Balsa wood bridge
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The building of balsa wood bridges is very often used as an educational activity. It may be accompanied by a larger project involving varying areas of study.
Typically classes which would include a balsa wood bridge project cover the subject areas of physics, engineering, static equilibrium, or building trades, although it may be done independently of any of these subjects. Building a balsa wood bridge can be done either after completing a section or unit covering a related topic or the process of design and building can be used to guide students to a better understanding of the desired subject area.
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[edit] Requirements
Although there is great variety between different balsa wood bridge projects, students are in general trying to build a bridge that holds the greatest mass before it fails. Other restrictions typically are included, but they vary widely from one location to another. Sample requirements include:
- restricting the maximum mass of the bridge
- requiring a minimum span
- requiring a minimum height of the roadway
- restricting the size of individual pieces of balsa wood
- limiting the amount of glue or balsa wood that can be used
- requiring a “driveable” roadway
- restricting the way pieces are placed on the bridge (for example no parallel joining pieces)
- Testing*
Bridges are most often tested by applying a downward force on the bridge. How and where the force is applied can vary from one project to the next. One common method of applying force on a bridge is by hanging a container (such as a trash can) from the bridge and loading weight into the can until the bridge breaks. This method also works by filling the container with water until the bridge breaks. This process provides a more accurate way to find the breaking point of the bridge. A second common method is by using a mechanical or pneumatic testing device that pushes down on the bridge with increasing force. The force can be applied to the roadway of a bridge or at the bridge’s highest point. If a testing device is used it if often equipped with a method of automatically gathering force data, such as a pressure sensor or load cell.
[edit] Scoring and Grading
[edit] Scoring
There are two chief ways that balsa wood bridge competitions may be scored. One way is simply by measuring how much weight each bridge can support. The second way is by structural efficiency, often expressed as a strength to weight ratio. The weight-only method is most effective where competitors are all building from a specified set of materials and are expected to use all the available materials. The strength-to-weight method is better when competitors are expected to use their materials as efficiently as possible.
[edit] Competitive Grading
Competitive grading compares how well each bridge does against bridges built by other participants to determine a grade. The top scoring bridge is assigned a maximum numeric grade (say, 100%) while the lowest scoring bridge that still meets all basic specifications is assigned a minimum grade (say, 70%)[1].
The method guarantee that some scores will be high even if the overall quality of bridges is low. Conversely, it also guarantees that some scores will be low, even in situations where all bridges are of high quality.
[edit] Standards-Based Grading
Standards-based grading sets certain standards that need to be met in order to earn a certain grade. An example of standards-based scoring would be to say that all bridges that hold 50 kg earn full credit; bridges that hold 25 kg earn half credit, and bridges that hold less than 10 kg earn no credit.
This method allows every bridge to potentially earn full credit if the standards are met. In situations where competition is weak, this method runs the risk of having many competitors receive no credit for their effort.
[edit] See also
[edit] References
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This article or section includes a list of references or external links, but its sources remain unclear because it lacks in-text citations. You can improve this article by introducing more precise citations. |
- Barron, B. J., Schwartz, D. L., Vye, N. J., Moore, A., Petrosino, A., Zech, L., et al. (1998). Doing with understanding: Lessons from research on problem- and project-based learning. The Journal of the Learning Sciences, 7, 271–311.
- Dods, R. F. (1997). An action research study of the effectiveness of problem-based learning in promoting the acquisition and retention of knowledge. Journal for the Education of the Gifted, 20(4), 423–437.
- Schneider, R. M., Krajcik, J., Marx, R., & Soloway, E. (2002). Performance of students in project-based science classrooms on a national measure of science achievement. Journal of Research in Science Teaching, 39(5), 410–422.
