The building of balsa-wood bridges is often used as an

educational technology Educational technology (commonly abbreviated as edutech, or edtech) is the combined use of computer hardware, software, and educational theory and practice to facilitate learning. When referred to with its abbreviation, edtech, it often refer ...

. It may be accompanied by a larger project involving varying areas of study. Typically classes which would include a

balsa wood ''Ochroma pyramidale'', commonly known as the balsa tree, is a large, fast-growing tree native to the Americas. It is the sole member of the genus ''Ochroma''. The tree is famous for its wide usage in woodworking, with the name ''balsa'' being ...

bridge cover the subject areas of

physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...

engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad rang ...

, static equilibrium, or building trades, although it may be done independently of any of these subjects. Building a balsa wood bridge can be done 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.

Requirements

Although there is great variety between different balsa wood bridge projects, students are in general trying to build a bridge that can withstand the greatest weight before it fails. Other restrictions are often applied, but these vary widely from one contest to another. Sample requirements include: *restricting the maximum

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...

of the

bridge A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or rail) without blocking the way underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually somethi ...

*requiring a minimum

span Span may refer to: Science, technology and engineering * Span (unit), the width of a human hand * Span (engineering), a section between two intermediate supports * Wingspan, the distance between the wingtips of a bird or aircraft * Sorbitan ester ...

*requiring a minimum height of the

roadway A carriageway (British English) or roadway (North American English) consists of a width of road on which a vehicle is not restricted by any physical barriers or separation to move laterally. A carriageway generally consists of a number of tra ...

*restricting the physical dimensions of the bridge *restricting the size of individual pieces of

balsa ''Ochroma pyramidale'', commonly known as the balsa tree, is a large, fast-growing tree native to the Americas. It is the sole member of the genus ''Ochroma''. The tree is famous for its wide usage in woodworking, with the name ''balsa'' being ...

wood *limiting the amount of glue or balsa wood that can be used *requiring a driveable roadway that allows passage of a vehicle of specified size *restricting the way pieces are placed on the bridge (for example no parallel joining pieces)

Testing

Bridges are usually tested by applying a downward force on the bridge. How and where the

force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a p ...

is applied varies from one contest to the next. There are two common methods of applying the test force to the bridge: *By hanging a container (such as a

trash can A waste container, also known as a dustbin, garbage can, and trash can is a type of container that is usually made out of metal or plastic. The words "rubbish", "basket" and "bin" are more common in British English usage; "trash" and "can" a ...

) from the bridge and loading known weights into the container until the bridge breaks. The tester could also slowly add water or sand to the container until the bridge breaks and then weigh the container, providing a more accurate way to find the breaking force. *By using a

mechanical Mechanical may refer to: Machine * Machine (mechanical), a system of mechanisms that shape the actuator input to achieve a specific application of output forces and movement * Mechanical calculator, a device used to perform the basic operations of ...

pneumatic Pneumatics (from Greek ‘wind, breath’) is a branch of engineering that makes use of gas or pressurized air. Pneumatic systems used in Industrial sector, industry are commonly powered by compressed air or compressed inert gases. A central ...

testing device that pushes down on the bridge with increasing force until the bridge breaks. If such a testing device is used it is often equipped with a method of automatically gathering force data, such as a

pressure sensor A pressure sensor is a device for pressure measurement of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually act ...

load cell A load cell converts a force such as tension, compression, pressure, or torque into an electrical signal that can be measured and standardized. It is a force transducer. As the force applied to the load cell increases, the electrical signal change ...

connected to a computer. This approach is more accurate at measuring bridges that have secondary structure that proves to be stronger than the primary structure. This method is also less damaging to the bridge, whose structure usually remains intact since the load is automatically relieved at failure. Of the two, the hanging container method provides by far the greater dramatic effect. Testing with a pneumatic press or ram is often complete in matter of seconds. Exceptionally strong bridges may take up to thirty minutes to test as weight (a combination of steel weights and sand is often used) is added to the suspended container.

Scoring and Grading

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.

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%). 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.

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 earn full credit; bridges that hold earn half credit, and bridges that hold less than 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.

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

BalsaBridge.com
Photos, records, tips from contest held annually since 1980 at Notre Dame Regional Secondary in Vancouver BC {{DEFAULTSORT:Balsa Wood Bridge Bridges Engineering education Physics education Science education Science demonstrations