Boomilever for 2013

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fishman100
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Re: Boomilever for 2013

Post by fishman100 »

iwonder wrote:Oh, we were talking about a non-tower crane design and the added members in that
Could work for that too, but I personally can't see myself building a boom like that. :P

Also, how can I calculate the moment of inertia of the entire compression structure? (This is in a 'box-beam' style boom.) My best guess is to use the general formula, but I would think it's more complex than that.
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SLM
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Re: Boomilever for 2013

Post by SLM »

fishman100 wrote: Also, how can I calculate the moment of inertia of the entire compression structure? (This is in a 'box-beam' style boom.) My best guess is to use the general formula, but I would think it's more complex than that.
If by box-beam, you mean a hollow rectangular section, then its Moment of Inertia equals to the Moment of Inertia of the outer rectangle minus the Moment of Inertia of the inner rectangle where each Moment of Inertia is calculated using the equation you wrote above.
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Re: Boomilever for 2013

Post by jander14indoor »

To add just a little, the moment of inertia is unique to a cross section. Properly it is an integral of the distance from the neutral line to the area at that distance across the cross section. Mid-level college mechanical engineering stuff. You can also find more correct/complete explanations on line.

But, while the proper calculation can be pretty advanced, the use is well within Middle or High School reasoning.

The formula you mentioned is the solution to that integral for a solid rectangular cross section. As SLM mentioned you can modify for a hollow rectangular cross section. If you look around on line you'll find solutions for other common shapes, round, oval, I-beam, etc. It is common engineering practice to deconstruct a complicated cross section to these known solutions instead of figuring out the integral. Engineers only break out the calculus for new shapes or special cases.

Really complex sections with multiple transitions and gaps like an open girder structure take a computer to solve properly.

BUT, you don't need to go that far. You can break down the problem into simple sections. Make sure each section doesn't buckle and the structure (most likely) won't buckle. Dig back through past years Tower, Bridge and Boom discussions for more on how to take advantage of this.

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