Cross bracing in a Balsa Tower..

[gcskhor]

Also, with a half lap or T joint, gussets could then be used. Bringing them in would really seem to solve multiple possible issues.

 

Gussets are a good idea, and I will be introducing them into the structure.

 

Half laps and T joints are good ideas as well, however I've actually already gotten a good design for my structure. I designed my pillars in a way in which the contact area with the glue is much higher than that of half lap or T joints. I have tested those out, but are unsuitable for thw wood I use (sticks of a cross-section 1/8" x1/8"). I can't reveal the design right now, but maybe in the close future :).

[Turtle]

Gussets are a good idea, and I will be introducing them into the structure.

 

Half laps and T joints are good ideas as well, however I've actually already gotten a good design for my structure. I designed my pillars in a way in which the contact area with the glue is much higher than that of half lap or T joints. I have tested those out, but are unsuitable for thw wood I use (sticks of a cross-section 1/8" x1/8"). I can't reveal the design right now, but maybe in the close future :eek2:.

 

Per my contract, I have to start with some pedantry on the term "T joint". Quite simply, it says nothing about the "joint" (the internal arrangement of contacting surfaces) and only would refer to the right angle & an end of one board meeting on the edge of another; it is not a qualified technical woodworking term. On the other hand, "half-lap" is a joint where two members are each reduced in thickness by half and overlapped at their common reduced sections.

 

I'm dubious of your claim of more glue surface area than a half-lap joint in such small pieces. In full scale contruction this would be got at by variations of mortoise & tenon joints. :eek2:

Anyway, no pressure to reveal your method before the big project is due as it seems this is a common school project with a competitive flavor. :) This is a destructive load test, yes/no? :) :)

[Turtle]

Further observations from a woodworker: :)

 

Per Modest's drawing in post #15, note that all the compressive & tensional loads between the members are carried by the glue. When a simple half-lap joint is used to join the cross-brace to the post (post & brace both let-in), then much of the compression load is transferred to & through the cross-brace where the insets abutt, rather than just the glue. Note the tension load is still carried by just the glue in this circumstance.

 

If the half-lap is made as a dovetailed-half-lap, then the tensional load is also partly transferred to the cross brace through the dovetail. The dovetail also gives some additional glue surface, and the more glue surface, the stronger the joint. :)

[modest]

Per my contract, I have to start with some pedantry on the term "T joint". Quite simply, it says nothing about the "joint" (the internal arrangement of contacting surfaces) and only would refer to the right angle & an end of one board meeting on the edge of another; it is not a qualified technical woodworking term.

 

butt joint :)

 

Further observations from a woodworker: :)

 

Per Modest's drawing in post #15, note that all the compressive & tensional loads between the members are carried by the glue. When a simple half-lap joint is used to join the cross-brace to the post (post & brace both let-in), then much of the compression load is transferred to & through the cross-brace where the insets abutt, rather than just the glue. Note the tension load is still carried by just the glue in this circumstance.

 

If the half-lap is made as a dovetailed-half-lap, then the tensional load is also partly transferred to the cross brace through the dovetail. The dovetail also gives some additional glue surface, and the more glue surface, the stronger the joint.

 

As wood glue is so much stronger than balsa, I wouldn't worry much about the joint. As I recall building towers in 'structural systems' at K State, the ones that actually broke (we didn't purposefully test them to failure (well, the instructor did if he knew it was going to fail:hihi:)) were the people who used hot glue or super glue because they ran out of time and people who had key points in the structure not connected to 3 orthogonal points.

 

If gcskhor uses wood glue and allows it to set and especially if he/she uses gussets then far more important will be the overall design. Don't you agree T?

 

By the bye, keep us updated gcskhor.

 

~modest

[Turtle]

butt joint :)

 

Gesundheit. :hyper:

 

 

 

As wood glue is so much stronger than balsa, I wouldn't worry much about the joint.

With few exceptions of wood species, the glued joint is usually stronger than the wood itself. (Presumes a "good" glue job with proper spread, pressure, & dry time and a tight mechanical fit).

 

As I recall building towers in 'structural systems' at K State, the ones that actually broke (we didn't purposefully test them to failure (well, the instructor did if he knew it was going to fail:hihi:)) were the people who used hot glue or super glue because they ran out of time and people who had key points in the structure not connected to 3 orthogonal points.

 

If gcskhor uses wood glue and allows it to set and especially if he/she uses gussets then far more important will be the overall design. Don't you agree T?

 

By the bye, keep us updated gcskhor.

