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Researched by Nathan B.
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The purpose of this experiment was to determine which truss bridge design supports the most mass.
I became interested in this idea because my father works for the Department of Transportation and although he doesn’t design bridges he often talks with people who do. He has explained the importance of good bridge design to me.
The information gained from this experiment could help engineers build
better truss bridges in the future.
My hypothesis is that the bridge design with the most triangles will support the more mass and the design with the fewest triangles will hold the least mass.
I base my hypothesis on information in the book, Bridges, which says
the more triangles in a bridge the stronger it will be.
The constants in this study were:
* The length of the bridge span.
* The material the bridge is made of.
* The amount of mass per unit applied as load during testing.
* The glue that holds the bridge together.
The manipulated variable was the number of triangles in each type of truss bridge construction.
The responding variable was the amount of mass in grams supported by each bridge before failure.
To measure the responding variable I continued to load each model bridge
with pennies until it failed and then determined the mass of the pennies.
| QUANTITY | ITEM DESCRIPTION |
| 5 | jars of pennies |
| 40 | 1 X 120 cm sticks balsa wood |
| 1 | 1 gallon bucket |
| 1 | 100 cm piece of twine |
| 1 | bottle of C.A. Quick Dry glue |
1. Get out all needed supplies for constructing bridges.
2. Build a platform out of 2 by 4's that’s approximately 15 centimeters high for the bridges to rest on, with a span of 26 centimeters
3.All of the bridges will have the same span of 30 centimeters and a width of 7.5 centimeters and will not have a deck.
4. Build the 12 deck frames for all six bridge designs with the measurements in step 3.(Two bridges for each design.)
5. Construct two of every bridge design using balsa wood and C.A. quick
dry glue.
5 (see drawing on next page)
6. Take all the bridges to the test platform.
7. Put the bridge being tested on to the test platform.
8. Place two extra pieces of balsa wood across the center of the deck frame to support the load.
9. Loop a piece of twine around the two pieces of balsa wood so the extra string hangs down, but about 8cm from touching the floor.
10. Tie a lightweight, small bucket to the hanging end of the twine.
11. Make sure the bridge is securely on the platform and the twine tied to the bucket is tied tight.
12. Start pouring pennies into the bucket slowly.
13. When the bridge deck frame cracks or completely breaks record the mass data in experiment log.
14. Clean all the remaining bridge pieces off the testing platform.
15. Repeat steps 7-14 on all the remaining bridges.
16. Average the 3 results in each design.
The original purpose of this experiment was to determine which truss bridge holds the most mass. It attempted to compare the strength of designs based on the number of structural triangles.
The results of the experiment were that the six triangle bridge, design F, on average held the most mass: 21,204 grams. The one triangle bridge, design A, on average held the least mass: 2538 grams. In general the more triangles in the design, the more mass the bridge held. The only exception to this pattern was bridge design E, with five triangle (but no center post.)
See the graph below.
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My hypothesis is that the bridge design with the most triangles will support the more mass and the design with the fewest triangles will hold the least mass.
The results indicate that this hypothesis should be accepted because bridge design F. had the most triangles and it held the most mass. Bridge design A had the least triangles and it held the least mass.
Because of the results of this experiment, I wonder if having a support post in the middle of the bridge would help other truss bridge designs hold more mass. I also wonder if bridges constructed of different materials, like metal, would show the same results. I also wonder if a bridge that is longer than 30cm but with the same height as the tested bridges would hold more, less, or the same amount of mass.
If I were to conduct this project again, I would have used larger increments of mass to break the bridges. It was very difficult to use pennies because I needed to use so many.
| RESEARCH REPORT
INTRODUCTION There are many bridges on roads and highways today. A bridge’s design is the most important part of the bridge. It determines how much weight the bridge will hold. Bridges are very important to society because they help us cross large rivers, streams, canyons, freeways, railroad tracks, and more. BRIDGE A bridge is a structure used by vehicles and people to cross obstacles. Bridges are often used on highways all over the world. Without bridges transportation would be much more difficult. Society is very dependent on transporting goods and people. Everyone is affected by bridges. THE HISTORY OF BRIDGES Logs or vines that were placed across small streams or rivers were probably the first bridges that people used. The first bridge that historians know about is an arch bridge built in Babylon around 2200B.C. Ancient Greeks, Chinese, Egyptians, and Romans built arch bridges too, mostly out of brick and stone. In the 1500’s truss bridges were invented. In the 1700’s cast iron and wrought iron were introduced into bridge building. In the late 1800’s steel was the most commonly used material in bridge building. Suspension and plate girder bridges came along in the 1800’s also. The first concrete bridge was built in 1869. In the years following pre-stressed concrete became an important bridge building material. The first modern cable-stayed bridge was built in 1955. BASIC PARTS OF BRIDGES A pier is the column shaped structure underneath the bridge. It sticks up off the ground and lets the bridge rest on the top of it. The abutments on the bridge are the parts of the bridge that rest on each side of the canyon, river, lake, etc. The trusses on a bridge are the triangle shaped supports. Not all bridges will have trusses. A girder is a long slab used as the roadway of the bridge. The girder usually rests on the pier of the bridge. MASS Mass is defined as the amount of matter in an object. Mass and weight are two different things. Weight is the force of an object due to the earth’s gravitational pull. Weight changes as the elevation gets higher because the object gets farther away from earth. Mass however does not change, no matter where you are, because it measures the amount of matter in an object. FORCES INVOLVED IN BRIDGE BUILDING Compression and tension are the two main types of forces in bridge building. Compression is the force that’s acts to shorten or compress the object it is acting on. For example when you lay thin slab of wood across two chairs and then add force, the bridge will bend causing it to shorten in length. Tension is the force which lengthens or expands what it’s acting on. For example when you lay a board across two chairs, just like with compression, when you apply force the bridge expands underneath. It also makes the sides of the board resting on the chair begin to lift up. SUMMARY There is much to learn about bridges before you can actually start
designing them safely. Engineers and other bridge designers are very
helpful to all of us.
"Bridge Site," http://www.bridgesite.com 1/05/00 Grivas , Dimitri A. "Bridge," Encarta 2000, disk 2, pp. 4 Macgregor, Anne and Scott. "Bridges," New York: Lothrop, Lee and Shepard Books, 1980, 24-31 "Michael Morrissey Bridge," http://www.howstuffworks.com/bridge.htm 1/11/00 "Truss Bridge Laboratory," http://www.ce.ufl.edu/activities/trusslab/trussndx.html 1/11/00 Videon, Fred F. "Bridge," World Book Encyclopedia 1991, 2, 610-613 |