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Name: Mark
Status: student
Grade: 9-12
Location: CA 
Country: N/A
Date: 5/31/2005

Dear scientists,

I am in a high school physics class. For our final grade, our teacher is having us design, build, and then report facts on a Balsa Bridge. However, she did not really tell us much of anything, and expects the report to be detailed on everything. My question is this; what equations can I use to predict how much weight my bridge will hold?


I am afraid this is a very tough question to answer and would depend on the type of bridge configuration you are trying to build and how you plan to load it. If you have the option, I would suggest that you design either a simple beam or a pin-connected truss because these are somewhat easier to analyze. If your physics text does not cover this, check your library for an engineering textbook on statics.

The other thing you need to find are the material properties of the balsa as far as its strength. Working with wood is somewhat complicated because its strength is different when measured along the grain or across the grain. (In scientific terms, this is called an orthotropic material.) To get more information on this, go to the USDA's Forest Products Laboratory's web site at Look under publications and download "Wood Handbook -- Wood as an Engineering Material". This is an excellent publication with information on all types of wood, including balsa. It also has a chapter on structural analysis equations which you will find very helpful. You may also want to download the latest West Point bridge design contest software from the US Military Academy's web site at This will give you a feel for how trusses behave.

On a practical note, I built some balsa bridges in college for competition that never did that well. The problem I had was not with the analysis, but with the behavior of the connections. I could never get the glued or pinned connections to behave consistently. The pin connections tend to fail by crushing around the pin because the stresses tend to concentrate at the pin. Of course, this is the point of the exercise - to learn that the real world is much more complex than the mathematical models. So, I am not the best person to give advice on building a successful balsa bridge. On the other hand, our concrete canoes were excellent...

Good luck,
Andy Johnson

You will need two things to be able to complete that project.

First: You will need some raw data. for the size of the balsa wood pieces you are using, how much weight (tension and compression) can they take before failing.

Second, you will need to apply some geometry. (yes, geometry!) The more highly angled a support is from the direction of the force applied, the less able it is to support that force. In other words, 2 pieces acting to support a load like vertical pillars will support 2 M. (M being the weight that causes a single piece to fail.) those same 2 pieces, angled at 45 degrees against each other like an arch will only support 1.414 M. lower those supports down to a 30 degree angle, and you only get 1 M.

It is somewhat difficult to explain this clearly without any diagrams. I would suggest you attempt to look up truss diagrams or stress diagrams for a better explanation. Once you have figured out what kinds of stress each component can hold, you can figure out the total load capacity of your bridge with relative ease.

Ryan Belscamper

First you need information on the material.

Here is a brief explanation on materials. Tension and compression are the critical factors. Picture a beam, with a large load, the beam sags in the middle. The bottom fibers are stretched, = tension. The top fibers are squeezed = compression. These "extreme" fiber stresses are what causes failure, different materials react to the stresses in different ways. Concrete is very poor in tension, yet very good in compression. Hence, the reason to add steel bars, the bars take the tension forces.

As far as equation, you have to decide on a shape for the bridge. Above, I discussed tension and compression. You can see if the beam was deep it would affect the fiber stresses. Truss designs are based on that principle. Rather than one piece, the design employees triangles to transfer the loads to the top and bottom members.

There are bridges using an arch, where the bridge deck is placed across the top of the arch.

So, you should start with the materials and choose a shape. Next, a critical factor is the connections, the loads, or forces have to be transferred from one piece to another. Failure, more often than not is in the connections. Are you going to glue ( glue represents welding in a big bridge), or use some pins (the pins represent bolting in a big bridge).

Bottom line, before you get to the design and equations, there are these decisions to be made.

There are other bridges, like the cable ones = suspension bridges. Chicago is famous for the bascule bridge, which opens in the center. I assume the teacher is leading to a trial of loading the model you are making. A key question, how will the load be imposed? Picture the bascule bridge with the opening in the middle, if the load will be from the middle, that puts a real burden on this type, since the bridge halves are cantilevers.

James Przewoznik

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