What is a Toothpick Bridge?
A toothpick bridge is a simplified demonstration of the mathematics involved with full-scale
bridges. The forces involved with toothpick bridges exemplify the forces that are involved in
Truss bridges built today. Through experiments and testing, you can reproduce the forces of a
full-scale bridge by building a toothpick bridge.
These forces include:
Compression: Compression is the downward force placed on a beam. For example, if you take
a short straw pinch it in the middle, it flats out. If you hold it length wise between your fingertips
and press, it takes a significant amount of pressure before the straw finally folds. That force is
called compression.
Tension: Tension is a force that pulls. For example, if you took a toothpick and tried to pull each
end until the toothpick broke, it would take a lot of force. That force is tension.
Torsion: Torsion is twisting. Twisting a toothpick to make it break is easier than pulling it apart
(tension). Not all toothpicks are the same and different forces on the torsion will break different
toothpicks.
Shear: Shearing is two opposing forces pushing on the same point. For example, if you hold a
piece of wood with both hands next to each other, and push up with one hand and down with
the other, you are applying shear to that piece of wood. Shear usually occurs horizontally, and
not vertically.
Girder Bridge - A girder bridge is the most common and likely, the most basic of bridges. A
fallen log across a creek is an example of a girder bridge. Steel girder bridges use I-beam
girders and box-girders. Picture to the right is the famous Coronado Bridge in San Diego, CA.
Truss – A truss bridge is a simple skeletal structure, typically made of an assortment of
triangles. The picture to the right is a railroad truss bridge over the Flint River in central
Michigan. Since most beams in a truss bridge are straight, toothpick bridge projects are perfect
for bridges as they are typically easy to build and demonstrate the strength these types of
bridges can support.
Rigid Frame Bridge – In the rigid frame bridges, the girders and support piers are all one
piece. These are required to support a lot of weight. The picture to the right is the George Bush
Bridge.
Arch Bridge - After girders, arches are the second oldest bridge type and one you see often.
Arch bridges are perfect for stone and were developed and used extensively by the Romans.
Arches are good choices for crossing valleys and rivers since the arch doesn't require piers in the
center. The arch bridge to the right is the Cold Springs Bridge near Santa Barbara, CA.
Cable Stayed Bridge- A cable stayed bridge is a continuous girder with one or more towers
erected above piers in the middle of the span. From these towers, cables are connected
diagonally and support the girder (the road or surface). The picture to the right is the Tatara
Bridge in Japan.
Suspension Bridge - The suspension bridge allows for the longest bridge spans. Although the
suspension and cable stay bridges look similar, they are very different. Suspension bridges are
in fact a very old form of bridge. In primitive cultures, simply ropes and wood planks are used in
a typical suspension bridge. To the right is the famous Golden Gate Bridge in San Francisco, CA
Toothpick Bridge Design. Information was taken was retrieved on January 23, 2013 at
http://www.toothpickdesign.com/toothpickbridgeforces.htm.