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How to Make a Girder Bridge

Girder bridges are one of several standard designs engineers use for creating spans. Other common types are arch, suspension, cantilevered and cable-stayed. Girder bridges are also known as beam or truss bridges, with the design having to withstand both compression and tension forces. The principles of girder bridge building can be learned from materials like toothpicks or balsa wood. Commercially constructed bridges are designed by civil engineers who must be qualified. Civil engineers often use multiple T-girders side-by-side in road bridge construction.

Things You'll Need

Toothpicks
White glue
Ruler
Wood blocks
Scale weights

Instructions

- 1
  
  Use regular toothpicks to build a girder bridge.
  
  Lay three toothpicks on a work surface to form an equilateral triangle. Use white glue to stick the tips of the toothpicks to one another. The glue should fix the toothpicks together in the way that rivets would be used with steel girder construction. When you have applied the glue at the three corners you should end up with a firm triangle shape.
- 2
  Construct another three similarly sized triangles from the glue and toothpicks. By the time you have finished you should have made four triangles from a total of 12 toothpicks.
- 3
  Place two of your triangles next to one another. Set the triangles so that one corner of each is just touching the other without them overlaying. The closest edge to you of each triangle should lie at 180 degrees to the other. Line them up against a ruler, or another straight edge, to help align them.
- 4
  Place a new toothpick in a parallel position to the closest edge to you of the two triangles. Set it so that the tip of each end connects with the two top corners of the triangles. Glue the toothpick to these two points. You should have constructed what looks like a trapezium shape with three internal triangles. Triangulation like this gives rigidity.
- 5
  Take your two remaining triangles and set them out, as you did the last ones. As before, use another new toothpick to make a second trapezium shape. Stand the two trapeziums up so that the long edge of each remains on the work surface. Set them in a parallel position to one another, one toothpick's length apart.
- 6
  Glue the tips of a toothpick from one trapezium to the other at each corner of each of the internal triangles in them. There should be three toothpicks that run along the lower-level, on the work surface, since there are three triangle corner points there. There should be two toothpicks at the upper-level, above the work surface, since there are two corner points there. Each of the five toothpicks you add should run cross ways between the trapeziums, at 90 degrees to them.
- 7
  Stand your girder bridge over two wood blocks to create a span. Test the bridge's strength by placing a small weight from a set of scales in the middle at the top. Add further weights and look for signs of stress in the bridge as you go.

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