19 Science: Designing Bridges

Objective:   Learn how engineers make bridges of different strengths using simple cables. And get ready to count your pennies as you test how much weight your bridge can hold. Credit: George Retseck

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Key concepts
Bridges
Forces
Load
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
• masking tape
• dental floss or thread
• 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
Illustration of two long straws taped together over a short piece of straw. 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. 
Illustration of this point.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. Illustration of this point.  
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.

Compression and tension forces on six different types of bridges: beam, arch, suspension, cable-stayed, truss, and cantilever

Carrying loads

If a bridge is unloaded, all it really has to do is support its own weight (the dead load), so the tension and compression in its structure are essentially static forces (ones that don’t cause movement), changing little from hour to hour or day to day. However, by definition bridges have to carry changing amounts of weight (the live load) from things like railroad trains, cars, or people, which can increase the ordinary tensile or compressive forces quite dramatically. Rail bridges, for example, bend and flex every time a heavy train crosses over them and then “relax” again as soon as the load has passed by.

Environmental forces

Bridges also have to bear ever-changing environmental forces. Arch bridges over rivers, for example, have to cope with water backing up behind them (their abutments often have strategically placed openings to let high flood water drain through). Suspension bridges that carry cars tend to bear the same loads all day long, though, often sited in windy estuaries, they also have to endure squalling gusts of wind, which can set up a twisting force, called torsion, in the bridge deck. (Modern suspension bridges tackle this problem by having decks with aerodynamically designed cross sections, tested in wind tunnels, and may be reinforced with trusses underneath.) Loads that cause a bridge to move back and forth can be particularly dangerous if they make it vibrate wildly at its so-called natural or resonant frequency. Resonance, as this is known, is what makes wine glasses shatter when opera singers get a bit too close; the “singing” of the wind can have equally catastrophic effects on a bridge.

Artwork: Balancing forces in a bridge: Different types of bridges carry loads through the forces of compression (“squeezing”—shown here by red lines) and tension (“stretching”—shown by blue lines): 1) A beam bridge has its beam partly in tension and partly in compression, with the abutments (side pillars) in compression; 2) An arch bridge supports loads through compression; 3) A suspension bridge has its piers (towers) in compression and the deck hangs from thick suspension cables by thinner cables, all of which are in tension. 4) A cable-stayed bridge is similar but the deck hangs directly from the piers from cables. The piers are in compression and the cables are in tension. 5) A truss bridge is a kind of reinforced beam bridge. Like a beam bridge, the top is in compression and the bottom in compression. The diagonal trusses are in tension and the vertical ones are in compression. 6) A cantilever bridge balances tension forces above the bridge deck with compression forces below.

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