Boomilever for 2013

[fanjiatian]

[quote="iwonder"]
A last suggestion... maybe a compression member made of two rectangular side supports(say, 3/16"x1/2") with the 1/2" vertical, and laminated with ultra high density balsa/bass, then put a truss in between the two side supports, it's yield a very high cross-sectional moment of inertia in both directions, because of the high density, but close in laminations, and the low density, far apart horizontal laminations. One thing to remember, the buckling strength goes up directly with the modulus of elasticity(basically, density(I might be wrong... pretty certain)), but it goes up with the cube of distance from the center(which is why the tube is nice, the wood close to the center does significantly less to improve strength than the outside wood), this, again, lends itself back to a low density, but large I-Beam, like what you describe.
/quote]

Interesting idea. Can you draw a sketch? I'm really bad at visualizing.

[iwonder]

As to the cross section limit, I agree with fishman100, that would be very shocking, also, if I'm not mistaken the limit was added in the second year of the event, it seems that first year builds are less strict and allow for teams to get used to an event before throwing some kind of kink in it(chinook, tower height bonus, etc)

Now, fishman100, there's a reason I choose to cut slots in the flanges, you're right that it'd give more surface area on the glue joint if you just put them on the sides, but, you also want to keep the tension member as straight as possible. To accomplish this, you generally angle in the tension members(because the distal end connection is wider than the base), if you kept the tension members on the side of the I-beam(which is perpendicular) you'd have a bend in the tension members right at the end of the flange, as they pull in to meet the base, this would(as we've seen in an earlier post) cause a premature failure in the member. The alternative, bending in the flanges, causes the gussets to be more complicated(different angles/sizes) and being able to do that would also mean that you over-engineered the I-Beam and it can hold more compression before buckling than it needs to(because you'd reduce the moment of inertia on one side, not on the other). By cutting notches in the flanges, you keep the I-Beam square, it can be lighter because it's not bent in, and the tension members aren't bent in as much, plus, the tension members still have glue down the thin edge of the flange, and, mainly, the distal end connection(which I assumed would be 1/4" thick, to hold the compressive force in the proper mode), 1/4" has proved to be more than enough in the past, so I didn't see a problem with it.

Lastly, I'll be able to draw out my original idea(with the lamination) soon... working on it now.

Alright... done with the sketches... and also

[SLM]

Using an I-beam as a compression member is an interesting idea. It would be useful to generate some performance data (with regard to buckling) for such a member.

As far as compression is concerned, the tubular shape is the most economical one; it resists buckling better than any other, comparable in size, shape.

An I-beam consists of 5 thin plates (the web, and 4 half flanges), as depicted below.


Generally, such thin plates are susceptible to buckling, especially if the member has wide flanges. Although the member may not buckle as a whole, the individual thin (flange) plates could buckle if the compressive stress in the member is relatively high. This kind of buckling is called local buckling and it looks something like this:



If this type of local buckling becomes an issue, the flanges can be braced by placing vertical plates (called stiffeners) between the top and bottom one, like this:

[himlynx]

A little off topic, but I have to ask since my last attempt was a disaster.

I plan on retrying to construct a balsa tube. I have the wood soaking now, and plan on wrapping it around a stainless steel handle and then fastening it with zip ties. How do you make sure that the tube is "straight"? Do you cut to size before (leaving room for error) or after it has taken the tube shape? I am assuming to do a very long lap joint to glue the wood together, roughly how much should the wood overlap?

The rules for the trial event in 2012 state "b. The entire Boomilever (except for the Attachment Base), including gussets, must be constructed of pieces
of wood no larger than ¼ inch high and ¼ inch wide in cross-section."

The wood used for a tube will have a cross section with one dimension greater than 1/4." Unless the rules change in 2013, I don't see how it will be legal.

[iwonder]

There's been some talk about that... It's very likely that the rules will change in order to allow the larger cross section. And SLM, what about x's down the length of an I-Beam instead of plates, would that be lighter and still effective?

[T-B]

If we want to try making balsa tubes for the compression member, how thin should we go to get it to roll? I presume 1/64th and it rolls into a long pencil-shape with the grain going the long way, right?

Isn't there a huge risk of failure related to using balsa in that thin? Even 1/32nd is quite thin, but I say that as one of the worst wood evaluating teams in all history. Basically, how thin does it have to be to roll and do you have to soak it first like you do when making an arch?

[SLM]

There's been some talk about that... It's very likely that the rules will change in order to allow the larger cross section. And SLM, what about x's down the length of an I-Beam instead of plates, would that be lighter and still effective?

I could carve out the inside of the stiffeners but would keep the outer box intact, as shown below.



But, I am not sure if placing x-bracings down the length of the beam would help with local buckling of the flanges.

[iwonder]

The second picture(hollowed out) is what I meant, only expanded so the center of the x is the web(it arose from concern about the web breaking where the loading block sits on it), so that when you look down the length of the tube you see x's. Also, what software are you using for the images?

[SLM]

The second picture(hollowed out) is what I meant, only expanded so the center of the x is the web(it arose from concern about the web breaking where the loading block sits on it), so that when you look down the length of the tube you see x's. Also, what software are you using for the images?

Yes, that makes sense.

I use SketchBook Pro on iPad for creating the hand drawings.

[iwonder]

If I had an iPad...

By the way, what's the reason behind splitting the I-Beam into 4 flanges instead of 2? and even with all this talk I tend to think the tubes will come out lighter...