Balsa Bridge Design Optimisation

AI Thread Summary
The discussion centers on designing a balsa wood bridge that spans 200mm and is intended to fail at a specific weight of 200N. Participants emphasize the importance of achieving a high strength-to-weight ratio while maintaining simplicity in design. Suggestions include using a 3D structure for improved strength and strategically placing joints to control failure points. The challenges of modeling and analyzing the bridge, particularly with balsa's material properties, are also highlighted. Overall, the focus remains on optimizing design while ensuring the bridge meets the specified performance criteria.
kuleke
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Hi,

I need to design a balsa bridge that will cover a distance of 200mm. It will be loaded centrally for testing. The objective of this however is not to test until failure, but to have it fail at a certain weight. Points are also given for high strength to weight ratios and simplicity of the design. I already have a design but the calculations are looking very tedious and I am hoping that a civil engineer out there can help me with a simplified design. The failure weight of the bridge is by default 200N but if calculated and stated beforehand, the performance criteria is adjusted to match the defined failure weight.

Thank you very much
 
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What are the dimensions of the balsa wood you have to hand?

What other materials are you allowed?

Are there any specific restrictions that should be highlighted before we start?
 
How do you define failure?
 
jarednjames said:
What are the dimensions of the balsa wood you have to hand?

What other materials are you allowed?

Are there any specific restrictions that should be highlighted before we start?

Allowed Dimensions: 2x2, 3x3, 4x4, 5x5, 6x6, 100x2, 100x3, 100x4, 100x5 and 100x6. They also have sheets but their dimensions weren't specified.

We're only allowed balsa wood and glue.

We require the structure to break at 200N, and the design has to make provision for the attachment of the bolts that will connect to the weight.
 
Studiot said:
How do you define failure?

The bridge failing completely. The idea is that all the components have the same chance of failure at the weight of 200N so it breaks down completely. We're trying to achieve a very high strength to weight ratio.
 
Hello?? :confused:
 
OK, sorry completely forgot.

So now, as this is school work, we need to see some effort from yourself.

What design do you currently have? What is its breaking point? What ideas do you have for increasing structural strength?
 
http://www.elpla.com/elpla_en/images/truss.gif

What I have is effectively that, I'm really stuck on how to optimize strength to weight and find the failing point (and possibly failure mode and location of failure). Also, I don't think that design is great. i chose it because they appear to emphasize simplicity. I'm really pressed for time at the moment on this so I would really appreciate some quick responses. Thank you :)
 
One other thing, what dimensions are you using? Never forget units!

Also, you're balsa wood should have three dimensions (it's not 2D is it?). Or is the third whatever length you require?
 
  • #10
All dimensions are in N, mm and effectively Mpa. the bridge has to span at least 200mm so it doesn't fall through to the ground, we're allowed to analyze as though its a 2d structure since there are no components in the direction of the third axis as far as the majority of designs go(also makes the concept of simplicity fail). The area of the balsa wood i believe we have but we just do a frameworks analysis until we need to estimate total weight/failure stress/load etc
 
  • #11
Given your wood is mainly square, I'd recommend you go with something like the following 3D structure:
[PLAIN]http://www.heightec.com/shop/images/tower-crane-rescue_1.jpg

It's slightly more complex, but it's a lot stronger.

If you'd prefer to go with something more 'square' for easier calculations, you're possibly going to need a fair bit more wood.
 
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  • #12
will it not be difficult to model/analyse this? I'm not very good with CAD/Simulation software :(
 
  • #13
It would.

A 2D design is all well and good to analyse, but your final model will be 3D.

That structure is a good strong one (hence the use on cranes).

I'm trying to think of one that would be simple to analyse but strong.
 
  • #14
From previous experience in balsa bridge building, keep in mind that you bridge will probably start failing at your joints rather than in the middle of a spar. This could make predicting when it will fail more difficult. I would recommend making two kinds of joint- one that is strong, and the other designed to fail. If you place your joints which are designed to fail at strategic locations, the whole bridge should come down at the load you designed it to. Repeatability will be key.

From an analysis standpoint, my guess is the easiest "bridge" to design would be a single simply-supported beam structure. Either an I-beam, hollow tube, or solid cross-section. Analysis would only require a simple beam-bending equation, and probability of failure could be tested ahead of time with test samples and a load cell.
 
  • #15
Mech_Engineer said:
From previous experience in balsa bridge building, keep in mind that you bridge will probably start failing at your joints rather than in the middle of a spar. This could make predicting when it will fail more difficult. I would recommend making two kinds of joint- one that is strong, and the other designed to fail. If you place your joints which are designed to fail at strategic locations, the whole bridge should come down at the load you designed it to. Repeatability will be key.

From an analysis standpoint, my guess is the easiest "bridge" to design would be a single simply-supported beam structure. Either an I-beam, hollow tube, or solid cross-section. Analysis would only require a simple beam-bending equation, and probability of failure could be tested ahead of time with test samples and a load cell.
Please refer to the available dimensions of balsa I mentioned earlier, I'd like to know how you think I can construct the I-beam because I don't think it will be able to handle 200N at the center. Thanks
 
  • #16
What are your own thoughts? Have you done any test configurations?

The project is yours, we can help but no one will do your work for you.
 
  • #17
http://www.elpla.com/elpla_en/images/truss.gif

That photo is what I had in mind (the dimensions are incorrect), I know how to find the internal forces, but I don't know how I'd optimise it and be able to predict its failure mode. I think the beam will be too difficult to construct and I also doubt I'll get any marks for simplicity if I were to do that
 
  • #18
OK here are a few pointers to start off with.

The material is Balsa.

If you are only allowed to glued joints, do you think the joints will be rigid?
If so will you analysis be valid?

Balsa is a very soft material, is the material as strong in compression as tension?
How does the answer affect the size and shape of members made from it?
Could any difference be exploited to define the failure member?
 

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