# 19 Science: Designing Bridges

Key concepts
Bridges
Forces
Engineering

Introduction
Have you ever ridden in a car driving across a suspension bridge? Suspension bridges, with their tall towers, long spans and gracefully curving cables, are beautiful examples of the work of civil engineers. How do the cables and towers withstand the load of the bridge—including you and the car you’re in? Can a suspension bridge carry a greater load than a simple beam bridge that doesn’t use cables? You can try to answer these questions in this suspenseful science activity!

Background
A beam bridge is the simplest type of bridge. It is typically supported by a raised part on either end. For example, a beam bridge could be as simple as a wood plank put down to cross a stream.

Suspension bridges are a bit more complicated. They comprise a deck (the long straight part that includes the road), cables and towers. The cables stretch between the towers and help support the weight of the deck and its load. In this type of bridge the cables are under tension and the towers undergo compression.

Suspension bridges might seem complicated, but for spanning long distances they can also be the most economical—they require less material per foot than would a simpler beam bridge. Because suspension bridges are relatively flexible, however, high winds and other forces can be a serious problem. The dramatic collapse in 1940 of the Tacoma Narrows Bridge in Washington State is an infamous example of this. (You can watch a video of the disaster here.)

In this activity you’ll build and test two types of bridges: a basic suspension bridge and a beam bridge. Which type of bridge do you think can support a heavier load?

What’s the secret of suspension?
A suspension bridge’s cables are beautiful to look at, but they also enable the bridge to cross large spans. Make a model suspension bridge to see how it works.

What You Need
• 7 drinking straws
• scissors
• 4 large paper clips
• paper cup
• pennies or metal washers
• ruler

Make a Prediction
After you test the strength of the beam bridge in Step 4, predict how many pennies your suspension bridge will support.

Try It Out
1. Cut two short pieces of straw, each 3 centimeters (about 1.25 in.) long. For each tower, tape two straws on either side of a short piece of straw, as shown. Tape the long straws together at the top, too.
2. Tape one tower to the edge of a desk or chair. Tape the second tower to a second desk or chair of the same height. Position the towers 17 cm (about 7 in.) apart.
3. Place another straw between the towers so its ends rest on the short pieces. This straw is the bridge deck. Now you have a simple beam bridge.
4. Make a load tester by unbending a large paper clip into a V-shape. Poke the ends of the paper clip into opposite sides of a paper cup, near the rim. Use a second paper clip to hang the load tester over the bridge deck. Record how many pennies the paper cup can hold before the bridge fails.
5. Now change the beam bridge into a suspension bridge. Tie the center of a 100-cm (about 4 ft.) cable around the middle of a new straw. Place the straw between the towers. Pass each end of the cable over a tower and down the other side.
6. To anchor the bridge, wrap each end of the cable around a paper clip. Slide the paper clips away from the tower until the cable pulls tight. Then tape the paper clips firmly to the desks. Test it again.

Explain It
Can you identify the forces acting on the loaded suspension bridge? Which parts of the bridge are in compression? Which parts are in tension?

Build on It
Can you design and build a straw suspension bridge that spans a gap twice as wide and supports the same amount of weight? What parts of the bridge design need to change? Try it.

Observations and results

Did the suspension bridge hold a greater number of coins compared with the beam bridge?

In this activity you should have seen that the suspension bridge was able to hold more coins than the beam bridge by around 150 percent, such as about 310 pennies (or 140 quarters) compared with about 200 pennies (or 90 quarters). When the beam bridge failed, this was likely because the deck straw bent downward as more coins were added until it bent so much that it slipped down between the two towers. As coins were added to the suspension bridge, the cable (that is, the thread) was under tension and reinforced the bridge deck straw, pulling it upward (while compressing the towers) and allowing the bridge to hold more coins. When the suspension bridge eventually failed, the bridge deck straw likely similarly bent into a V-shape, but because it was attached by the thread the straw couldn’t fall and instead the cup may have slipped off of the straw.

More to explore
Super Bridge: Build a Bridge, from NOVA Online, WGBH
Mysteries at the Museum: Tacoma Narrows Bridge, from the Travel Channel
Fun, Science Activities for You and Your Family, from Science Buddies
Keeping You in Suspens(ion), from Science Buddies

This activity brought to you in partnership with Science Buddies

## How bridges balance forces

Forces make things move, but they also hold them still. It’s far from obvious, but when something like a skyscraper looms high above us or a bridge stretches out beneath our feet, hidden forces are hard at work: a bridge goes nowhere because all the forces acting on it are perfectly in balance. Bridge designers, in short, are force balancers.

The biggest force in the universe, gravity, is constantly tugging things down, which isn’t such a problem for a skyscraper, because the ground underneath pushes straight back up again. But a bridge spanning a river, valley, sea, or road is quite different: the huge deck (the main horizontal platform of a bridge) has no support directly beneath it. The longer the bridge, the more it weighs, the more it carries, and the bigger the risk it’ll collapse. Bridges certainly do fall down from time to time, and quite spectacularly, but most stand happily still for years, decades, or even centuries. They do it by carefully balancing two main kinds of forces called compression (a pushing or squeezing force, acting inward) and tension (a pulling or stretching force, acting outward), channeling the load (the total weight of the bridge and the things it carries) onto abutments (the supports at either side) and piers (one or more supports in the middle). Although there are many kinds of bridges, virtually all of them work by balancing compressive forces in some places with tensile forces elsewhere, so there’s no overall force to cause motion and do damage.