Boomilever for 2013

[iwonder]

I don't know of any good truss analysis software... but there's this... http://www.nexote.net/nexote/3D%20Truss ... 0Notebook/ and SLM posted a pdf in the design thread(I believe it was... whichever one talks about boomilever near the end...) detailing the forces involved, but to sum it up, single out the end connection to the loading block as a point, with 3 forces acting on it(the load, straight down, the tension members, at whatever angle you choose(I've been assuming 15cm high and 40 cm long for now), and the compression member, perpendicular to the load. You know the load, and the angle of the tension members, so it's just an equilibrium calculation(or summation of forces... as they call it in my schools poor physics classes) to figure out the loads on the forces on the various members. If you do find better truss analysis software... please do tell... hope this helps!

[JimY]

I don't know of any good truss analysis software... but there's this... http://www.nexote.net/nexote/3D%20Truss ... 0Notebook/ and SLM posted a pdf in the design thread(I believe it was... whichever one talks about boomilever near the end...) detailing the forces involved, but to sum it up, single out the end connection to the loading block as a point, with 3 forces acting on it(the load, straight down, the tension members, at whatever angle you choose(I've been assuming 15cm high and 40 cm long for now), and the compression member, perpendicular to the load. You know the load, and the angle of the tension members, so it's just an equilibrium calculation(or summation of forces... as they call it in my schools poor physics classes) to figure out the loads on the forces on the various members. If you do find better truss analysis software... please do tell... hope this helps!

You don't need truss analysis software for boom unless your compression member is not straight, such as the curved design shown in AIA's boom guide. For all straight leg designs, what you learned in Physics class is sufficient. Then while knowing the forces is fine and dandy, it won't necessarily allow you to design and build a great boom in one season. Rather, I think is it more useful to compare forces from one season to the next as the dimensions are changed. For example, back in 2001-3, the forces increased something on the order of 60% from the first year to the third, so if you used the same design, you'd need roughly 60% more material for the third year than the first.

[SLM]

Theoretically speaking, tension and compression are not the only forces present in boomilevers. Since the load is applied in the perpendicular direction to the central axis of the structure at some distance from the support wall, the boom also undergoes measurable bending stress, particularly at the joints. I would not ignore stresses due to bending unless it can be shown they are inconsequential for a particular boom design.

If you have decided to use a simple triangular shape (like the one shown below) and you wish to consider tension and compression forces only (no bending), then you probably don't need to use a structural analysis software to calculate member forces. You can simply set up two equilibrium equations and solve them for tension in the diagonal member and compression in the horizontal member.



On the other hand, if you plan to compare several potential geometric configurations for your boom before committing to one, then you could benefit from using a structural analysis software. For example, the analysis of the boom shown below requires more than what one learns in a typical physics course. A structural analysis software is the right tool to use here.



If you end up using a software tool, I suggest analyzing the boom as a frame not as a truss. The difference is that a truss can only carry tension and compression forces where as a frame can carry bending as well as tension and compression forces. A boom made of balsa sticks attached together using glue acts more like a frame than a truss. In this case, a two-dimensional frame analysis software will do. Unless you are willing to pay for such a software, use this one. It does the job and it is free.

[fanjiatian]

Thanks!

How do you build a testing surface for boomilevers?
Looking at old boomilevers, it seems that the testing surface is vertical as opposed to horizontal for bridges or towers.

[Faustina]

Yes, the testing surface is vertical. I’m not sure how to build a competition-worthy test surface, but if you want to test at home, it’s not too hard. Just find a sturdy piece of vertical wood to drill a hole in, like this: http://gallery.scioly.org/details.php?image_id=928. At my house we have some shelves in our garage made of 2 by 4’s, and I drilled a hole in a vertical piece. Then I used a ¼ inch bolt, a washer, and a wingnut to attach the base.

Also, I just finished testing my second boomilever, and it has not been going very well. Both booms broke at 14 lbs, and both broke in the same spot. The location of the break was on the tension pieces, about 3 mm before they entered the base. The base and compression were perfectly intact both times. The tension pieces were 3/32 square basswood, so I don’t think the tensile strength was exceeded. However, I noticed that the base “slid down” during testing. The hole in the base that accommodates the bolt was larger than it needed to be, and at the beginning of testing it was centered with the hole in the wall. But when the boom broke, the base had slid down so that the top of the hole in the base rested on the top of the bolt.

