Wood

*Wood

While some structure challenges have required the building of structures with such diverse materials as pasta, paper and duct tape, wood and glue are the typical building materials specified in the structure challenge.

At times the challenge may limit the variety and sizes of wood that teams can use to build their structures. In recent years Destination ImagiNation® challenges have allowed the use of any woods in any size.

Teams should read the challenge and make sure they are using only materials that are allowed by the challenge. Composite materials such as particle board, plywood, hardboard, etc. have not been allowed in the past but it is possible that this may change in the future.

Hobby shops typically carry three types of wood: basswood, spruce and balsa. You can find other varieties of wood at Lumber Yards or Home Improvement Centers. You can also find specialty wood shops by checking the yellow pages or doing on-line searches.

Properties of wood

Wood has a grain to it and will react differently depending on the way the grain runs in the wood. An easy way to demonstrate this is to take a piece of 4″ x 4″ x 1/8″ balsa wood and try to bend it one direction and then try to bend it in the other direction. You will find that the sheet bends easily when you bend it with the grain of the wood and that it is more difficult to bend the wood against the grain of the wood. When designing and building your structure keep in mind how the grain affects how the wood performs.

Tensile Strength

If you have long thin strips of wood that you are going to work with you might want to test all the strips for tensile strength before you begin to cut them up into pieces for your structure. You can do this with a simple deflection test. Note that you can only run this simple test on wood sizes up to about 1/4″ x 1/2″ cross section.

To test deflection you need:

  • A table with a straight flat edge (no lip)
  • A yard stick or meter stick
  • A small piece of clay (no more than a 1″ cube)
  • A paperclip bent into a hook and inserted into the cube of clay

Steps

  1. Use the yardstick to measure the height of the table from the floor (assume 30″ for this example)
  2. Hold a strip of wood with all but 6″ of the wood hanging off the edge. Lay a book across the wood strip even with the edge of the table. Hang the paperclip hook with the clay on it about 1″ from the free end of the strip of wood. If you are testing wood with a very small cross section (for instance 1/8″ x 1/16″) it may not be necessary (or possible) to hang the clay on the end of the strip. With other cross section sizes you may have to use a different size pieces of clay.
  3. One person should hold the yard stick vertically on the floor at the end of the stick. Write down the height of the end of the wood strip from the floor. Subtract this number from the height of the table (measured in step 1) and this gives you the amount of deflection for the particular wood strip you are measuring.

The wood with the least amount of deflection is the wood that has the greatest tensile strength. Group the wood of similar tensile strength together. You could label the wood lightly in pencil with the amount of deflection. The smaller the number the greater the tensile strength.

This is the wood that you should use to construct your structure. If you have limited supplies of the wood strips then use the strips of wood with the greatest tensile strength for the longest pieces of your structure. Then move toward the strips with lesser tensile strength for the shorter pieces of the structure.

What wood should I use?

Destination ImagiNation structure challenges typically allow the use of any natural wood. As you are trying to decide what woods to use, spend some time with the various types just playing with them.

  • How porous is it?
  • How easy (or hard) is it to cut?
  • How consistent is it?
  • How easy is it to identify the grain in the wood?
  • How strong is it in compression? tension? shear?
  • How easily does it bend?
  • Do certain glues work better with it?

Other Considerations?

Moisture in the air affects how the wood performs. All woods absorb some moisture from the air – especially balsa wood. This moisture also affects the weight of the structure. It is possible for a wood structure weighing less than 20 grams to absorb as much as 1 gram of moisture from the air. Brainstorm ways to control this moisture absorption and to remove moisture from the structure if necessary.

* Copied from: “Diary of a Balsa Goddess” http://structure.txdi.org/node/27  ©2009 Heather Compton

Diary of a Balsa Goddess by Heather Compton is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.

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