NOVA scienceNOW: Smart Bridges
Model a deck-truss arch bridge. The bridge in the segment is an under-deck-truss arch design. To help students understand what gives a bridge its strength, and the specifics of the Minneapolis bridge design, introduce this bridge's characteristics. Before class, gather the following items for a demonstration or for teams of students: two heavy books or blocks of the same thickness, several pieces of paper, toothpicks, mini marshmallows, and pennies or other small weights.
Begin by presenting an image of the bridge and discussing its configuration (see the Links section or: en.wikipedia.org/wiki/Image:I35W_Bridge.jpg). Point out the arch, the curved structure that spans a gap and supports weight with all parts in compression (rather than under tension, as in other kinds of bridge designs). Set up a very basic model of an arch by placing the books or blocks about four inches apart. For comparison purposes, first lay a piece of paper across the gap. Then place one of the weights on the paper and watch the bridge collapse. Using the same piece of paper, place it so that it is sitting between the two books, forming an arch. Now place one of the small weights on top of the arch. It may sag, but it should hold. In fact, it should be able to hold a number of small weights. Have your students discuss what is happening. (The arch of the paper is transferring the force down the sides of the paper and into the table and outward against the books.) How do arches work? (All the components of an arch are pushing on each other; they are under compression. As a result, an arch pushes outward at its base and needs to be restrained there.)
Next, point out the triangles in the bridge. Tell students these are called trusses, ridged structures made of one or more triangles. Show that a triangle is a very strong shape. Using the toothpicks and marshmallows, make a square, and show how it bends and flexes. Then make a triangle and show that it is much more stable. Finally, build a pyramid and demonstrate that by combining the triangles we can make a structure that is considerably more stable.
Explain that "under deck" simply means that the structure (and strength) of the bridge is below the road instead of above it.
Build a truss bridge.
The segment discusses how we can monitor bridges as they age to make sure they are safe. To understand this, students need to construct their own bridges, test them, and examine how and why they fail. Gather the following materials: toothpicks or stirring straws, carpenters glue, string, paper cups, and pennies or other small weights.
The day before showing the segment, have students work in teams to construct bridges that can span a six-to-ten-inch gap between two desks. Let them experiment freely, or you can provide them with some guidelines and/or limitations. For example, give them only 50 toothpicks and place limits on the amount of glue they can use. Or try suggesting to them that triangles are very strong structures, or that straws can be flattened and inserted into other straws to strengthen them not only at the joints but also along their lengths. Allow the bridges to dry overnight.
During the next class period, watch the program segment. Then test the bridges. (You may wish to make a contest out of it.) Place a bridge across the gap between two desks and hang the paper cup from the middle of the bridge. Then add weights one at a time. As the bridge starts to collapse, look for and discuss the points of weakness. Test each team's bridge this way. Which bridge supported the most weight? What feature(s) of its design or construction might account for this?
Extension: Extend the challenge to more rigorous and careful bridge construction by offering your students more time in or out of class to design, test, and redesign their bridges.
|Safety Warning: Wear goggles, and have students wear goggles, when performing the Measuring Strain activity.
Measure strain. The segment focuses on monitoring bridges and their safety. As presented in the segment, one means of accomplishing this is a system of sensors called strain gauges. They are welded onto critical spots on a bridge and measure the stretching of the metal. In the segment, Finn Hubbard explains that this stretching is "sort of like the stretching of a rubber band." In fact, rubber bands can serve as a model in the classroom.
Before class, gather the following materials: rubber bands of various sizes, types, and ages, force meters (if available), rulers or meter sticks, scissors, and safety glasses. (NOTE: When a rubber band snaps under tension, it can give a nasty sting to hands and can be dangerous to eyes, which should be protected by safety goggles.)
Ask students to put on goggles, then have them stretch and observe some rubber bands. What happens to a rubber band when it is stretched? (It gets longer, narrower, and thinner. The color may lighten. Any flaws in the band become more visible. A damaged area of the band may show additional stress–for example, it may be even narrower or lighter than the rest of the stretched band. The band may even snap.)
After discussing the results as a class, explain to students that just as they can detect flaws in rubber bands as they are stretched, so too can gauges on bridges sense increasing strain. Unfortunately, such gauges can only provide information about stresses near the locations where they are attached. If the bridge (or rubber band) is in trouble somewhere else along its length, the gauge can't detect the problem.
Extension: Challenge your students to experiment with factors that affect the stretching of a rubber band. For example, does heating the rubber bands have an effect? Does freezing? Does exposure to sunlight? What about repeated stretching? What other creative ideas can your students suggest and test?
