Have you ever driven over a bridge and wondered what was holding it up? Learn about different types of bridges and the science that holds them up in this issue!
Build a Paper Bridge
Do you think you can you build a bridge out of paper? Try it out and then see if you can make it stronger with these tips.
Arch Bridge Activity
Arches are one of the oldest types of bridge. Try this activity to learn more about them.
The stacks of books act as abutments or supports to keep the ends of the arch from collapsing from the weight you put on it. Without them, there was nothing to support the weight of your hand and keep the ends of the arch from pushing outward and falling down.
For another simple project, use sugar cubes to build an arch. Taper the sides of the cubes with an old nail file (or scrape them with a butter knife) so that they fit snugly against each other to form an arch. Be sure to use an odd number of cubes, so that there's only one cube in the middle. If you used two cubes in the middle, the frictional force between those blocks would be overcome by the load force on the arch, causing it to buckle.
Suspension Bridge Activity
Make a model of a suspension bridge and learn why anchors for the cables are so important!
When the string was tied around the tops of the books, there was nothing to support the books when weight was applied to the loose string. When you changed the design by putting the ends of the string securely under stacks of books, the books helped anchor the string and stabilize the upright books. This model is very similar to how a real suspension bridge is designed. The books standing upright are like the towers at each end of the bridge and the string is like the cable that is anchored at each end. Depending on the size and thickness of the books you used, the books may still fall over, but they should stay up better than the first time you tried it (without the anchors). You can experiment more with different sizes of books, different kinds of strings, and even using two strings a few inches apart from each other. Take a look at the picture to the right - can you tell where the cables are anchored at each end of the bridge?
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Types of Bridges
Arch bridges were one of the first types of bridge. The Romans used arches in their bridges and aqueducts as well as in their triumphal arches and other great architecture. See a picture of Constantine's Arch here. Arches are designed with an odd number of stones, with the top, middle one being the keystone on which the bridge's weight rests. These stones have slanted sides so that they fit tight against each other. The downward force of the bridge's weight spreads out to the stones on either side of the keystone. Arches used in bridges also have supports at each end of the arch called abutments. These supports keep the stones in the arch in place even when weight is put on top of them.
Suspension bridges use a combination of strong cables and tall, solid towers to achieve a balance of forces. The roadway of a suspension bridge is suspended from the tower and held up by incredibly strong cables. Most suspension bridges today use steel wires and cables, because steel is very strong. A steel wire only 1/10th of an inch thick can hold more than 1,000 pounds of weight without breaking! To make the cables used in bridges, thousands of strong steel wires are twisted and bound together to make them even stronger. The Brooklyn Bridge in New York is one of the most famous bridges, now not so much for its size as its beauty. Built in 1883, it was the first great suspension bridge and a feat of engineering. At the time, it was the longest suspension bridge ever built (though several larger ones have been built since). The stone and steel that Brooklyn Bridge was built with combined to make it both solid and graceful. See a picture of it here.
Learn more about another famous suspension bridge, the Golden Gate Bridge, located in California.
Beam bridges are the simplest kind of bridge. A beam bridge is usually supported by a "pier" at each end, with a beam stretching across them. The weight of the beam pushes down on the piers, and the weight of cars or anything that will cross the bridge pushes down on the beam. Both the beam and the piers must be strong enough to support the weight that will be put on them. Beam bridges are often built over highways, rivers, and valleys. However, they cannot stretch across very long distances, since they are only supported by the piers at the ends of the beam, with nothing supporting the center. To make longer bridges, lots of beam bridges can be linked together in a row; each beam is still supported by a pier at each end.
Truss bridges utilize the strength of triangles to spread out force. A web system of sturdy beams between two girders forms a zigzagging triangle pattern, stiffening the bridge and spreading out weight so that no one section bears all the weight at that spot. Trusses are also used to support other types of bridges, like beam or suspension bridges.
To learn more about different types of bridges, check out this site.
Architects & Engineers
In general, bridges are designed by two different types of people. Architects are responsible for how a bridge looks while engineers are responsible for how the bridge works and is supported. There are a lot of things to consider when designing a bridge. Physics is a field of science that studies matter, energy, forces, and more. People who design bridges need to understand some important physics concepts in order to make a bridge that will be strong, stable, and secure over time.
Some physics concepts that are considered when bridges are designed are compression and tension. These are both forces that happen every day on bridges and can act differently on different materials. Compression presses down or shortens an object. Try putting a marshmallow on the counter and pressing down on it with the palm of your hand. The force from your hand compressed the marshmallow! Tension stretches out or lengthens an object. When you stretch a rubber band, a toy slinky, or pull two ends of a string tight, that's tension.
In addition to being able to support lots of weight, a bridge must also be able to stand up to lots of different types of weather, like high winds and sometimes even earthquakes!
Sometimes architects and engineers design special bridges that combine two or more types of bridges together to make the best possible bridge for a location. They have to be very careful to make sure the bridge design will still be stable and strong enough. Bridge designs must be tested quite well before a bridge is ever built.
Printable Worksheet & PDF
Use this worksheet to help kids review types of bridges. It also includes two simple activities.
To view a printable PDF version of this newsletter and the worksheet together, click here.