How to make a bridge out of balsa wood

Written by tracy morris
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How to make a bridge out of balsa wood
Building a model bridge can teach students what's involved in the design of real bridges. (bridge image by pearlguy from Fotolia.com)

Building a bridge from balsa wood is such a popular educational activity that there are competitive contests for building model bridges. A balsa wood bridge-building project touches on principles of building and construction trades, engineering, physics and static equilibrium. The main goal of building a balsa wood bridge is to construct a bridge that withstands the greatest amount of force before falling. Competition balsa wood bridges have been constructed that hold up to stresses greater than 181kg.

Skill level:
Moderate

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Things you need

  • Graph paper
  • Waxed paper
  • Masking tape
  • 20 balsa sticks measuring 1/8 by 1/8 by 48 inches
  • Balsa sheet measuring 1/8 by 2 by 16 inches
  • Balsa sheet measuring 1/16 by 3 by 8 inches
  • X-Acto knife
  • Straight pins
  • Wood glue
  • Clothespins

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Instructions

  1. 1

    Sketch out a rough design for your bridge on paper. These sketches will help you develop different ideas of bridge design before you begin to cut your balsa wood. Your design should have a superstructure and a substructure. This view does not have to be to scale, since it is only a rough design. One style of bridge that many designers use is the Warren truss style, which is favoured by railroads for its load-bearing capabilities.

  2. 2

    Sketch a three-view sketch, which will include views of your bridge from the top, sides and end. Draw this sketch to size, using the graph paper to ensure your measurements are precise. Since your wood is 1/8 inch wide, your structure should have 1/8-inch-wide trusses. The bridge should also be open enough that you can push a simulated vehicle across the bridge span.

  3. 3

    Sketch out your bridge nodes in larger-than-life detail. Nodes are the points of connection where your bridge will endure stress. Stress points are the most probable point of fracture for your bridge when it is put under testing conditions. Attention to details, such as factoring in how to cope with stress along the nodes, is the key to a successful bridge design.

  1. 1

    Attach your three-view drawing to a hard surface, such as a piece of cardboard. You will build your bridge on this surface using your drawing as a guide. Over this, tape the waxed paper. The waxed paper will prevent you from accidentally gluing your project to your guide.

  2. 2

    Cut the pieces of wood so that each piece fits the outline of the bridge you've drawn. To ensure a perfect fit, cut each piece of wood slightly longer than the guide. Then line up the wood with the guide, and trim it to match.

  3. 3

    Pin the pieces of wood in place, then glue them together. Once the major trusses are glued in place, glue in the secondary trusses. Wait until all glued spots are dry before removing pins and setting aside your bridge's sides.

  4. 4

    Repeat the previous step to build your bridge's cross braces. Once the cross braces have dried, attach them to the sides at the roadway and the top of the bridge. Use clothespins to hold the bridge and cross braces together while they dry.

  5. 5

    Check all the nodes to ensure they are tightly in place once all the glue has dried on your bridge. Your bridge is now ready to be graded.

Tips and warnings

  • Though wood glue is the most common glue used in constructing a balsa wood bridge, other types of glue that can be used include gorilla glue or epoxy. If you're constructing your bridge for a competition, check the competition rules to determine the types of glues allowed.
  • X-Acto knives are extremely sharp. Use caution when cutting your balsa wood.

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