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In our last video, we looked at the simple designs of beam and arch bridges.

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Now let's move into the modern age with the truss bridge.

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Truss bridges make use of a large frame, called a truss, that sits on top or below the bridge deck.

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In this case, it is on top.

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While it may seem like we are only adding weight to the deck,

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the design of the truss distributes the load through the frame so that the deck does not experience as much of a load.

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Each segment of the truss experiences different loads of either tension or compression.

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We apply two equal loads to the deck and calculate the loads in each segment,

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which are shown as percentages of the total load.

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You can see that the largest loads are on the end and top segments,

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while the middle segments have none.

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Remember that when we do the compression test. Spoiler alert!

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Let's see if this convict gets shot out of his truss jail.

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So how do you think the truss will break?

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Discuss.

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Thanks for coming, Yoda. I love your work.

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As you can see, the outer segments of the truss are the first to break

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because they were handling the largest part of the load.

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The diagram showed that the outer and top segments had the same loads.

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Why didn't the top break?

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That's because the top pieces are aligned along the grain of the wood

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and wood is stronger in that direction. Adding the truss allowed the same deck

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length to hold 32 pounds, which is 25% stronger than the beam bridge of the

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same length. The final type of bridge we'll discuss is the iconic suspension

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bridge. Although the only suspension bridge around us is less than iconic, but

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the same principles apply. Suspension bridges utilize thick steel cables that

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support the deck and transfer the load to the towers and to the anchors at the end of

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the bridge.

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Supporting cables are used to suspend the bridge deck from the main cables.

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The main cables and supporting cables of the bridge are always under tension.

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The cables transfer the load to the towers, which experience compression, and also to

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the anchors at the end of the bridge.

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In our model, we used wires for the main cables and supporting cables.

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Some of the construction is not ideal because it is difficult to simulate some of the joining

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points and anchors on a small scale.

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For this test, we need a full cast of characters.

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The Misfits versus the Bike Gang.

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Oh, there's Crazy Guy again.

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Classic Crazy Guy.

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As force is applied, the cables transfer the load out to the towers and anchor points at

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the end.

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The full force distribution maintains the integrity of the deck so that even when it does break, it doesn't really launch anyone.

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Unfortunately, I really wanted to see the crazy guy get launched.

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This bridge supported 32 pounds, which is the same as the truss bridge.

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The truss and suspension bridges were stronger than the long beam bridge, but weaker than the arch bridge.

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This may have been unexpected, but the real advantage of truss and suspension bridges

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are that they can span longer distances than beam and arch bridges.

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Now, what we've all been waiting for.

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Crushing a Lego Man.