Cross bracing in a Balsa Tower..

[gcskhor]

I'm doing a project on balsa tower strength, and I'm trying to get the strongest structure out of the least weight used in the structure.

 

I've thought of cross-bracings, but I am unsure if the two separate braces should be glued together at the intersection point.

 

Can anyone give any help on this?

 

Thanks in advance.

 

 

[Turtle]

I'm doing a project on balsa tower strength, and I'm trying to get the strongest structure out of the least weight used in the structure.

 

I've thought of cross-bracings, but I am unsure if the two separate braces should be glued together at the intersection point.

 

Can anyone give any help on this?

 

Thanks in advance.

 

 

 

Yes; glue the joint to make the whole structure rigid. Otherwise the legs are free to move independently at the upper ends and the structure is free to fold & fall. :)

[gcskhor]

That helped alot, thanks :smilingsun:

[belovelife]

are you making multiple skyscrapers and linking them together to stabalize and allow for higher building?

if so you should look up the japanese temple style for earthquake proofing

 

(just to throw that in, i know in large buildings they put the center pillar and only fasten it to the first 4 floors, thus allowing for the pillar to absorb the energy of the quake)

[gcskhor]

I haven't actually thought about that, but now that you mention it, it does sound like a good idea.

 

I couldn't really find much information, so could you please shed some light on the topic?

[Turtle]

I haven't actually thought about that [the japanese temple style for earthquake proofing], but now that you mention it, it does sound like a good idea.

 

I couldn't really find much information, so could you please shed some light on the topic?

 

While pagodas have a unique construction method as belovelife points out, it is not a lightweight matter of affairs as your balsa tower project description. Nonetheless, here's some info for you. Good luck on the project. :Glasses: :smilingsun:

Pagodas

...The weight of the upper story, pushing down on the inner ends of the taruki, would cause the outer ends to rise if there were no counterweight. The heavy tiled roof of the eaves performs precisely this function. In short, the taruki functions as a lever arm, while the top of the gawabashira serves as the fulcrum.

 

The story above bears down on the inner end of the lever, and the overhanging roof balances this load at the outer end.Or, to put it another way, the heavy eaves are in effect supported by the story above. When one reaches the uppermost level, of course, there is no story above to counterbalance the overhang. Here, however, the tall copper or iron spire, or finial, performs that function. The finial of the Horyuji pagoda, we are told, weighs a full three tons. ...

[gcskhor]

Thanks for all the help :)

 

But adding on to this, I just wanted to get some opinon on a design I've created.

 

The two separate braces on the horizontal beams are actually separated by an empty space approximately the width of a stick of balsa. Therefore, in order to make the structure rigid, I will have to joint the two braces together somehow.

 

My options:

1- (Due to the flexibility of balsa wood) Bend the two braces towards each other and glue

 

2- Stick a small piece of balsa to fill in the empty space of the intersection point of the two braces

 

3- Get a new design in which there is no gap between the braces.

 

 

However, there are problems even with those options:

1~ Since the wood is being constantly held in such a position, it may be weaker than traditional cross bracings.

 

2~ What I've realised is that short pieces of balsa wood tear along the grain easily when pulled apart. And since commercially produced balsa sticks are cut mostly parrallel to their grains, it is difficult to get a balsa stick with its grain perpendicular to the stick

 

3~ Well, it may take some time to think of something else XD

(correct me if I'm wrong about some of these)

 

 

So, if you could, please tell what is best, or give an additional idea.

 

And again, thanks for all help so far :D

[belovelife]

you could steam curve the balsa support to add the strength of arc theory to the braces

[Turtle]

Thanks for all the help :)

 

But adding on to this, I just wanted to get some opinon on a design I've created.

 

The two separate braces on the horizontal beams are actually separated by an empty space approximately the width of a stick of balsa. Therefore, in order to make the structure rigid, I will have to joint the two braces together somehow.

...

So, if you could, please tell what is best, or give an additional idea.

