety first kids. You can see the simple design. Now, let's see what happens when we apply forces to it. To do this, we'll use a compression tester that can apply up to 20,000 pounds of force, although we probably won't get that high on this test. The compression tester will apply a load, or in other words, a weight, and measure the strength, or just how far the material moves in response to a load. Also to help us on this test will be these LEGO people, who will measure the ability to be launched from a bridge when it breaks. Critical information for bridge designers. Officer. Hello. You see that as the load increases the bridge bends. And it eventually breaks at a load of 115 pounds. But now, what if you want to span a longer distance? Here we use the same deck material on top, but we have doubled the length of the bridge. This time, our main test subjects will be the happy former chef, hat man, bike guy with no helmet-- shame on you. You can see that the longer bridge bends even more, and it breaks a much lower load, only 25 pounds, which is 80% percent weaker than the short beam bridge. Plus, the LEGO people get launched into space. That isn't very good. We want the bridge the sport a lot of weight over a long distance. So how can we make bridges that span longer distances? There are other bridges designs that allow us to use the same deck material and span longer distances. This takes us to the curviest of bridges, arch bridges. An arch is a great way to evenly distribute a load, and has been used in bridge building for a long time. In this example, we just have a beam bridge with an arch underneath. The load is distributed through the arch into the ground. Notice how only compressive forces are present, and no tension forces. This is great for wood and stone bridges, because they are much stronger in compression than tension. In our model arch bridge, we used the same long bridge length, with added arch supports on both sides. To help us is tricycle man, helmet-- good job-- classy businesswoman, and this, crazy guy. Let's go. You can see that the beam does not bend as much with the arch underneath. And that reaches a higher load before it fails, 100 pounds, which is 500% stronger than the long beam bridge. Here, we have seen that beam bridges get weaker as they get longer. Adding an arch makes the bridge very strong. But arches have length limitations. Beam and arch bridges historically have relied on wood and stone as building materials. These materials are usually only strong in compression, and not in tension, which restricted the designs that could be used. However, with the discovery of steel bridge builders now had the ability to add structural units that would be strong in tension. This led to more intricate styles of bridges that we will explore in our next video. [MUSIC PLAYING]