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Bridge Design (and Destruction) Part 1

6 minutes

(Describer) A machine burns lines into wood, drawing rectangles that form the letters MIT. They turn into a red and grey logo. Title: Massachusetts Institute of Technology.

[whirring]

(Describer) Title: Bridge Design (and Destruction!) part 1.

(male narrator) When did you last do this...

(Describer) ..ride an elephant or drive a car through water.

or this to get across a stream, river, fjord, or lava pit? Probably never because we have these things called bridges. But what are all these different types of bridges? Why don't they look the same? Let's find out.

The most basic type of bridge is a beam bridge.

(Describer) It’s straight across with supports at the ends.

It has the most simple design,

with a beam, also called the bridge deck, that is supported at both ends. Our diagram mode shows 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 beam is pushed together, which is called compression.

The bottom is pulled apart, which is called tension.

(Describer) The machine that drew the logo cuts through more wood.

Here we are using lasers to build model bridges out of balsa wood and plywood. Wear purple gloves while handling balsa wood. Safety first, kids.

(Describer) The constructed model with a block in the middle is placed under a column and clipped to the base.

You can see the simple design. Now let's see what happens when we apply forces. We'll use a compression tester that can apply 20,000 pounds of force, although we probably won't get that high on this test. The compression tester will apply a load or weight and measure the strength, which is how far the material moves when loaded. Also to help us will be these LEGO people, who will be launched from the bridge when it breaks. Critical information for bridge designers. Officer. Hello.

(Describer) The column end is lowered.

You see that as the load increases, the bridge bends, and it eventually breaks at a load of 115 pounds. What if you want to span a longer distance? We'll use the same deck material on top, but we have doubled the length of the bridge.

(Describer) It’s clipped and the block is placed in the middle. Lego figures are added.

This time, our main test subjects will be the Happy Farmer Chef, Hat Man, Bike Guy with no helmet.

Shame on you. [horn honks]

(Describer) The column is lowered.

The longer bridge bends even more, and it breaks at a much lower load--only 25 pounds, which is 80% percent weaker than the short beam bridge.

(Describer) The bridge snaps.

Plus, the LEGO people get launched into space. That isn't very good. Bridges must support a lot of weight over a long distance. How can we make bridges that span longer distances? Other bridge designs allow us to use the same deck material and span longer distances. This takes us to the curviest of bridges, arch bridges.

(Describer) ...with arch supports along the bottom.

An arch is a great way to evenly distribute a load and has been used in bridge building for a long time.

(Describer) Other examples are shown.

In this example, we just have a beam bridge with an arch underneath.

(Describer) One section, with one arch, becomes part of a diagram.

The load is distributed through the arch into the ground. Notice how only compressive forces are present and no tension forces.

(Describer) The lasers cut more bridge parts.

This is great for wood and stone bridges because they are much stronger in compression than tension. In our arch bridge, we used the same long bridge length with added arch supports on both sides.

(Describer) It’s placed under the column.

To help us is Tricycle Man, helmet--good job, Classy Businesswoman, and this, Crazy Guy. Let's go.

(Describer) The pressure is added.

The beam does not bend as much with the arch underneath. That reaches a higher load before it fails, 100 pounds,

(Describer) It snaps.

which is 500% stronger than the long beam bridge.

(Describer) The tests are shown again.

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 styles of bridges that we will explore in our next video.

(Describer) Beside two Lego figures, titles: Ian Fein, Filmmaker. Alan Lai, Nerd/Scientist. Accessibility provided by the U.S. Department of Education.

Accessibility provided by the U.S. Department of Education.

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This two-part video introduces the different types of bridges. In part one, simple beam and arch bridges are introduced. The design of each bridge is shown schematically along with the load distribution. The advantages and limitations of each type of bridge is also discussed. Technical concepts covered include tension, compression, bending, span, force balances, material selection, and beam/cable design. Part of "Bridge Design (and Destruction)" series.

Media Details

Runtime: 6 minutes

Bridge Design (and Destruction)
Episode 1
6 minutes
Grade Level: 7 - 12
Bridge Design (and Destruction)
Episode 2
5 minutes
Grade Level: 7 - 12