Tower bracing

[Prashun Patel]

I am coaching my kids' Odyssey of the Mind team this year and they need to design a balsa tower to support as much weight as possible.

I'm walking a careful line here, and must not lead them into a particular design or assist them unduly.

That being said, I've been teaching them general construction and physics concepts.

i'm stuck on one:

Are towers stronger if the bracing is in the same plane as the vertical members? Placing them in the same plane seems like a more efficient transfer of forces, but it also relies more on butt joints or at least reinforced butt joints. Cutting precise butt joints that hold and clamp well is a challenge for 5th grade hands.

It is far easier and stronger to utilize lap joints. However, I cannot find any info that says one is better than the other.

Any ideas?

[Dave Richards]

I think considering the materials, lap joints would be a good option. the key thing is triangles. How is the weight to be put on the tower?

[Myk Rian]

There is a reason towers are built with triangles.

http://farm3.static.flickr.com/2199/2208678688_325f8f50b5.jpg

[Andy Booth]

Triangles, always triangles.

[Rick Potter]

How about showing them pic of an early oil rig made of wood.

[Lee Schierer]

It is far easier and stronger to utilize lap joints. However, I cannot find any info that says one is better than the other.

Lap joints are stronger because you are gluing face grain to face grain. Butt joints are end grain to face grain.

The triangles keep the legs from twisting or bowing that is why they are there. If the legs twist or bow they will break. As long as they stay perfectly straight they will hold a lot of weight.

[Tom Ewell]

Is gluing required?

When I was a kid we used to build 'full sized' towers for our campsites with bamboo poles that were lashed together, no joints just laid the poles together and wound rope around the intersections.

Bunches of us could climb all over the thing during construction and upon completion to hang our banners.

The Japanese built their scaffolds the same way (well don't know about now but back then they did)

They were virtually earthquake proof allowing for flex.

Don't recall our specific technique but similar, I'm sure, to what is pictured.

326291

[Patrick McCarthy]

Pashtun, my engineering-challenged mind has nothing to offer re the tower bracing . . . . But props to you for getting involved with your kid's education. They are what life is really all about, at least IMHO.

Best regards, patrick

[Bill ThompsonNM]

I am coaching my kids' Odyssey of the Mind team this year and they need to design a balsa tower to support as much weight as possible. I'm walking a careful line here, and must not lead them into a particular design or assist them unduly. That being said, I've been teaching them general construction and physics concepts. i'm stuck on one: Are towers stronger if the bracing is in the same plane as the vertical members? Placing them in the same plane seems like a more efficient transfer of forces, but it also relies more on butt joints or at least reinforced butt joints. Cutting precise butt joints that hold and clamp well is a challenge for 5th grade hands. It is far easier and stronger to utilize lap joints. However, I cannot find any info that says one is better than the other. Any ideas?

It's not that one joint is better than the other-- In a balsa tower if the lap joint is stronger than the load on the joint than the load is successfully transferred to the next piece.
So all in the same plain gives easier load transfer but will be weaker for any loads not in the line of the supports. I suspect most of the balsa towers collapse due to uneven loading.
I'd use lap joints but note that to get a given height, lap joints take more material, the overlap. In line might get an extra support.... At greater risk of collapsing due to cross loading. A great example of an engineering decision.

[Prashun Patel]

Thanks for the responses.

My dilemma is that a butt-jointed center support would provide good resistance to inward buckling force of the vertical members (since balsa has good resistance to compression force), but poor resistance to outward buckling, since the glue joint is weak.

A lap joint would have moderate resistance to inward and outward buckling because of the stronger glue joint. I suspect a good lapped glue joint would be stronger than the balsa's ability to resist the shear(?) force to the center support that's applied by the buckling of the vertical pieces it supports. Balsa's shear resistance is less than it's compressive resistance, so I've read.

Anyway, having built a couple prototypes with my team, I can say that it's easier to make a GOOD lap joint, than a good butt joint (which is hard to cut and clamp perfectly), so we'll probably go with that.

[Greg Hines, MD]

As a Scoutmaster of a Troop, we build towers all the time. Here is a listing of different designs: http://www.pioneeringprojects.org/projects/index.htm


(http://www.pioneeringprojects.org/projects/index.htm)

[Robert LaPlaca]

Prashun, who gets to select the balsa material used for the tower?

The reason I ask, is balsa varies greatly in density and strength..The prized A grade 'contest balsa' is very light and quite soft, the C and D grades are quite a bit heavier and stronger, they are probably as hard as white pine! When building traditional built up R/C airplane wings, one would try to find the harder denser balsa for the spars and leading or trailing edges, the contest stuff was prized for sheeting..

[Prashun Patel]

I can select the wood. That is, my team can, and I can teach them where/how to do it.
Know any reliable sources?
p

[Robert LaPlaca]

It been a really LONG time ago, but Balsa USA and Sig Manufacturing were the Heane Hardwoods and Groff and Groff of the balsa world..

[Pat Barry]

Yes - lap joints. Keep in mind that because of the taper of the tower front bottom to top, each lap joint will be a complex angle though. Additional diagonal braces are also going to be needed depending on the weight required to support and the loading (torsional, lateral). Since its not going to be all that high or out in the wind these diagonal brace don't need to be elaborate. Do triangles if you wish - I don't think its a requirement, just balance the bracing equally between left and right tilt. How tall do you expect this monster to be? What is the base size? Also, what size balsa wood can you use?

[Myk Rian]

How about showing them pic of an early oil rig made of wood.
Like this one? Looks to me glue isn't needed. Bolt/screw it together.

http://www.westkern-oilmuseum.org/images/new_woodderrick_partial6.jpg