What makes bridges strong

Or put a long strip of cardboard between two short blocks. That's a beam bridge too. All bridges can hold a certain amount of weight, but what happens if we put too much weight on a beam bridge?

Let's find out. It collapses. So a bridge that carries trucks and cars, which are very heavy, would have to be stronger than a bridge that carries bikes or people on foot, which are lighter. So how do we make stronger bridges? Well, over time, people have learned that certain shapes can be used to make stronger bridges.

Take a look at this railroad bridge. It has to be strong because it carries trains. What shape do you see? That's right, triangles. And that's not by accident. The fact is, triangles are really strong shapes for building. If you put force on one side of a triangle, it bends. But if you put force on its point, it keeps its shape. That's because the two sides of the triangle are pushed down by the force and the bottom gets stretched out to both sides.

Each side feels the force, but none of them bends, and this makes the triangle a really sturdy and stable shape. This is why you'll see lots of triangles in bridges, both above the part that you actually travel on, called the deck, and below it.

The long string of triangles that you see in a bridge is called a truss. Trusses help a bridge spread out the weight that it has to carry. But not all bridges are made of trusses.

If a bridge has to cross a really wide body of water, it might be too difficult or expensive to build a truss bridge. So engineers designed another kind of bridge called a suspension bridge. The Golden Gate Bridge in California is a great example of a suspension bridge. Suspension bridges work by using a force called tension. Hands-On Activity 30 min. Scroll for prep. Please wait…. This video is having trouble loading. You may have lost your Internet connection.

Step 1: Click to Reload this page. Step 2: Click to Try our other video player. Step 3: contact support if trouble persists. Or, dismiss this message. Full Screen. Exit Full Screen. Video trouble? How did it go? Extend this lesson:. Video Camera Icon. Sign up now for more great lessons!

Sign up. Reading: Read about why that bridge collapsed. Activity: Discover what it feels like to be a bridge. Activity: Gumdrops and toothpicks make great trusses. Going Further: Additional resources on paper bridges.

Online resources: Learn more about the Golden Gate Bridge. Online resource: Experiment with bridge building online on Building Big: Bridges. Paper Bridges To learn more about paper bridges, check out these sites. Golden Gate Bridge Learn more about this world-famous bridge. Lesson Image. Golden Gate by James W. Shepp , used under Public Domain. Exploration 22 min Hands-On Activity 30 min. I taught both 3rd grade classes this lesson, and they had an amazing time learning from their mistakes and building stronger bridges.

We went on a field trip to San Francisco a week after I taught this lesson, and all the kids started shouting, "Look, a suspension bridge!!! With pillars!! It was a proud moment for me! They tried and failed and tried again. They loved this!!! The emphasis on the "test and redesign" model worked well--this is an important process for students to start to grapple with, and can be applied to any inquiry or research opportunity that we encounter.

We'll be able to connect back to this lesson as a touchstone for many things we do in the future. During the bridge design activity, I noticed my students using problem solving skills, persistence, and teamwork in order to make the strongest bridge. Their favorite was the Golden Gate Bridge because it was familiar to them. They also liked the competitive nature of building the strongest bridge. The videos were a perfect way to introduce the lesson and the paper bridges were easy and taught the engineering design process!

I've used this lesson before, and the students are able to work in groups independently after watching the videos.

They love this lesson! I love how the lessons promote thinking and collaboration. They were engaged during the video and enjoyed the entire lesson. I appreciate how complete the lesson is, with teacher tips, materials, videos, and all directions given clearly.

I also did the extension activity with gumdrops. They loved this as well. The experiment morphed into a research and writing project to learn about more types of bridges and about building designs. The activity also allowed my second language learners to demonstrate their incredible intelligence.

Their bridges were amazing! The whole bridge design project encourages greater thinking and problem solving. They had so much fun and were completely engaged! After the first bridge collapse, both of my boys were designing their next one, with changes in mind. My 9 year old son, Cody, made a folded bridge with 4 pillar supports. That bridge held coins! How fun! I ran out of coins for some of the paper bridges. I extended this with making human bridges. They loved experiencing the weight, pulls and pushes of the structures.

They really got creative within their groups. I wish I could show you the great pictures I got!! Building, re-designing, learning from our failures- all of it was great! They worked together building bridges and testing their ideas, then trying out a different method or way learning from their mistakes. A great activity. I learned much myself. He had fun building bridges out of paper. Bridges are often made using arches of concrete.

Concrete has very high compression strength, and an arch is almost entirely under compression instead of tension. This is due to the geometry of the arch, which channels the load downward into the ground. Most smaller bridges, such as those used for freeway overpasses, use a trick that combines the tensile strength of steel with the compression strength of concrete.

Reinforced and pre-stressed concrete are building techniques where steel reinforcements are added to concrete where it will be under tension. Little or no reinforcing is needed where the structure is only under compression forces. In a bridge span, the steel will be on the bottom and the concrete on the top.

As the bridge is loaded, the concrete compresses, and the steel pulls.