Balsa wood bridge - AbsoluteAstronomy.com

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

Educational technology

Educational technology is the study and ethical practice of facilitating learning and improving performance by creating, using and managing appropriate technological processes and resources." The term educational technology is often associated with, and encompasses, instructional theory and...

. It may be accompanied by a larger project involving varying areas of study.

Typically classes which would include a balsa wood bridge cover the subject areas of physics

Physics

Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

, engineering

Engineering

Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...

, 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 big looking bridge that withstands the greatest force 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
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

of the bridge
Bridge
A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...
requiring a minimum span
Span (architecture)
Span is the distance between two intermediate supports for a structure, e.g. a beam or a bridge.A span can be closed by a solid beam or of a rope...
requiring a minimum height of the roadway
restricting the physical dimensions of the bridge
restricting the size of individual pieces of balsa
Balsa
Ochroma pyramidale, commonly known as the balsa tree , is a species of flowering plant in the mallow family, Malvaceae. It is a large, fast-growing tree that can grow up to tall. It is the source of balsa wood, a very lightweight material with many uses...

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

Force

In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

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) 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
Machine
A machine manages power to accomplish a task, examples include, a mechanical system, a computing system, an electronic system, and a molecular machine. In common usage, the meaning is that of a device having parts that perform or assist in performing any type of work...

or pneumatic 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
Pressure sensor
A pressure sensor measures pressure, typically 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 acts as a transducer; it generates a signal as a function of the...

or load cell
Load cell
A load cell is a transducer that is used to convert a force into electrical signal. This conversion is indirect and happens in two stages. Through a mechanical arrangement, the force being sensed deforms a strain gauge. The strain gauge measures the deformation as an electrical signal, because the...

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 impact. Testing with a pneumatic press or ram is often complete in matter of seconds. Exceptionally strong bridges may take up the to thirty minutes to test as weight (a combination of steel weights and sand is often used) is added to the suspended container.

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 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.

External links

BalsaBridge.com Photos, records, tips from contest held annually since 1980 at Notre Dame Regional Secondary in Vancouver BC
Model Bridge Design
Canyon High School Balsa Bridge Competition