Problem Statement: Each year, over 800 earthquakes with a Richter Scale magnitude between 5 and 6 are detected world-wide. A magnitude 5.5 quake can have an intensity of up to VII on the Modified Mercalli Intensity Scale. Depending on the structure, this can cause light to moderate damage in well-built buildings; considerable damage in poorly built buildings.
Request: In this lab, you and your engineering team will design and build a model bridge planned for an earthquake-prone area. You will test to see how well it withstands simulated stress, strain and shearing from seismic waves. Your bridge must: • weighs less than 250g
• spans a gap of 400mm (16 inches) and is 510mm (20 inches) long
• is less than 100mm (4 inches) wide
(Click on the first picture to flip through the Earth Science students building seismic safe spaghetti structures that can withstand stress, strain and sheering)
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| 20120125 Seismic Spaghetti |
Explore: Draw a plan of your bridge on paper first. Use your imagination; the bridge can be any shape you like as long as it is the right weight, width and length. (Hint: triangles are a strong shape!)
Engage: Build your bridge out of spaghetti and glue. The glue can only be used on overlaps, intersections and joints. Check it weighs less than 250g and record the actual weight. Measure your bridge to make sure it is the correct length and width.
Evaluate: Testing time! Place the bridge between two tables or boxes 510 mm apart. Place the rod halfway along the bridge so it runs across the center. Hang a bucket off the rod. Slowly add more and more sand to the bucket until the bridge breaks. Weigh the sand, rod and bucket your bridge held. This is the load. Divide the load by the weight of the bridge. This is your load/weight ratio. The higher the load/weight ratio, the stronger your bridge. EX: Taryn and Jasmine made a bridge weighing 75g that held 675g. This means the load/weight ratio was 9.
Explain: Two important forces you have to consider are compression and tension. Compression is a force that will try to squeeze or shorten the thing it is acting on. Tension is a force that will try to lengthen or expand the thing it is acting on. A bridge needs to be able to handle these forces without buckling or snapping. A bridge will buckle when the force of compression is greater than the bridge's ability to handle compression. A bridge will snap when the force of tension overcomes an object's ability to handle tension. A good bridge design will spread the force over a greater area, or move the force from an area of weakness to an area of strength. For example: using triangles in your bridge can create both a very rigid structure and one that transfers the load from just one point to a much wider area. Bridges are designed and constructed by civil engineers. Civil engineers work on things built in or on the ground. As well as bridges, this can include railways, harbors, roads, dams, power projects, and more.
