Balsa_wood_bridges

A balsawood bridge being tested by loading weight into a metal trash can that is suspended from the bridge\'s roadway. The pictured bridge\'s mass was 96.24 grams and it held 46.27 kilograms (102.0 lb)

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.

Building balsa wood bridges as a part of a unit on statics, structures, forces, or construction trades is used by teachers to make the learning environment hands-on and to give students a real-world example of material covered in the class

Contents 1 Requirements 2 Scoring 2.1 Competitive 2.2 Standards-Based 3 References 4 External links 5 See also

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)

A balsa bridge breaks after holding a mass of 89 pounds (40.4 kg). Note that the failure of the entire bridge was a result of the failure of just the roadway.

• 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 it breaks. A second common method is by using a pneumatic 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.

Scoring

There are two main ways to score a balsa wood bridge. One way is by how much it holds and the other way is by its efficiency.

Competitive

Competitive scoring compares how well each bridge does compared to the bridges built by other participants to determine a score. The score earned by a bridge holding 10 kilograms, for instance, might vary widely from one year to the next depending on the level of competition. If, for example, 10 kg is the least amount of mass held one year out of the entire class, but the greatest amount of mass held the next, the scores earned the first year will be much higher because of weak competition.

The method does 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 years with high levels of competition.

Standards-Based

Standards-based scoring sets certain standards that need to be met in order to earn a certain score. 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.

References

• 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.

External links

• Balsa wood bridge building guide
• BalsaBridge.com
• Balsa Bridge Competition
• Model Bridge Design

See also

• Bridge
• Balsa
• Trusses
• Statics
• Physics
• Civil Engineering
• Problem Based Learning

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