Could this be the reason for early failure? Is the solution to make the hole in the base smaller? (BTW, I tried to tighten the wing nut as much as possible.)

Any help would be greatly appreciated.

[iwonder]

I certainly wouldn't tighten the wingnut very much, it seems you would risk crushing the base.

As for the hole being to large, that's worth a look... If the base slides down the wall while testing, it would shorten the boom becuase the compression member wouldn't move, and change the angle the tension members need to be glued at, this bend would most likely happen at one end(the base end in this case) and could cause a premature failure... If its not to hard to build another, I'd just drill a smaller hole, snug the bolt up to the base, and try again.

[SLM]

...
Also, I just finished testing my second boomilever, and it has not been going very well. Both booms broke at 14 lbs, and both broke in the same spot. The location of the break was on the tension pieces, about 3 mm before they entered the base. The base and compression were perfectly intact both times. The tension pieces were 3/32 square basswood, so I don’t think the tensile strength was exceeded. However, I noticed that the base “slid down” during testing. The hole in the base that accommodates the bolt was larger than it needed to be, and at the beginning of testing it was centered with the hole in the wall. But when the boom broke, the base had slid down so that the top of the hole in the base rested on the top of the bolt...

Since I don’t know what your boom looks like and how exactly it fails, it is difficult to make a sound assessment. But, here are a few potential reasons for the failure.

1) If the boom is triangular but is missing the vertical member connecting the tension and compression members at the base, then the movement of the bolt in the hole could cause additional stress at the connection. If however, the vertical member is present, then the movement of the bolt most probably is not the cause of the failure. Regardless, you can always move the boom and the bolt downward as much as possible before tightening the nut. This way, the boom or the bolt has no more room to slide down once the structure is loaded.

2) The connection between the tension member and the base is inadequate, not enough glue surface. The tension force in the member has to be transferred to the base through the glue. If the glue surface is not sufficient, the concentration of stress increases at a transfer point causing failure.

3) The applied load causes the tension member to bend downward. As a result, additional stresses develop at the base of the member. This could cause failure in the glue (if the connection is weak) or in the member itself (if the member does not have adequate depth). Perhaps you can increase the depth of the tension member close to the base (as shown below) in order to increase its moment of inertia, there by increasing its bending strength at that critical location.

There could be other reasons for the failure, but without being able to examine the boom closely it is difficult to say.

[Faustina]

SLM, thanks for the input! Yes, my boom lacked any vertical members between the tension and compression. The design was essentially a box boom built to 2008 specifications. I don't think surface area was the problem; I carefully designed the base to have a ton of surface area for the tension member. But I will definitely take your advice about increasing the depth of the tension member close to the base. And I'll make sure that the base can't slide down, either by moving it down or having a smaller hole. I'll work on this over the next few days and post the results.

[jander14indoor]

Make sure you don't miss that you probably need a vertical member between the base and the compression members. Doesn't need to be large, but these structures tend to be marginally stable due to the edge of failure design needed to win. If the compression member slides more than the rest so it isn't perpendicular to the wall, bad things happen. Shouldn't take much to stabilize it as theoretically there is no stress there, assuming nothing distorts. But as distortion occurs, forces climb rapidly.

Also, DON'T increase section as shown. Those corners are stress risers and you'll fail there. Instead increase section smoothly. Think of a taper.

Jeff Anderson
Livonia, MI

[fishman100]

I seem to be having issues with the tension members bending, but that's because I used 1/8"square balsa (didn't have any bass on hand). Has anyone tried balsa for tension rather than bass? Even though it only held ~1.5kg before shearing, I'm starting to wonder if I can get away with using larger pieces of balsa instead of bass.

Thanks!

How do you build a testing surface for boomilevers?
Looking at old boomilevers, it seems that the testing surface is vertical as opposed to horizontal for bridges or towers.

As Faustina said, testing a boom requires nothing more than a vertical surface and a drill. You could copy nickfastswim and drill a hole in a plank of wood and clamp it to a post, etc. (I built a rig, but only because the school doesn't have a good place to test.)

Also, DON'T increase section as shown. Those corners are stress risers and you'll fail there. Instead increase section smoothly. Think of a taper.

I think I may be misunderstanding you, but wouldn't tapering a member subject it to more stress in the tapered spot?