Expert: Jeanette A - 1/7/2009

Question
Hi Jeanette. I am doing a science fair project on a toothpick bridge to see how much weight it will hold. I was wondering what a toothpick bridge design would need to hold quite a lot of weight. Please reply.

Answer
Hello Matthew,

Well firstly you are given specifications I assume, so considering your specifications you will then need to decide on a bridge design. There are various bridge designs, and we all know that a truss is the most strongest configuration in civil engineering. Below is a link which will give you some guidance;

http://pghbridges.com/basics.htm

Once you have decided on a bridge, keep in mind, the simpler the configuration the less material you will need (glue, toothpicks etc) which will also lessen the weight of the design. You will then need to decide on the type of toothpick and glue (flat/round,wood glue/ araldite glue) which will give you good bonding. You will need to be aware of joints so not to create weak joints - good glue joints make for good strength. Also keep in mind some glues require 24hrs complete drying time. To reduce drying time, do initial gluing with small drops of glue (applied with an extra toothpick). You can go back later and reinforce these small drops with more glue after everything dries.

The spaces between the intersections of toothpicks affects strength of the design. You should test each toothpick before you use it. For flat toothpicks roll the toothpick between your fingers to see if it can withstand some pressure without breaking. For round toothpicks apply some pressure to bend it without breaking it. Therefore, consider the strengths and weaknesses of each single toothpick under  COMPRESSION, TENSION and SHEAR. Design your toothpick bridge to use the strengths of each toothpick as much as possible.

The bridge's strength is also dependent on its vertical height.   Although, this is not to say "the higher the better", as there are specifications to be met.

Be aware of stress risers. For example, before you bridge fails, it will give and bend under the load of the test weight. If you have created some parts of the bridge so strong and hard that it will not give, the bridge will fail in some other area much quicker than if the whole bridge were able to adjust.


I think you are now able to get started on building a bridge, just keep in mind its a learning process and go from there. Enjoy it and goodluck with it! I hope this has helped.


Warm Regards,
Jeanette