Boomilever B/C

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Re: Boomilever B/C

Postby iwonder » December 19th, 2012, 9:11 pm

What do you mean by distances between the two long trusses? I assume you mean the members in tension(It's not technically a truss, to my knowledge)? Aia's boomilever guide(the link should be here somewhere, it's on the wiki) has a lot of good starting information, and it talks about the spacing between the tension members, if that's what you mean. I'm not sure why this would vary from boom to boom, it seems like the spacing Aia talks about is pretty much accepted as what you want...
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Re: Boomilever B/C

Postby Balsa Man » December 21st, 2012, 7:42 am

I've read through most of this and found it all very useful and interesting, but there is one thing I haven't seen addressed (and if it has I missed it), but what are the average distances you guys have been using between your two long trusses on a tension boom? I am looking for a starting point, so even if you've nothing to back up your chosen distance, it'd be a useful starting point. Thanks! :)
What do you mean by distances between the two long trusses? I assume you mean the members in tension(It's not technically a truss, to my knowledge)? Aia's boomilever guide(the link should be here somewhere, it's on the wiki) has a lot of good starting information, and it talks about the spacing between the tension members, if that's what you mean. I'm not sure why this would vary from boom to boom, it seems like the spacing Aia talks about is pretty much accepted as what you want...
Hmmm…, and I thought he must be referring to the compression member(s). Correct, common vocabulary would be helpful.
Don’t think anyone’s going to be able to say anything about “average” spacing (whether tension or compression members)- just what they’re using.

So, assuming the question is on the compression members, and we’re talking two compression members and two tension members….
The obvious/common distance apart is 5cm (outside edge to outside edge), so they support the load block (as in Aia’s boom guide). You certainly don’t want them any wider apart than that. Closer together has the (weight) advantage of shorter bracing pieces between them-bracing to shorten effective column length/strengthen buckling strength in the horizontal plane-see earlier discussion on this). The limit on this is getting the tension members to clear the load block; you want the tension members to join the compression members at the centerline of the load block.

Figuring out how best to get needed clearance for tension members at the distal end takes you to how you do the wall attachment. There are two…..apparent options; running both from one bolt, or running them from two bolts.

For a reason I’ll get into in a second, if you go the two bolt direction, you want to use the 10cm spacing you get from using the center and one of the ‘side’ bolt holes, not the two side holes (which are 20cm apart). As with virtually all design options, there are tradeoffs- pros and cons, and you want to make those tradeoffs in ways that maximize structural efficiency-least weight to get you the strength needed.
The apparent advantage of two T-members from two bolts is stability against side-to-side sway in the loading process. What are disadvantages?

First, they need to be a little longer than they would need to be if they came from a single bolt- that means a little heavier- going out to the side bolt holes (20cm apart) would mean even longer/even heavier.
Second, that additional angle to the side means the tension force goes up a bit, compared to T-members running from a single bolt; not a lot, but a bit, and that means stronger- denser, heavier wood; an additional weight increment.
Third, the angle to the vertical plane of the compression member needs to be dealt with by using an ‘attachment wedge’- a triangular piece; one side glued to the compression member, one side, at an angle to the compression member side, to which the t-member is glued. Both of those glue joints are in shear loading. On the side of the ‘attachment wedge’ that joins the compression member, the shear is parallel to the grain. On the side of the ‘attachment wedge’ that joins the T-member, the shear is at an angle to the grain- a bit ‘cross-grain.’ As seen in the article on wood properties that I posted a link to earlier, shear strength in wood is maximum when parallel to the grain- as the angle to the grain increases, shear strength falls off; the more cross-angle there is, the stronger the parallel to the grain shear strength needs to be, to provide enough shear strength at a cross-grain angle, which means higher density, which means higher weight.

These factors, taken together suggest advantages to a single bolt approach. Lengths are shorted, tension loads are lower, and attachment wedges strong enough to carry cross-grain shear loading aren’t needed. For reasons discussed before, you want the T-members to run as close to the edge of the washer as possible- so ¾” separation. Given that, that means C-member separation that gives you outsides ¾” apart, or insides ¾’ apart.

Hopefully, this gives you some food for thought- an understanding of the tradeoffs you have to play with.
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Re: Boomilever B/C

Postby engineeringmaniac » December 21st, 2012, 9:11 am

Has anyone done any testing with a lenticular compression beam? I'm no civil engineer, but it seems to be the most efficient. The two arches with vertical bracing inbetween (I was thinking one or three centered) would brace against vertical deflection. I used a similar design with bridge and had success.
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Re: Boomilever B/C

Postby Balsa Man » December 21st, 2012, 12:11 pm

Has anyone done any testing with a lenticular compression beam? I'm no civil engineer, but it seems to be the most efficient. The two arches with vertical bracing inbetween (I was thinking one or three centered) would brace against vertical deflection. I used a similar design with bridge and had success.
It may well be a viable solution; but no reason I'm aware of to say, "the most efficient." One difficulty with curved compression members is getting enough symmetry to work. Forces in those curved members are quite a bit more than they'd see in a bridge....
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Re: Boomilever B/C

Postby NHills777 » December 21st, 2012, 1:25 pm

what is the length and OD of the eye bolt? I read and reread the parameters but couldn't find anything on it. (I know that this isn't where to ask for rules clarifications) Do you think they would allow our team to bring in an eye bolt that meets all the parameters so we know it will actually work?

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Re: Boomilever B/C

Postby A Person » December 21st, 2012, 1:49 pm

what is the length and OD of the eye bolt? I read and reread the parameters but couldn't find anything on it. (I know that this isn't where to ask for rules clarifications) Do you think they would allow our team to bring in an eye bolt that meets all the parameters so we know it will actually work?
Look at section 4.b.iii. It states 1/4" wide and 3" long eye bolts.
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Re: Boomilever B/C

Postby NHills777 » December 21st, 2012, 2:18 pm

That states the parameters for the three bolts that would be for the three holes in the testing wall not the eye bolt that attaches the chain to the mounting block. All it says in 4.c is that it has to be 1/4" threaded eyebolt.

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Re: Boomilever B/C

Postby Balsa Man » December 21st, 2012, 2:55 pm

That states the parameters for the three bolts that would be for the three holes in the testing wall not the eye bolt that attaches the chain to the mounting block. All it says in 4.c is that it has to be 1/4" threaded eyebolt.
You're right it doesn't specify the length, but it does spec the o.d- 1/4"

While you can get longer and shorter, what to expect at competition is a 'standard one'; don't have in front of me, 3 inches or so.. Length shouldn't be a will work/won't work issue. Supplying your own, rules say ES to provide, so probably not a way to go
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Re: Boomilever B/C

Postby A Person » December 21st, 2012, 6:03 pm

Depends if they're asking about the loading block or the testing apparatus. For testing it is 3".
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Re: Boomilever B/C

Postby fanjiatian » December 22nd, 2012, 6:34 am

How can you figure out an appropriate geometry for bracing between compression and tension members without relentlessly testing the milllions of possibilities?


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