Use sound to determine a material's stability. In the segment, Eric Flynn explains how he uses a tone to differentiate between sturdy and damaged structural plates. When he sends a high-frequency wave through a healthy plate, he hears a clear tone. Damaged plates add extra tones, making a distinctly different sound. Your students may not be able to experiment with sonar, but they can investigate the tonal qualities of various materials and how they change.
Before class, gather string, tape, metal hangers, and same-sized tin cans. Check the cans for sharp edges, and take off all the labels. Then in class, have students experiment with making sounds, using either the cans or the hangers. Have them attach string to the cans and hangers, then hold up the string and tap the (intact) cans and hangers. When different parts of these metal objects are tapped, how does the tone vary? When they are held in different ways (such as hanging them from a string), how does the tone vary? Cans and hangers can be deformed and damaged, just like the plates in a bridge. Have your students bend and dent the cans and hangers in various ways, keeping at least one of each intact for comparison purposes. How does this damage change the tone? Once they've explored, have your students challenge each other to identify the health of a can or hanger just by the sound. One student closes his/her eyes or turns his/her back, and another student selects a can or hanger to tap. The first student then tries to guess if the tapped item is intact, dented, or otherwise damaged.
Probe for flaws. What indicators show that a bridge is under stress? Scientists and engineers in the video are developing systems to monitor bridges. Can your students come up with their own detection systems? Present your class with a very basic bridge–a piece of corrugated cardboard–and challenge them to come up with ideas for how to monitor the bridge.
Before class, obtain corrugated cardboard and cut strips of about 2 inches wide and ten or more inches long. Cut the cardboard along the corrugation, so that when it is viewed on end, you can see the repeating triangles, or wave pattern, of the corrugation. Also gather weights such that two or three will bend but not collapse your cardboard strips. Then in class, have your students begin by laying the cardboard strips across a gap, placing two or three weights on the center, and observing and recording the indicators of stress. (The ends of the bridge will probably rise off the tabletop; a crease or creases may appear in the surface of the cardboard; the bottom of the corrugation may look stretched while the top may look compressed, etc.) Next, have students brainstorm ways that they might be able to detect these indicators of stress before they become this extreme. (For example, what about an alarm or buzzer that would sound as soon as the cardboard started to lift off the tabletop? Might a thin layer of paint or plaster on the cardboard show cracking before the cardboard collapses?) Have students try some of their brainstormed ideas.
Research local bridges.
What bridges are there in your local area? How old are they? How well are they maintained? What, if any, stress monitors or early warning systems are in place on them? Are they safe? Bridges don't collapse every day, but as Congressman Oberstar points out in the video, "We have 76,000 structurally deficient bridges in the United States [in 2007]. That's almost two and a half times as many as we had twenty years ago.
Have students list as many bridges in their area as they can. Include bridges over rivers and other natural terrain, and bridges spanning roads, railroad tracks, and other constructed features. Supply the class with local maps, if possible. Next, have students research those bridges online. To begin, check your town Web site for information, or link to the National Bridge Inventory Database at www.nationalbridges.com.
Offers resources related to bridge structure and safety, including additional activities, streamed video, and reports by experts.
American Society of Civil Engineers
Offers information on publications, educational programs, and other civil engineering resources.
CNN: Bridge Collapse
Offers news reports and articles on the Minneapolis bridge collapse and its aftermath.
Star Tribune: After the I-35W Collapse
Presents local news reports on, images of, and stories about the Minneapolis bridge collapse and its aftermath.
Tacoma Narrow Bridge
Provides information on the bridges that span or spanned the Tacoma Narrows in Washington, site of a dramatic collapse of the Tacoma Narrows suspension bridge.
by David Macaulay.
Houghton Mifflin/Walter Lorraine Books, October 2000.
Outlines various developments in building technology, including bridges.
Fantastic Feats and Failures
by the editors of YES Magazine/Kids Can Press, Ltd, 2004.
Presents designs for various structures, including both successes and failures.
The Great Bridge: The Epic Story of the Building of the Brooklyn Bridge
by David McCullough.
Simon & Schuster, 1983.
Tells the compelling story of the construction of the Brooklyn Bridge.
Teon Edwards is a curriculum developer with a background in astrophysics, mathematics, and the use of technology and multimedia in teaching and learning. Since 1996, she has developed numerous science and mathematics education materials for school, home, and informal learning environments.