Building Lab, Part IV: Paper Bridges and Loads

Vocabulary

Loads create a force on a structure. 

Dead load The weight of the permanent, non-movable parts of a structure, such as the towers, cables, and roadway of a bridge.

Live load The weight of a structure's non-permanent, movable parts, contents, or "users," such as the traffic, people, and seagulls on a bridge. Environmental loads, such as wind, rain, and earthquakes, that can affect a structure temporarily are also live loads.

Activity

Have your students look around the room and make a list of as many different loads affecting the room as they can. 

List everyone's loads on the board. Next, have them decide whether each load is live or dead. Dead loads include the weight of the walls, ceiling, floor, and any permanent fixtures such as ceiling lights, wallpaper, paint, and windows. Live loads include things that are not fixed, such as furniture, people, signs and pictures hanging on the wall, plants, and wind blowing outside.

Have your students estimate the room's load, assuming the floor and walls weigh 50 lbs./sq.ft. and the ceiling weighs 30 lbs./sq. ft. Use a scale to weigh small furniture.

Building a Better Bridge

A bridge must support its own weight (the dead load) as well as the weight of anything placed on it, like the pennies (the live load). Changing the shape of a material can change the way it resists forces. Although a piece of paper seems flexible and weak, it can be folded, rolled, twisted, or otherwise altered to support quite a bit of weight. Students may accordion-pleat the paper, roll it, or cut it into strips and weave them together. The paper clips can be used to stiffen folded paper. Folding the paper helps it to resist bending forces created by the live load of the pennies on top of the bridge. The paper can be folded into the shape of an I-beam or accordion-pleated. Rolling the paper around the pennies and fastening the ends with paper clips is another possible solution. Students will probably find that the bridge can support more weight distributed along the bridge than at a single point.

Hold up a single piece of paper. Ask: How many pennies do you think a bridge made out of this paper can hold? After kids make some guesses, lay the sheet of paper flat across two books placed 20 cm (about 8 in.) apart. With the kids keeping count, place pennies on the bridge, near the middle, until the bridge fails.  Now introduce the activity challenge.

What can you do to the paper to make it stronger?

Materials

plain paper (such as photocopier paper)

5 paper clips

ruler

2 books or blocks

at least 100 counters, pennies, metal washers, or other small weights

scissors

Explain to your students that their task is to make a bridge out of one sheet of paper. The rules are that they can also use up to 5 paperclips, but nothing else. The sides of the bridge must rest on two equal materials such as books, or as in our case, two storage containers, with about 8 inches between the sides. They cannot use tape to tape down the sides. 

After thinking and discussing the problem, have your students describe how they think the bridge should be constructed in order to support its dead load plus the live load of the pennies.

Place the bridge they make across two supports that are 20 cm apart. Remember that the space below the bridge must be clear to allow boats to pass! To test your bridge, load it with pennies one at a time, until it collapses. Record how many pennies your bridge supported. As they test their bridges, suggest that they observe the bridges closely to determine where they fail.

Describe how well your bridge supported its dead load and the live load you placed on it. Was the bridge as strong as you thought it would be? Where did it fail?

Redesign your bridge and test it again, using a new sheet of paper. How does your second attempt compare? How can engineers test their plans for building a full-size bridge?

Is there a difference in the load your bridge can hold if you put the load in the center of the bridge compared to spreading it out along the bridge? Make a prediction and test it. 

Have a discussion about different types of bridges kids have seen. How long were they? How tall? What were the bridges designed to transport (e.g., trains, cars, people)? What other considerations went into designing the bridges (e.g., earthquakes, boat traffic)?  Use this opportunity to discuss that while engineers cannot build multiple full-size bridges to test their ideas, they use models and computer simulations to test and redesign structures.

(To see the results of our testing, click here.)

An Additional Project

Have your students each choose a large bridge to study. After research, have him create an advertisement for his bridge that highlights what has learned about the bridge, including its construction, its  use and anything else that is unique to this bridge.