Designs

[LKN]

SLM or Balsa Man,

BalsaMan, thanks for insight on your experiments. And great explanation in the pdf SLM. As of now I have been using built up sections with 1/16 or 3/32 squared and 1/16x1/4, but I plan on switching to two rectangular members for the legs. Your posts made me think about densities to use for the design.

For built up sections, would you want the denser section to be the "vertical" (against the natural failing mode) facing leg or the "horizontal" (directed along the natural failing mode) facing leg? I have been thinking this over, and here is what I have come up with.

1. If you treat the built up section as "one unit", and not two separate pieces of wood carrying a load, I would think that the horizontal pieces would need to be denser. Would this should help them against compression forces in the direction they would tend to go in?

2. If you look at the built up section as two difference pieces of wood, then naturally you would want denser vertical pieces that are already against the natural failing mode. The horizontal piece would serve just as more of a laminate and an extra gluing surface area for trusses, therefore it would not need be too dense.

3. OR both densities should be similar.

Your thoughts? I know trial and error and rebuilding is the best way to find out what "works", but if I had an idea where to start it would save a good bit of time.

[Balsa Man]

Sorry, LKN, I can't follow your "vertical" and "horizontal" description to get a picture to make any comment on....

A couple thoughts, things to consider that might go to what you're wondering, though:

In a leg, any (shape/density) "built up" (2 or more pieces glued/laminated together) is going to "act as one piece", in terms of its behavior as a thin column- its buckling strength under axial compression loading. It will act in accordance with Euler's Buckling Theorem. The two factors in his equation you're playing with are the modulus of elasticity (which is effectively a function of density), and the cross sectional moment of inertia (which is determined by the.....geometry/sizes of the pieces). You need to look at these factors in multiple planes, If the ....pieces used are the same & symmetrical (e.g., 4- 1/16ths put together as a 1/8th square), then the buckling strengths in various planes are symmetrical; strength against buckling toward any of the 4 faces, and the higher strength against buckling toward any of the diagonal corners. Put such a piece under a true axial load, and it will always fail (buckle) toward one of the faces.

Going to 'built up" configurations - as reflected nicely in SLM's attachment - things get very different in different planes. Looking at a "T" configuration; let's say a T with a wide top, and narrow upright, if both elements are the same density (modulus of elacticity), the strength in the plane of the T-top will be much higher than in the plane of the T-upright (same mod of elasticity, higher cross sectional moment of intetia; strength is the product of both). The difference in bracing intervals in the two sides in SLM's attachment correctly reflects, and is consistent with the difference in strengths in the two planes. Higher density in either piece will increase the buckling strength in that piece's plane.

So, while I understand what you have to analyze to understand the relative strengths of different cross-sectional layups and different densities and how the noted factors vary, the actual math of such analyses is way beyond what i know how to do. Serious nuts and bolts structural engineering stuff. If you can find yourself a good structural engineer, and they could talk you through it all, though....

Bottom line, though, and SLM and I seem to be on the same page on this, finding/using a single piece of the right wood, cross section, density, and bracing is probably your optimal path. If you have the time to carefully evaluate multi-element layups, you may come up with slightly better, or you might not.

[SLM]

SLM or Balsa Man,
For built up sections, would you want the denser section to be the "vertical" (against the natural failing mode) facing leg or the "horizontal" (directed along the natural failing mode) facing leg? I have been thinking this over, and here is what I have come up with.

1. If you treat the built up section as "one unit", and not two separate pieces of wood carrying a load, I would think that the horizontal pieces would need to be denser. Would this should help them against compression forces in the direction they would tend to go in?

2. If you look at the built up section as two difference pieces of wood, then naturally you would want denser vertical pieces that are already against the natural failing mode. The horizontal piece would serve just as more of a laminate and an extra gluing surface area for trusses, therefore it would not need be too dense.

3. OR both densities should be similar.

Your thoughts? I know trial and error and rebuilding is the best way to find out what "works", but if I had an idea where to start it would save a good bit of time.

I echo Balsa Man's comments on this.

For most cross-sectional shapes (i.e., rectangular, T, L, ...) a member could buckle in one of two ways. The Euler's buckling equation is an excellent aid for determining the direction of buckling. I've put together two examples illustrating the use of Euler's equation for deciding how to brace the legs and how to choose a section. To keep the examples simple, I've omitted a few details and have made a few simplifying assumptions. I hope you find them useful.