 

And again, thanks for all help so far :(

 

I'm having trouble visualizing the situation from your description. :) Your previous drawing has no horizontal members. If the little gap is between the crossing of the two diagonals you asked about gluing, then just glue in a little filler block (make it only as big as needed to fill the gap.) Perhaps you would make another drawing to help clarify if I haven't called this correctly?

 

In terms of general construction process, if there is a gap that isn't part of the design, then either the design is faulty or something has been cut too small. Measure twice, cut once. :D

[modest]

The two separate braces on the horizontal beams are actually separated by an empty space approximately the width of a stick of balsa.

 

Speaking of horizontal beams—you need some. Above and below the X shape in your diagram should be a beam running horizontal and parallel to the ground. This would be true regardless if the tower is expecting more of a vertical load (as with the towers on a cantilever bridge) or if it is expecting significant lateral loads and shear forces (like a crane tower).

 

Perhaps your design already has them and you've omitted it in the diagram.

 

As far as how to secure all the connections with strength and rigidity (and no gaps), I'd recommned something like the top of this model bridge:

 

http://www.garrettsbridges.com/images/pratt-truss-bridge.jpg

 

It sounds like you might be joining your beams to the inside and outside of the tower legs rather than planar with them. It would be better to implement something closer to the image above where all the truss elements meet the legs head on and the connections are strengthened with a bit of paper.

 

If your X pieces (the red and blue pieces in your diagram) are very much thicker than those in the image above then it may be necessary to cut one of them in half (rather than bending around the other). That would be fine, and I think preferable.

 

Also, I don't know how far along you are, but have you considered making the base larger than the top? If this tower is going to be very tall and have much lateral load then that would be best. You might take a look at the jacket sections of offshore oil platforms for inspiration along those lines.

 

Oil, Gas and Chemicals - Exploration and production facilities. Jacket structures

 

specialized in offshore and onshore oil & gas field development - RichTech

My options:...

 

2- Stick a small piece of balsa to fill in the empty space of the intersection point of the two braces..

 

2~ What I've realised is that short pieces of balsa wood tear along the grain easily when pulled apart. And since commercially produced balsa sticks are cut mostly parrallel to their grains, it is difficult to get a balsa stick with its grain perpendicular to the stick

 

A point of historical interest more than a criticism of your design—putting a 'spacer' beam between two load bearing members and having the beam tear was the exact reason the Hyatt Regency hotel catwalk collapsed awhile back killing some dozens of people. :)

 

But, the connection you're talking about in your design is nowhere near as structurally important as the connection holding up a catwalk. :)

 

Good luck,

 

~modest

[belovelife]

multiple desighns

 

Umeda Sky building, at Umeda, Osaka.

 

amd i know that one of the ultimate desighns was 4 pillar going up almost to the top floor

all the pillars were fastened to the first 4 floors of i think an 80 story building

although i'm sure its more i just can't remember it was a few years ago

but the main concept was the pillars

the absorbed the energy from the bas

so the structure didn't shake

it was the pillars

and they can always be refurbished after the point

[gcskhor]

In terms of general construction process, if there is a gap that isn't part of the design, then either the design is faulty or something has been cut too small. Measure twice, cut once. :evil:

 

Well actually the gap is part of the design. The vertical beams on the tower are actually thick, in a way, so it would be inappropriate to place the braces right next to each other for them to be touching. Sorry for being brief. Down here's a bit more detail to it, what it would look like more or less from the side.

 

 

Speaking of horizontal beams—you need some. Above and below the X shape in your diagram should be a beam running horizontal and parallel to the ground. This would be true regardless if the tower is expecting more of a vertical load (as with the towers on a cantilever bridge) or if it is expecting significant lateral loads and shear forces (like a crane tower).

 

Perhaps your design already has them and you've omitted it in the diagram.

 

Yes, my design has them.

 

Also, I don't know how far along you are, but have you considered making the base larger than the top? If this tower is going to be very tall and have much lateral load then that would be best.

 

Actually my tower will not be tall at all. I'm making the tower a minimum of 7 inches and maximum of 9, so a base larger than the top will be unneccessary. Also, from previous designs, I've noticed that when force is applied directly downwards to balsa towers not entirely perpendicular to the ground, the structure will collapse inwards, as compared to one which is straight and will compress downwards until it finally collapses to the side or twists.