 

~modest

 

Well, I don't know. I have never actually done this project and as the conversation has passed, I am convincing myself that the joints I have suggested would make a stronger tower than the regular glue jobs everyone is doing, with or without gussets. I'll go out on a limb & guess that no one is making the joints I suggest. Do ya think? :eek2: :naughty:

 

Butt if and until someone knowledgeable & skilled enough to make a lot of teensy-weensy high-precision joints as I have described in1/8"x1/8" balsa stock, then our hunches are just hunches.

So I would say central to my overall design approach would be close-fitted glued half-lap joints and then only by experiment & comparison to the others would we have an indication of the importance/efficacy of such joinery in this project.

 

Again, my designing is predicated on a destructive test where whoever builds a tower that holds the greatest load before breaking wins. If the point is just to support a static load of a few pounds, then my design is hardly worth the effort.

 

That's all I got.

[modest]

Butt if and until someone knowledgeable & skilled enough to make a lot of teensy-weensy high-precision joints as I have described in1/8"x1/8" balsa stock, then our hunches are just hunches.

 

While I've made many architectural models with lap joints, few of them were with balsa and none tested for strength. As far as cutting an 1/8th or 1/16th inch notch in balsa—it's not hard. Two cuts and a few lateral scrapes with the blade until the right depth reached will do. Many of the more serious model makers in my studio class had a lot of files rasps and rifflers for that sort of thing, but for the simplest of notches, an exacto would suffice.

 

~modest

[Turtle]

While I've made many architectural models with lap joints, few of them were with balsa and none tested for strength. As far as cutting an 1/8th or 1/16th inch notch in balsa—it's not hard. Two cuts and a few lateral scrapes with the blade until the right depth reached will do. Many of the more serious model makers in my studio class had a lot of files rasps and rifflers for that sort of thing, but for the simplest of notches, an exacto would suffice.

 

~modest

 

Roger the serious skills; cynic that I am, I presume young 'Joe/Jane' student is lacking them. :) I found a YouTube vid of the kind of thing I have seen in this project line.

 

My joinery experience is in furniture, cabinetry, and framing & the balsa at this scale is just a gedanken for me. In fairness I should note that full-scale timber framing is not glued. :naughty:

 

PS gcskhor, what grade level is this?

 

YouTube - science olympiad balsa tower explosion http://www.youtube.com/watch?v=i14jTAays6o

[GAHD]

how much time do you have? what are your material constraints (max length of pieces before cuts)?

 

assuming you want to build it to minimal material spec:

 

-use 3 walls, not 4:

-have 3 'boxes' with a single cross-brace running corner to corner in each box, as this will force rigidity.

-attach boxes to each other bhy their sides for your 3 faced tower(you can build the other 2 walls off the first if you have enough clamps...)

-if materials are "too short" to span total distances I'd suggest gluing multiple sticks together in 2 layers so that any "joints" are halfway through it's neighboring ply, and then filling in the ends with half-peices where necessary.

-should you have lots of time, gluing sticks into i-beam, T-beam, or hollow triangular cross-sections will greatly increase load constraints with significantly less material weight.

 

 

Depending on your math background you might find the rest of this info useful...

euler beam equation

 

 

Hope it helps

[gcskhor]

Yes, this is a destructive load test.

 

Well actually, before I viewed that website you provided, I assumed that my idea for the joints were original :)... But now I now realise that it's kind of like some of those joints... not sure which of them though (I'm no carpenter, haha).

 

The joint I created is made of several pieces of wood glued and interlocked together at a certain point

[gcskhor]

how much time do you have? what are your material constraints (max length of pieces before cuts)?

 

assuming you want to build it to minimal material spec:

 

-use 3 walls, not 4:

-have 3 'boxes' with a single cross-brace running corner to corner in each box, as this will force rigidity.

-attach boxes to each other bhy their sides for your 3 faced tower(you can build the other 2 walls off the first if you have enough clamps...)

-if materials are "too short" to span total distances I'd suggest gluing multiple sticks together in 2 layers so that any "joints" are halfway through it's neighboring ply, and then filling in the ends with half-peices where necessary.

-should you have lots of time, gluing sticks into i-beam, T-beam, or hollow triangular cross-sections will greatly increase load constraints with significantly less material weight.

 

Hope it helps

 

Would you mind elaborating on these points? I'm not terribly familliar with these things.. It will help alot.

 

Thanks :)

[modest]

Roger the serious skills

And you, yours. I've seen some of your grade A carpentry at the local woodshop :hyper:

 

I found a YouTube vid of the kind of thing I have seen in this project line.