Example I: Square Base
Example II: Rectangular Base

[Balsa Man]

Good stuff, SLM (as usual); thanks!

[baker]

SLM, nice job...

[iTzDiamondFirexD]

For most cross-sectional shapes (i.e., rectangular, T, L, ...) a member could buckle in one of two ways. The Euler's buckling equation is an excellent aid for determining the direction of buckling. I've put together two examples illustrating the use of Euler's equation for deciding how to brace the legs and how to choose a section. To keep the examples simple, I've omitted a few details and have made a few simplifying assumptions. I hope you find them useful.

Example I: Square Base
Example II: Rectangular Base

I understand your thinking, but on the diagrams on the last page on both examples, does the left side of the Z Brace touch the middle of the T Member or on the long side of the T Member? Or does it not matter at all?

Btw I'm a beginner at tower building so I'm still getting used to types of bracings and wood and that sort of stuff

[SLM]

For most cross-sectional shapes (i.e., rectangular, T, L, ...) a member could buckle in one of two ways. The Euler's buckling equation is an excellent aid for determining the direction of buckling. I've put together two examples illustrating the use of Euler's equation for deciding how to brace the legs and how to choose a section. To keep the examples simple, I've omitted a few details and have made a few simplifying assumptions. I hope you find them useful.

Example I: Square Base
Example II: Rectangular Base

I understand your thinking, but on the diagrams on the last page on both examples, does the left side of the Z Brace touch the middle of the T Member or on the long side of the T Member? Or does it not matter at all?

Btw I'm a beginner at tower building so I'm still getting used to types of bracings and wood and that sort of stuff

It really does not matter. If you are going to use Z bracing, the bracing can be glued on the outer surface of the two adjacent legs (on the top side of T), or it can be glued to the inside of the T. If you are going for X bracing, then one of diagonals can be glued on the outside of the legs and the other one can be glued on the inside of the legs. The important factor is to have adequate glue surface between the bracing and the leg.

[flyingwatermelon]

Hi guys,

Quick question,

What is a good way to prevent Towers from being too uneven/unbalanced? Is it just a simple matter of sanding and then checking to see if it is level or designing the towers so that will be unlikely to topple?

I know this is unrelated but, is 7 grams chimney and 4 grams base for a 70cm tall tower in Division C a good efficiency or should I modify my design? Thanks!

[Balsa Man]

Hi guys,

Quick question,

What is a good way to prevent Towers from being too uneven/unbalanced? Is it just a simple matter of sanding and then checking to see if it is level or designing the towers so that will be unlikely to topple?

I know this is unrelated but, is 7 grams chimney and 4 grams base for a 70cm tall tower in Division C a good efficiency or should I modify my design? Thanks!

Take the time to read what's been posted- lots of good discussion. Jigs are how. Lots of info how, and why. Check the Gallery, too, for pictures.
7+4=11 grams is not bad at all for weight

[minniemouse]

For most cross-sectional shapes (i.e., rectangular, T, L, ...) a member could buckle in one of two ways. The Euler's buckling equation is an excellent aid for determining the direction of buckling. I've put together two examples illustrating the use of Euler's equation for deciding how to brace the legs and how to choose a section. To keep the examples simple, I've omitted a few details and have made a few simplifying assumptions. I hope you find them useful.

Example I: Square Base
Example II: Rectangular Base

I understand your thinking, but on the diagrams on the last page on both examples, does the left side of the Z Brace touch the middle of the T Member or on the long side of the T Member? Or does it not matter at all?

Btw I'm a beginner at tower building so I'm still getting used to types of bracings and wood and that sort of stuff

It really does not matter. If you are going to use Z bracing, the bracing can be glued on the outer surface of the two adjacent legs (on the top side of T), or it can be glued to the inside of the T. If you are going for X bracing, then one of diagonals can be glued on the outside of the legs and the other one can be glued on the inside of the legs. The important factor is to have adequate glue surface between the bracing and the leg.

Bracing one inside and one outside is good for X bracings? I haven't heard of this before. Also, when using the X bracings, do you overlap the two pieces in the middle?