How often do you worry about the roof collapsing on your head? We assume that you never worry about such things. This is an example of how we have an inherent trust in the engineering designs in our daily lives, often without even being aware that we are doing this. When an engineering design fails, it is on display for the whole world to see. The collapse of the I-35 bridge in Minnesota in August, 2007 (see Discussion Questions below), was a glaring example of the failure of something we take for granted everyday. Thirteen people died and 145 people were injured. There was not a large earthquake that triggered this bridge collapse. The reason for failure found by the NTSB (National Transportation Safety Board) was a design flaw. If engineers had designed the bridge to properly withstand the compression and tension forces acting on the bridge, this disaster could have been averted. Compression and tension are forces that cause a material to be pressed together or pulled apart. You compress a chair when you sit on it. Squeezing a sponge is a compression force. Can you think of others? We think of tension in a rubber band as it stretches. But just similar to compression in a chair that you don’t see when you sit on it, you don’t see tension in the chair or cables hanging lights or plants from the ceiling. Two of the common forces that building components experience are compression and tension. When properly designed, a bridge or building will be built to withstand any potential load that the structure may experience.
Observe the wood beams that are on the front edge of the top porch. Among the left and right vertical beams, and the boards that form an ‘X’ shape in the front, which of these do you think helps support the roof above? Discuss this with a friend and decide which beams you think are important for structural support.
Ceilings or walls cover most of the structural beams in your house or school. Look in the gym or cafeteria at school to see if there are exposed beams. Can you tell which parts experience tension? Compression? If you home has an attic, maybe you parents will examine the structures visible in you home. To help you examine visible structures, look at the picture of the exposed beam in a restaurant I visited. The overall structure of this truss or framework is a triangle. Draw a triangle and draw the other beams as shown. Notice the difference between the beam pointed to by the white arrow and the beam pointed to by the red arrow.
What happens to the weight from the roof on the beam with the white arrow? Notice it forms a V with another beam. The compression from the room pushes these two beams together in the middle. They will not move. The horizontal beam supports the V shaped beams.
What happens to the beam with the red arrow? If it had to support any weight, it would slide and fall down. The beam with the red arrow is just for decoration and supports no weight. Can you see the difference?
Next Generation Science Standards
Scientific and Engineering Practices:
– Asking questions/defining problems
– Structure and function<
1. Listen to and read the Minnesota Public Radio account of the bridge collapse: http://minnesota.publicradio.org/collections/special/2007/bridge_collapse/multimedia/.
2. Examine images of the bridge and discuss the design. Compare it to bridges in your community: http://www.bing.com/images/search?q=I-35+bridge+in+Minnesota+in+August%2c+2007&qpvt=I-35+bridge+in+Minnesota+in+August%2c+2007&FORM=IGRE
3. One of the images is copied below. Draw the triangular structures shown in the photograph of the bridge before it collapsed. Where are the compression forces in these beams? Compare this bridge structure to the restaurant truss discussed above. See the similarities? What has been added in the bridge design? The blue arrow shows a vertical support in the V design from the restaurant. What kind of force is on this support?
Look at the long beam pointed out by the orange arrow. What is its function? Think about it before moving to the next paragraph.
This beam seems to be holding up everything between two posts. If this beam were a piece of licorice so that it was not strong and stable but weak and bendy, then you could see what is happening to this bridge beam. If you held the licorice with two hands and bent it in the middle, you could see the top is being compressed and the bottom is being pulled apart or put in tension. You can see the bottom being stretched.