1 00:00:00,000 --> 00:00:16,769 When was the last time you had to do this? 2 00:00:16,769 --> 00:00:21,769 Or this, to get across a stream, river, fjord, or lava pit? 3 00:00:21,769 --> 00:00:26,769 Probably never, because we have these things called bridges. 4 00:00:26,769 --> 00:00:28,769 Bridges are awesome! 5 00:00:28,769 --> 00:00:32,770 But what are all these different types of bridges that you see around? 6 00:00:32,770 --> 00:00:34,770 Why don't they all look the same? 7 00:00:34,770 --> 00:00:36,770 Let's find out! 8 00:00:36,770 --> 00:00:37,770 Engineering! 9 00:00:37,770 --> 00:00:43,719 The most basic type of bridge is a beam bridge. 10 00:00:43,719 --> 00:00:50,460 It has the most simple design, with a beam, also called the bridge deck, that is supported 11 00:00:50,460 --> 00:00:52,780 at both ends. 12 00:00:52,780 --> 00:00:57,340 Let's drop into our diagram mode to see how the forces act upon this bridge. 13 00:00:57,340 --> 00:01:02,979 When a load is applied to these bridges, they bend and experience two types of forces. 14 00:01:02,979 --> 00:01:07,019 The top of the beam is pushed together, which is called compression. 15 00:01:07,019 --> 00:01:15,099 The bottom of the bridge is pulled apart, which is called tension. 16 00:01:15,099 --> 00:01:21,500 Here we are using lasers to build model bridges out of balsa wood and plywood. 17 00:01:21,500 --> 00:01:24,099 Always wear purple gloves while handling balsa wood. 18 00:01:24,099 --> 00:01:26,060 Safety first, kids. 19 00:01:26,060 --> 00:01:27,819 You can see the simple design. 20 00:01:27,819 --> 00:01:31,299 Now let's see what happens when we apply forces to it. 21 00:01:31,299 --> 00:01:33,700 To do this, we'll use a compression tester that 22 00:01:33,700 --> 00:01:37,939 can apply up to 20,000 pounds of force, although we probably 23 00:01:37,939 --> 00:01:40,379 won't get that high in this test. 24 00:01:40,379 --> 00:01:43,959 The compression tester will apply a load, or in other words, a weight, 25 00:01:43,959 --> 00:01:48,959 and measure the strain, which is how far the material moves in response to a load. 26 00:01:48,959 --> 00:01:53,019 Also to help us on this test will be these LEGO people, who will measure the ability 27 00:01:53,019 --> 00:01:56,239 to be launched from a bridge when it breaks. 28 00:01:56,239 --> 00:01:59,099 Critical information for bridge designers. 29 00:01:59,099 --> 00:02:00,099 Officer? 30 00:02:00,099 --> 00:02:01,099 Hello. 31 00:02:01,099 --> 00:02:08,539 You see that as the load increases, the bridge bends, and it eventually breaks at a load 32 00:02:08,539 --> 00:02:11,439 of 115 pounds. 33 00:02:11,439 --> 00:02:14,560 And now, what if you want to span a longer distance? 34 00:02:14,560 --> 00:02:21,939 Here we use the same deck material on top, but we have doubled the length of the bridge. 35 00:02:21,939 --> 00:02:27,479 This time, our main test subjects will be the happy farmer chef, hat man, bike guy with 36 00:02:27,479 --> 00:02:31,580 no helmet, shame on you. 37 00:02:31,580 --> 00:02:36,159 You can see that the longer bridge bends even more, and it breaks at a much lower load, 38 00:02:36,159 --> 00:02:41,979 only 25 pounds, which is 80% weaker than the short beam bridge. 39 00:02:41,979 --> 00:02:45,319 Plus, the LEGO people get launched into space. 40 00:02:45,319 --> 00:02:46,639 That isn't very good. 41 00:02:46,639 --> 00:02:50,139 We want the bridge to support a lot of weight over a long distance. 42 00:02:50,139 --> 00:02:53,740 So how can we make bridges that span longer distances? 43 00:02:53,740 --> 00:02:57,580 There are other bridge designs that allow us to use the same deck material 44 00:02:57,580 --> 00:03:00,379 and span longer distances. 45 00:03:00,379 --> 00:03:05,460 This takes us to the curviest of bridges, arch bridges. 46 00:03:05,460 --> 00:03:10,000 An arch is a great way to evenly distribute a load and has been used in bridge building 47 00:03:10,000 --> 00:03:13,460 for a long time. 48 00:03:13,460 --> 00:03:20,199 In this example, we just have a beam bridge with an arch underneath. 49 00:03:20,199 --> 00:03:24,039 The load is distributed through the arch into the ground. 50 00:03:24,039 --> 00:03:29,719 Notice how only compressive forces are present and no tension forces. 51 00:03:29,719 --> 00:03:33,979 This is great for wood and stone bridges because they are much stronger in compression than 52 00:03:33,979 --> 00:03:35,740 tension. 53 00:03:35,740 --> 00:03:40,780 On our model arch bridge, we used the same long bridge length, with added arch supports 54 00:03:40,780 --> 00:03:43,060 on both sides. 55 00:03:43,060 --> 00:03:52,099 To help us is Tricycle Man, Helmet, Good Job, Classy Business Woman, and this crazy guy. 56 00:03:52,099 --> 00:03:54,060 Let's go! 57 00:03:54,060 --> 00:03:58,539 You can see that the beam does not bend as much with the arch underneath, and that reaches 58 00:03:58,539 --> 00:04:05,639 a higher load before it fails, 100 pounds, which is 500% stronger than the long beam 59 00:04:05,639 --> 00:04:08,539 bridge. 60 00:04:08,539 --> 00:04:12,300 Here we have seen that beam bridges get weaker as they get longer. 61 00:04:12,300 --> 00:04:17,939 Adding an arch makes the bridge very strong, but arches have length limitations. 62 00:04:17,939 --> 00:04:23,160 Beam and arch bridges historically have relied on wood and stone as building materials. 63 00:04:23,160 --> 00:04:28,439 These materials are usually only strong in compression and not in tension, which restricted 64 00:04:28,439 --> 00:04:30,600 the designs that could be used. 65 00:04:30,600 --> 00:04:35,819 However, with the discovery of steel, bridge builders now had the ability to add structural 66 00:04:35,819 --> 00:04:39,220 units that would be strong in tension. 67 00:04:39,220 --> 00:04:44,439 This led to more intricate styles of bridges that we will explore in our next video.