# Balsa Bridge help

1. Dec 11, 2011

### pasterino

Im building a balsa bridge for class but I'm struggling to find a design. The bridge must hold 20 pounds and have a trapezoid clearance. the minimum length is 14cm and the max is 20cm, min to max height is 5-8cm. Width must be 3cm. Here is my design so far, I would build two of these faces and connect them with horizontal lengths of balsa. I have 24 feet of balsa so please critique, advise, and even reconstruct my design. Thanks

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2. Dec 12, 2011

### Simon Bridge

... and you are presumably limited to how much balsa you can use? (Otherwise you can just use a thick plank.) Does it only have to support a 20lb weight?

What level is this to be done at? Like a high-school project gets a different kind of answer to a college engineering class.

There are infinitely many designs - consider what happens when you put the balsa plank edge-up: pretty rigid right? You'll need at least two of these with the gap narrower than the width of the test weight. After that you can put anything you like up for the surface ... but I am imagining a std hobby sheet of balsa - if you have a pile of thin sticks it's trickier.

So you see how important the constraints are?

BTW: there is a simple design here which supports 220kg (485lbs).
The same site also has lots of tips and other designs, physics links etc.

Last edited: Dec 12, 2011
3. Dec 12, 2011

### pasterino

thanks for the reply, im limited to small sticks of balsa, i believe they are around 1/8 inch but im not sure. This is for my high school AP physics B course

4. Dec 12, 2011

### netgypsy

Find out of laminating is allowed. If it is, this is a way to really increase the strength.

5. Dec 13, 2011

### Simon Bridge

OK - check out the balsa-bridge website for designs - those folk were restricted by weight.
You may like to try making an arch. You can build long beams out of short lengths like building a wall.

At least now you know your design need only be quite short and the load light :)