Boomilever for 2013

SLM
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Re: Boomilever for 2013

Post by SLM »

For the simple triangular design, the vertical member that connects the tension member to the compression member carries very little axial force. Well, theoretically speaking the applied load causes no axial force in that member, if the compression member is horizontal.

The (vertical) member mainly acts as a bracing. It restrains the vertical (up/down) movement of the free end of the compression member at the wall. Either the butt or the lap configuration can be used for connecting the vertical member to the compression/tension member without adverse consequences. Either type of connection, if done properly, provides adequate strength.

Here are a few possible ways to connect the members.

If the compression member is tubular:
Image

If the compression member consists of two separated rectangular sections:
Image

If the two compression members align vertically with the tension members:
Image
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Re: Boomilever for 2013

Post by SLM »

iwonder wrote:...but I always thought that putting a force on the tension member like that would take the member out of pure tension and allow it to fail much sooner than other members... is that true?
True. But, you may not have any choice if you need to control buckling in the compression member using bracings. Axial force in a bracing is generally not large, compared with the force in the main compression/tension members. In most cases the force exerted on the tension member by a bracing is not large enough to cause failure, but, it is always a good idea to check it mathematically, or in our case experimentally.
Last edited by SLM on August 24th, 2012, 10:56 pm, edited 1 time in total.
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Re: Boomilever for 2013

Post by SLM »

_HenryHscioly_ wrote:...maybe it is possible that people have tested and concluded those vertical supports may be able to reduce more weight from the compression part of the boom than they would add to the boom( their own mass + using slightly stronger tension members.)
Can anyone confirm this? but this would most likely vary and depend on the boomilever design.
Axial force in the compression member is not going to be affected in any significant way by the presence of the vertical member at the wall. So, you will not be able to reduce the size of the compression member by placing a vertical member at the wall. However, in addition to restraining up/down movement of the compression member at the wall, the vertical member could influence rotation at the joints of the boom thereby having a significant (or measurable) effect on the deformation of the entire structure.
Last edited by SLM on August 25th, 2012, 4:59 am, edited 1 time in total.
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Re: Boomilever for 2013

Post by _HenryHscioly_ »

A vertical piece in the middle of the compression member connected to the middle of the tension member would help brace the compression member from buckling, but the force is not very big so may not affect the tension member that much? Did I interpret the posts you posted correctly?

Also, how would you place the loading block on a boomilever where the tension members are attached on the inside? I'm not really seeing how that would work.
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Re: Boomilever for 2013

Post by SLM »

_HenryHscioly_ wrote:A vertical piece in the middle of the compression member connected to the middle of the tension member would help brace the compression member from buckling, but the force is not very big so may not affect the tension member that much? Did I interpret the posts you posted correctly?


Also, how would you place the loading block on a boomilever where the tension members are attached on the inside? I'm not really seeing how that would work.
Yes, but not just one vertical member, more like this:
Image

However, it may not be necessary to use bracings at alll if the compression member has adequate buckling strength on its own, if its cross-section has a large moment(s) of inertia.

If the tension members are attached on the inside of the compression member, I probably do something like this:
Image
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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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Re: Boomilever for 2013

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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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