 

However, similar to the jacket structures, there are more than just a single 'X' in the design (if my earlier explainations were too brief).

 

A point of historical interest more than a criticism of your design—putting a 'spacer' beam between two load bearing members and having the beam tear was the exact reason the Hyatt Regency hotel catwalk collapsed awhile back killing some dozens of people.

In that case, could I assume that my Option 1 is the better choice of the lot, in that it would not tear as early on as Option 2?

[Turtle]

Well actually the gap is part of the design. The vertical beams on the tower are actually thick, in a way, so it would be inappropriate to place the braces right next to each other for them to be touching. Sorry for being brief. Down here's a bit more detail to it, what it would look like more or less from the side.

 

...

In that case, could I assume that my Option 1 is the better choice of the lot, in that it would not tear as early on as Option 2?

 

Roger clarification. I think the spacer is the better choice than the bending, for the same reasons you mention above about towers with bowed members. Saturate the spacer with glue. If you're using white glue, then you can thin it a little with water & get it to soak into the wood spacer deeper (more deeply? :naughty:) This will mean a longer drying time as well.

 

Last thing is just to say the little paper joint-overlays are referred to as "gussets". :evil: >> gussets - encyclopedia article about gussets..

[gcskhor]

That's a really good idea :evil:

I've never thought about it that way..

 

Well, I'll just wait around for some other opinions to emerge (if they do).

 

Thanks for the opinon Turtle

[modest]

If you’re set on your design, then I’d also say the spacer is probably best, but without seeing your tower it seems impossible to say for sure.

 

The vertical beams on the tower are actually thick, in a way, so it would be inappropriate to place the braces right next to each other for them to be touching.

 

If any given column has beams in tension on one side with beams in compression on the other side under a certain load then the column will shear or twist. Consider a plan view of one of your tower legs:

 

 

There is a lateral load on the top of the tower pointing right. The red and blue rods are the red and blue lines in your diagram. Under this load, blue will be in tension and red in compression. The column (or tower leg) will then have a shear force on it.

 

If the pattern repeats itself up and down the column then the whole leg will twist counterclockwise. If another column has the same tendency under a given load then the whole structure could twist.

 

So, I don't necessarily agree that it would be inappropriate for the braces to be planar or close together. I think they should be. But, again, without seeing your tower it's impossible to say for sure.

 

~modest

[Turtle]

...So, I don't necessarily agree that it would be inappropriate for the braces to be planar or close together. I think they should be. But, again, without seeing your tower it's impossible to say for sure.

 

~modest

 

A full scale tower meant to support a downward bearing load, such as a water tower, uses some specialized woodworking joints if timber-framed. Not only do pieces close-fit, they interlock. As you say, without a better idea of the actual sizes of the material it is hard to judge what is the best method.

 

While the balsa seems unlikely to handle mortoise/tenon joints, not to mention some very specialized teeny-tiny tools to make them :), I think the terminal joints of the cross-braces as you have drawn them in relation to the post might be "let-in" using half-lap joints. With the cross-braces let in on either side of the posts, they can be brought to touch where they cross. Alternately, if the cross braces are attached on the same side of the post, then the crossing can be half-lapped to allow them to lay in the same plane. No more than an Exacto knife and a straightedge required. (Oh; and bandaids. :) :doh: ;))

 

The joint is mechanically stronger and gives a larger glue surface area than a simple flat butting/crossing. Just the carpenter in me.

 

This page has some illustrations of half-lap & crossing half-lap joints. >> Google Image Result for http://www.rabbitwoodworks.com/images/Joinery/corner-half-lap-joint.jpg

[modest]

I think the terminal joints of the cross-braces as you have drawn them in relation to the post might be "let-in" using half-lap joints. With the cross-braces let in on either side of the posts, they can be brought to touch where they cross. Alternately, if the cross braces are attached on the same side of the post, then the crossing can be half-lapped to allow them to lay in the same plane.

Absolutely ;)

 

I think either option would be better.

 

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

 

~modest