Design and Build a Strong Toothpick Bridge or Balsa Wood Bridge

In science classrooms around the world, students construct balsa wood bridges and toothpick bridges. Whether for extra credit or bragging rights, they always hope that their bridge will hold the most. This information will help you to ensure that your bridge is competitive.

You will want to complete your bridge in three separate stages. You will want to create two identical sides and a strong roadbed, and you will then want to anchor them all together. Before starting construction on your bridge, you should create a diagram. This will help you to visualize what you will be doing and prevent any potentially disastrous errors.

Step One: Constructing the Sides of the Bridge

Your bridge should be symmetrical, as this will result in the best possible weight distribution and lessen the stress placed on any particular joint. If possible, you should construct an underhang for your bridge; this will help to alleviate many of the compression forces during the testing period.

Once you have the diagram of your sides completed, you are ready to begin construction. You should take care not to simply cut the pieces quickly and glue them together according to your diagram. Rather, you should be cutting certain pieces at an angle so that when they are placed against other pieces, they lie flat. This will make the weight your bridge needs to support less reliant upon your glue and more reliant upon the actual bridge structure. If you have time, you may even want to consider using notches to fasten your pieces together. Don’t rush through the construction of your sides, as they will bear the majority of the pressure put on your bridge.

Step Two: Constructing the Roadbed

People generally tend to devote far too much or far too little time to their roadbeds. You don’t want your roadbed to be so weak that it fails when weight is placed upon it, but the reality is that your sides will bear the majority of the weight. You should therefore ensure that the roadbed is sturdy enough to handle the weight, but once you are confident, it’s time to move on. I used a pattern of small rectangles that handled the weight quite well. Several roadbed styles will work, but a main one to avoid is one in which the wood stretches the full distance of the bridge lengthwise. The tension on these pieces of wood will be much too strong, and this will cause your bridge to fail. There is nothing more tragic than to see your bridge ruined with the first few pounds added to the roadbed.

Step Three: Anchoring it all Together

Connecting the sides of your bridge to your roadbed is arguably the most important step of all. You may be thinking that you will simply glue the three pieces together and be done with it, but doing this would be a huge mistake. The correct way to anchor the roadbed to the bridge is only slightly more complicated, and is outlined in these two steps:

1. Connect the two sides by gluing braces across the width of the bridge, ensuring that the roadbed will be able to fit snugly. You will want to have these braces every quarter of an inch to ensure the best possible weight distribution. Don’t simply glue the braces between the sides; rather, you should be resting the cross braces on the sides before gluing

2. After you have attached these cross braces, you will now want to glue your roadbed on top of them, making sure that your bridge now remains in the proper position while the glue dries.

Using this method of connecting your bridge will help it to last well beyond the failure point of the glue.

Finally, you should also use braces to connect your sides together along the top of your bridge; this will help your bridge last longer by preventing it from torquing.

Good luck!

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