Designs B/C

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bernard
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Re: Designs B/C

Post by bernard »

IvySpear wrote:Our bridge always seems to tear away at the bottom right before we make it to full load. Basically our sides and top areas are intact, but the wood at the bottom just pops off. We tried adding gussets, but the gussets tear in half as well. We tried a different glue but that snapped as well. We use normally use Loctite. Is this a design problem or is it a wood problem? Any suggestions?
Since this seems to happen with every bridge, I would say it is a design problem. But I can't be sure since I have almost no idea what your bridge looks like.
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Re: Designs B/C

Post by bernard »

IvySpear wrote:At regionals this Saturday, we saw a couple of bridges that held their weight from the top, what advantages would that have? Our team won 1st in Varsity and 1st and 2nd in JV :D
It depends on the design. Placing the loading block on the bottom wouldn't work for me since I only have tensile members on the bottom, which can't hold much.
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Re: Designs B/C

Post by SOCoach »

In a bridge loaded at the top . . . the top beams would be in compression while the bottom member would be in tension. If the load was moved to the bottom, the two swap places correct? If loaded at the bottom, the bottom would be in compression and the top in tension. Does anyone know of a good computer simulation for determining which pieces are undergoing which force?
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Re: Designs B/C

Post by chinesesushi »

SOCoach wrote:In a bridge loaded at the top . . . the top beams would be in compression while the bottom member would be in tension. If the load was moved to the bottom, the two swap places correct? If loaded at the bottom, the bottom would be in compression and the top in tension. Does anyone know of a good computer simulation for determining which pieces are undergoing which force?
Incorrect. The bottom would still be in tension and the top is in compression. Though that is quite generalized, as it really does depend on your design. But in general, the top is always compression and the bottom is always tension. I suggest JHU bridge builder.
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Re: Designs B/C

Post by SOCoach »

So regardless of where the load is placed (generally) . . the top beam is always in compression and the bottom is in tension.

but if the load is placed on the bottom are the vertical supports from the top to the bottom of the bridge in tension? It would seem to make a lighter bridge you'd want as many pieces in tension as possible because those pieces can be lighter.
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Re: Designs B/C

Post by chinesesushi »

SOCoach wrote:So regardless of where the load is placed (generally) . . the top beam is always in compression and the bottom is in tension.

but if the load is placed on the bottom are the vertical supports from the top to the bottom of the bridge in tension? It would seem to make a lighter bridge you'd want as many pieces in tension as possible because those pieces can be lighter.
I suggest you use the simulation program I suggested, may answer some questions. If not, let me know.
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Re: Designs B/C

Post by dholdgreve »

Keep in mind that by minimizing the number of pieces in compression within the design, you may be increasing the amount of compression force that the remaining pieces need to carry...
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Re: Designs B/C

Post by SOCoach »

Is there an optimal height for a bridge? My students are struggling to get the weight down and it occurs to me making it shorter (not as tall . . . not length) would reduce the weight and help prevent buckling.
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Re: Designs B/C

Post by chinesesushi »

bernard wrote:
iwonder wrote:
Friedoyster3 wrote: The structure isn't so much twisting and breaking as it is deforming downwards. Across the entire span the deformation is actually quite noticeable. But since the break patterns aren't very conclusive, I can't really be 100% sure that's all that is happening to cause failure without high speed footage.
Ahh the lengths I went to trying to get high speed footage of my structures breaking....

From how it sounds you could probably try horizontal braces, if the whole thing deforms try making it taller (I haven't read the rules enough to know if you can...?)
No rules about maximum/minimum heights of bridges, though it is used as a second tiebreaker. One issue I encountered when making my bridges taller while still trying to maintain a low mass was that all my members were longer and less dense so they buckled more. So far my best efficiencies are from my 10cm tall bridges (I've tested bridges from 10cm tall to 17cm tall).
SOCoach wrote:Is there an optimal height for a bridge? My students are struggling to get the weight down and it occurs to me making it shorter (not as tall . . . not length) would reduce the weight and help prevent buckling.
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Re: Designs B/C

Post by SOCoach »

So taller helps lessen the effects of downward deformation . . . . but you run into more buckling issues?
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