 

Yup, good vid. My Jr. High did the same with bridges—oodles of fun. Perhaps we should start a hypography model bridge / tower contest :)

 

In fairness I should note that full-scale timber framing is not glued.

 

Or.. not only glued. Construction adhesive seems to go on about everything anymore, especially if the framing is steel or laminated lumber as they can't be nailed.

 

Gcskhor, I think we're all curious to see this thing. Any chance of posting a pic? :naughty:

 

~modest

[Turtle]

... Construction adhesive seems to go on about everything anymore, especially if the framing is steel or laminated lumber as they can't be nailed.

 

Gcskhor, I think we're all curious to see this thing. Any chance of posting a pic? :hyper:

 

~modest

 

Roger on the construction adhesive. I gotta wonder how it's holding up in the long haul. :)

 

Anyway, on the balsa towers, I tried to catch the frame right at failure in the vid, but no luck. I think an important part of these classes, if it's not already, would be to analyze each destroyed tower to see where/why it failed. If the glue joints aren't where the failures are, then fancy joinery would be wasted.

 

Also love to see some vids or pics Gcskhor, and still curious to know what grade level u are. :winter_brr: :)

 

PS I'm with Gahd on the triangular tower; three legs, not four. :naughty:

[gcskhor]

Well... I've already crushed my first few stuctures, and right now I'm out of wood. I'm from Singapore, and ordering balsa wood from here is VERY expensive. The cost of the shipment here from the US costs quite a bit more than the wood itself. So right now I'm collecting my idea's well before I actually start constructing again, instead of wasting tons of wood. And sorry, I got no pics right now :)

 

And also.. from previously seeing three-legged structures, I would think that an equaliteral triangle shape would be best? But unfortunately those are hard to do, especially the joints. Would you have any ideas on how to create that without splitting a (1/8" x 1/8") stick of wood?

[Turtle]

Well... I've already crushed my first few stuctures, and right now I'm out of wood. I'm from Singapore, and ordering balsa wood from here is VERY expensive. The cost of the shipment here from the US costs quite a bit more than the wood itself. So right now I'm collecting my idea's well before I actually start constructing again, instead of wasting tons of wood. And sorry, I got no pics right now :(

 

And also.. from previously seeing three-legged structures, I would think that an equaliteral triangle shape would be best? But unfortunately those are hard to do, especially the joints. Would you have any ideas on how to create that without splitting a (1/8" x 1/8") stick of wood?

 

On the triangle tower, equilateral footprint, yes. I think you can make the joints by just trimming the brace's end's a bit and gluing to legs? Go with the paper gussets over the joints too I think. I couldn't find a good image of a triangular tower, but they use that form a lot for tall radio/TV antenna masts. You might search for 'antenna mast designs' & see what you can find.

 

In the mean time, I found a very nice page with a variety of bracing schemes for wood water towers. :hyper: :( >> Water Towers - Custom Designed Plans for Towers

[gcskhor]

Well.. right now I'm lacking an idea for the thee-legged tower. And frankly, I don't think I have the skill to be doing that, so I suppose I'll have to stick to a four-legged tower.

[GAHD]

right a bit of a delay on my part but maybe this diagram will help?

 

here I show cross sections of 2 "beam" types, a diagram of "overlapping" or "plying" shaort peices of wood together to make a longer one, and a view of the simplest possible stable 3-legged tower. each box requires ONE cross brace to insure rigidity.

 

Take all your broken peices, split them down the middle to be thinner peices, then glue them back together in that overlapping style on a flat surface, they should be about as strong as new.

 

If you then take those this plys and glue them into the "t" or "H" shape beams they will be pritty much as strong as a SOLID peice as wide and tall as the outside of the t/H shape. a TRIANGULAR and HOLLOW beam is FAR stronger than either H or T beam shape AND has better torsion resistance.

 

build it, you will BLOW AWAY the compitition. (trust me I have built high-rise apartment complexes for a living for several years)

[gcskhor]

Well.. I'm back after, um, 3 weeks.

 

Right now my plan is to have a 4-legged (three is too hard for me :confused: ) tower with cross bracings that lie on the same plane, connected together with cross halved joints instead of bending the balsa around (finally countered it :evil: )

 

Now I'm taking on a new challenge.. I'm not sure what they're called, but for now I'll call them internal bracings for now. They're basically braces inside the tower, stretching from one bottom corner inside the structure, to the top corner diagonally opposite to it. And for obvious reasons, I can only have two of them. For a better idea I've got pictures (yay).

 

But of course, I'm wondering if this is actually useful for the structure. Can anybody help with this?

 

Thanks.