Ongoing Contest(Scores)

[dholdgreve]

Northern Colorado Regionals
B-Div
1228.5- 12.21gr, carrying 15kg
276
249
227
The rest were below 200

C-Div
1083.7- 9.5gr carrying a little over 10kg
1075.3- 13.95 carrying 15kg
835.8
823.2- 12.39 carrying 10.2kg
557
519
287

The B-Div winner was a tube w/ single tension member.
C-Div winner was a 3-legged "tower in its side" design w/ 2 small tension members- 2nd thru 4th were tubes w/single tension member.
We were 2nd and 4th in C-. Our 4th place one had been tested to 12.5kg; we expected it to go full load; a little error in bolt hookup alignment way over-stressed the bolt connection.

Wow! Hard to imagine that much "space" between 1st and second... Glad to see someone is pushing the envelope with a very respectable score. At some point in time, I'm hoping to see a pic of one of these tube designs. I've been to several Boom competitions here in Ohio, and no one has used it yet.

[iwonder]

That's frustratingly close to first place congrats!

[Balsa Man]

I'll try to get some pictures w/in a couple weeks.

Yeah, that's the closest I've seen in a while- about 100gr of sand.
Good drama, good show for the spectators; nice setup for State; see if we can defend our Towers champoinship from last year.

[UQOnyx]

What do you mean be tube?

[Balsa Man]

What do you mean be tube?

What I mean by tube is...tube.
A single, tubular compression member. I've mentioned this in passing earlier.
We're running 11/16th" o.d. - 1/32nd" thick wall. The tube is constructed from 2 pieces of wood. The two other schools are running a little bigger diameter (about 3/4"). They're using a rolled tube approach, with 1/32nd sheet. There are a few differences in the distal end and wall end treatments. All running single tension member; again some differences in attachment to the wall.
This all started from early season discussions on how to keep things simple, and the technology has proliferated

[dholdgreve]

I still cannot imagine how one can use a single tension member and create shear at the distal end. In a double tension rod design, the tension rods can be brought down on each side of the loading block, aligned with the eye-bolt, virtually eliminating the shear forces of either cantilevering out beyond the connection, or setting inside the connection. With a single rod, it seems that the load would have to set out beyond, creating the cantilever effect, and thus the shear. If you got THAT figured out, my hat is off to you Colorado Dudes! Well done!

[Balsa Man]

I still cannot imagine how one can use a single tension member and create shear at the distal end. In a double tension rod design, the tension rods can be brought down on each side of the loading block, aligned with the eye-bolt, virtually eliminating the shear forces of either cantilevering out beyond the connection, or setting inside the connection. With a single rod, it seems that the load would have to set out beyond, creating the cantilever effect, and thus the shear. If you got THAT figured out, my hat is off to you Colorado Dudes! Well done!

You're quite correct that load center beyond the tension/compression connection point ....creates a cantelevered situation - best described, I believe, as a bending moment (semantics clarification - shear, as I understand, is different - what you have between a piece in tension (the t-member) and what it is glued to- the shear force is parallel to the tension in the t-member. I've talked in other posts to the importance of aligning tension/compression connection with load center; if the center of load is....outboard or inboard of the t-c connection, it does act to put a bend in the c-member.
So why is that not a problem with the tubes?
It comes from two factors working together. Primary one is distance - the moment arm length/distance at work. The eye-bolt hole is immediately beyond the end of the tension member - end of t-member is right against the bolt. The moment arm length is 1/2 the diameter of the bolt; 1/8th inch. That's pretty small. How much bending that moment arm is going to create in the compression tube is a function of the vertical (Y-plane) cross-sectional dimension of the beam/member/tube its acting on- "h sub Y". In 11/16th" or 3/4" diameter tubes, that "h" is....pretty darn large; "I sub Y", second moment of inertia in the Y-plane is very large- large I with a short moment arm L means essentially no deflection, hence no induction of bending in the tube. There are some small details that vary in the distal end treatment between the two design approaches, but both work because of short moment arm and large Y- cross section. Last little factor - which comes into play w/ bucket swing to/away from the wall (with the potential to put cantelevered load/bending moment in farther out); In both approaches, the end of the tube is only about a cm beyond the outside (distal side) of the bolt hole, so a cm/cm and a half of the load block hangs over the end.

The question now becomes, how far can this tube approach be pushed. We have 6 weeks to see. Our 12.5gr one failed early only because of a.....wall hookup error (video review now confirming what we saw in the failed joint). Design calcs and testing are saying grams can come off, and still hold full. We'll just have to see. Do I think this approach can push 2000..... not quite....probably. Better than 1500 (for C-)? I do think that's possible. But, as what happened at regionals reiterates, you never know till you hang em.....

[dholdgreve]

I must as dense as oak... It seems to me that at a minimum, on a single tension rod structure, the eye-bolt would need to sit out from the tension to compression connection 25 mm (2.5 Cm) just due to the size of the regulation loading block, unless you are building it with the tension rod perpendicular to the testing wall, and the compression member angled, and below it.

It has been my experience that without fail, the compression members always need to be heavier than the tension members... Designing the compression members to support from below seems counter-intuitive then, since these angled members would be longer than the lighter tensile perpendicular members...

Clearly I must have stepped onto the tracks at platform 9-3/4!

[Balsa Man]

Ah, I wish it were easy to post a sketch. No magic, just Muggle Geometry.
Draw it out from this, and I think you'll see:
The overall configuration is parallel (horizontal) compression tube, with the t-member angling in from the top. Tube diameter- vertical height- is 11/16ths
With C- geometry, 40.1cm back from the eye bolt center (at the wall), the t-member is a bit less than 16cm above the bottom side of the tube (it is attached on top of the bolt). It enters the top of the tube on a line that intersects the bottom of the tube immediately inboard of the bolt hole. What that means is the point it enters the top of the tube is more than 2.5cm inboard from the center of the eyebolt hole. (with B-dimensions/angle, its closer, but still doesn't run into the block) The block sits on top of the tube. The tube extends a little over a cm past the eye bolt hole- so the inboard half of the block sits on the tube up to the bolt hole, and about 1cm beyond the bolt hole sits on top of the tube, and the last cm+ of the block just hangs over the end.

[UQOnyx]

I hate to interrupt this lovely magic conversation but my NJ state finals are tomorrow. Thank you ALL for helping us, and we couldn't have done it without everyone on this forum!