Designs

[KBS]

and one final note, i can certainly attest to the effectiveness of x bracing in the base and z bracing in the chimney.

Does anyone have an understanding/theories on why the chimney was OK with Z bracing? Towers usually fail as much in torsion as they do buckling. Z-bracing must be weaker in torsion than X-bracing...

[lllazar]

Well really, the fact of the matter is, your tower is NOT going to break due to the bracing in the chimney. It will break due to the bracing in the base, or if you selected bad wood, it will just break due to collapse of the columns (which is pretty rare, i've had 0 of those so far after testing over 15 towers).

In each of the slow mo vids of our tower tests, the bracing of the base has come apart, allowing for the tower to then completely tear apart at all braced parts, chimney included. Basically the point is, the chimney bracing doesn't need to be strong. It just needs to be strong enough, but more importantly light.

This happens because the force in the base legs is simply more than that in the chimney, due to the angle at which they are at from vertical. This makes them want to slide apart, and so the bracing comes apart.

Now this is from the point of view of a person who's just speaking from testing experience and a little physics knowledge, i suggest you consider this but wait and see if someone more experienced posts.

[Jo_squirrel]

I don't know if this has been discussed or mentioned earlier but for the chimney part of the tower, are straight legs better than slightly angled ones?

[diamondplatinum]

so, i've heard and read that triangular bases and towers generally are stronger and lighter, if built correctly. i had a triangular tower that was 19 grams, but it broke holding >=7 kilos because of a joint failure and/or misplaced braces. what would be the best way to brace the base of my tower? (i saw the v bracing methods, and i had already been using that for the 'chimney'.)

[SLM]

and one final note, i can certainly attest to the effectiveness of x bracing in the base and z bracing in the chimney.

Does anyone have an understanding/theories on why the chimney was OK with Z bracing? Towers usually fail as much in torsion as they do buckling. Z-bracing must be weaker in torsion than X-bracing...

Here is my take on this subject.

A bracing system has three uses here. It
(1) helps keeping the tower upright,
(2) controls the nodal displacements of the structure, hence improving the overall stability of the tower, and
(3) prevents main structural members from buckling.

Depending on the section and material properties, a structural member can undergo various types of buckling. In the case of towers, I think, you need to watch for:
1. Overall (flexural) buckling. This is the type that generally happens when the member is tall and slim. The Euler’s buckling equation predicts the critical load for overall buckling.
2. Local Buckling. This is similar to overall buckling but it happens only in a part of the member. (see video link below).
3. Torsional Buckling. This type of buckling generally occurs in compression members that have an open section with large width-to-thickness ratio. Angles (L), Tee, I, and Channels(C) are considered open sections since they have unsupported edges. In contrast, closed sections, like a tubular one, generally are not susceptible to this kind of buckling.
4. Flexural-Torsional Buckling: This is a combination of (1) and (3) above.
I’ve prepared three short video clips that show overall, local and torsional buckling in balsa wood members. You can view them at: http://nexote.net/nexote/Snotes.

Assuming that the tower’s chimney consists of either three or four main (vertical) compression members each designed so as not to buckle, I think it is not likely that the entire chimney would undergo torsional buckling.

Although X bracing offers more stability than Z bracing, it is probably the case that the Z bracing used is strong enough to partially compensate for the added stiffness an X bracing system provides.

To see how Z and X bracings could affect the behavior of a structure, I’ve prepared three short video clips. The first clip shows the base of a tower without any Z or X bracings. The second clip shows the same base with a Z bracing. The third clip shows the base with an X bracing. They can be viewed at: http://nexote.net/nexote/Snotes.

[jpmacguy]

i don't know if this has been posted before, but i was wondering if the strength of a x-brace decreases if one member of the brace is bent over the other. I'm using 1/16th wood for the braces and i'm gluing at the endpoints and the piont where the members cross. I'll make a diagram when i can.

[SLM]

i don't know if this has been posted before, but i was wondering if the strength of a x-brace decreases if one member of the brace is bent over the other. I'm using 1/16th wood for the braces and i'm gluing at the endpoints and the piont where the members cross. I'll make a diagram when i can.

This, in part, depends on length of the two members; the longer they are the less significant the overlap would be.

Also, gluing the two members at their point of intersection, if it was not considered in the force analysis of the tower, could cause a force redistribution in some of the structural members. This may not be that significant in the grand scheme of things, but you need to be aware of it.

[matthew8817]

I'm new to towers but last year my partner and i blew away the cometion in bridges at state. Does the tower have to touch every side of the square hole in the test base or does it sinmply not hav to fall through? Reading through this forum i just had an idea to build a shorter and lighter version of my bridge and then glue the "top" portion of my tower on top of that. I understand that would drasticly increase the chances of it rocking along the y- axis but i found a way to fix that last year that i know will help again. But would it be legal to have the base of my tower to only touch two sides of the base?

[Littleboy]

I'm new to towers but last year my partner and i blew away the cometion in bridges at state. Does the tower have to touch every side of the square hole in the test base or does it sinmply not hav to fall through? Reading through this forum i just had an idea to build a shorter and lighter version of my bridge and then glue the "top" portion of my tower on top of that. I understand that would drasticly increase the chances of it rocking along the y- axis but i found a way to fix that last year that i know will help again. But would it be legal to have the base of my tower to only touch two sides of the base?

The rules say that it must span the 20 cm. I personally perfer square bases over triangle or rectangular bases (for a veriety of reasons mentioned in this forum.

[lllazar]

What density balsa are you guys using for the cross bracing?

Also, how can you effectively use 1/32 inch thickness balsa for cross bracing? And as i was building yesterday, i had an idea, i'll use the base as an example.

For the very bottom of the base, to brace two sides together, i placed a strip of 1/32 by 1/16 on one side and another strip on the other side. This might make sense and then again it might not, but to consider the following: You have two sticks of balsa lying next to each other. You brace them together with a strip of balsa. Then, on the opposite sides of the same two sticks, you brace it again. I guess it would kind of look like a sandwich.