Elevated Bridge B/C

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Bjsong
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Re: Elevated Bridge B/C

Post by Bjsong » January 25th, 2010, 9:20 pm

thanks for your help andrewwski

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Re: Elevated Bridge B/C

Post by cypressfalls Robert » January 25th, 2010, 9:26 pm

Bjsong wrote:My problem is that I messed up gluing the wood in the first time and I tried to glue it the second time so is there anyway to quickly take out the glue before it permanently stays in the wood like washing it off?
would'nt applying water to the balsa on the bridge be a bad thing?

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Re: Elevated Bridge B/C

Post by Balsa Man » January 26th, 2010, 5:46 am

evbassboy13 wrote:Another advantage to using 1/64 x 1/4 is that you have a lot more surface area for a solid lap joint without using gusset plates, which saves building time and (a trivial amount of) weight, and should make your joints sturdier. I think. Just thought of that while I was reading.
More surface area- absolutely correct, and important. And with pairs vs a single piece, each joint is carrying 1/2 the load
dragonfly wrote:I think the key for you first is to nail down a good design and then plan out densities and wood sizes second.
Again, absolutely correct. Its having a design (good, bad, or otherwise) that allows you to understand what the forces on each member are (using, for example, the jhu bridge Builder app, or something else). That information becomes the basis for selecting the wood needed to carry the design loading; you need to know what a piece needs to do- how much force it has to carry -before you can decide what to use. Otherwise, you're shooting in the dark. Through trial and error; a lot of building and breaking, you'll eventually get there, but its a lot more efficient to know the loads, and know your wood (or do piece testing), and be able to build with pieces that are pretty close to what you'll need. Knowing the loads various pieces need to carry will give you choices in how to select them, and what you're looking for is mostly what the lightest choice is, along with any buildability advantages/disadvantages.
dragonfly wrote:no matter what your crosspieces (to connect your bridge) will be lighter/smaller, since for most designs it works out that way.
I can't think of a design where this isn't true. They are just to keep the 2 sides flat and aligned (whether parallel or leaned in), and to provide bracing against column buckling/failure in compression pieces. If the two sides are flat and symetrical, the loading these "ladder" pieces will see are VERY small. You can see/feel this by taking a stick, push down on the end, see it start to bend in the middle, back off, then put 2 fingers at the middle, bracing your hand so they don't move, push down again. As it just starts to bow against one finger, it takes very little pressure/force to keep it from bending, and you can put a lot more pushing force on the top. The more it bows, the more force it takes to keep it from bowing further; to stop the first bit of bowing takes very little force.
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Re: Elevated Bridge B/C

Post by evbassboy13 » January 26th, 2010, 9:21 am

Is it better to use thicker (1/4 instead of 1/8, etc.) low-density pieces for compression?

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Re: Elevated Bridge B/C

Post by Balsa Man » January 26th, 2010, 10:10 am

evbassboy13 wrote:Is it better to use thicker (1/4 instead of 1/8, etc.) low-density pieces for compression?
It's all about trade-offs.
Google up Euler's Buckling Theorem to understand more.
For compression pieces, we're talking about column failure; buckling, where the piece bends then breaks.
Simplifying, there are three factors that deternine the compression load under which a column fails:
the length - actually the exposed length; the length between braced points,
the cross-section; thicker cross-sections increase the load it can carry, and'
the inherent strength/stiffness- which, simplified, boils down to the density.

At a given length, both a fatter, lighter piece , and a denser, skinnier piece will carry the same load. We are not necessicarily talking the same weight for these two alternatives, though.

The only way to KNOW what the weight comparisons for a given strength are is testing.
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Re: Elevated Bridge B/C

Post by StampingKid » January 27th, 2010, 7:28 am

On the subject of glue, has anyone tried the Gorilla Glue CyA? it is supposed to be rubberized for shock strength.
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Re: Elevated Bridge B/C

Post by Balsa Man » January 27th, 2010, 12:08 pm

StampingKid wrote:has anyone tried the Gorilla Glue CyA? it is supposed to be rubberized for shock strength
Old (yellow) Gorilla Glue, yes; works well; slow; newer (white - quicker; works well, but still way slow vs CA), yes; GG "CyA"- haven't seen such a thing.

As to glue that flexes.... its conceivable that in certain, limited situations/places having a little glue flex could work, but in general/most situations you want rigidity, where an attached piece can't move around.... There is flexible CA out there (from other that the GG folk); have played with it, but only used at cross-over points of X-bracing on a tower... once.
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Re: Elevated Bridge B/C

Post by lllazar » January 27th, 2010, 3:35 pm

What do u guys think about Super Glue? Cyanoacrylate?

Is it pretty good?

Also, are cross pieces the pieces that connect the two sides of the bridge?

By 2 sides, i mean the front and back sides if u were looking at it head on.
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Re: Elevated Bridge B/C

Post by nejanimb » January 27th, 2010, 7:04 pm

CA glue is, I think, the best choice for most joints in a bridge.

And yes - when we say cross pieces, that is exactly what we mean.
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Re: Elevated Bridge B/C

Post by Deathshadow5 » January 29th, 2010, 7:23 pm

im not in this but if it is not breaking the rules a double arch would be effective, ill let you wonder on that now that its probably to close to change it.................... :? lets see what you come up with plz post images of your bridges! :) :D :mrgreen:

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