Strength of Torsion Boxes

[Dave Lame]

Looks like I will be building 3-4 torsion boxes in near future. Would like some information about the design of torsion boxes.

1. What is the relationship, if any, between height of lattice and strength of the box?

2. Does the thickness of the skins matter to any significant degree? If so, is there an optimum thickness?

3. Should the lattice be composed of squares or rectangles? If rectangles whats the optimum ratio of length to width?

4. Is the thickness of the lattice significant? If so what is optimum?

I understand, I think, that continuous contact between the lattice and the skins is quite important. Is this correct?

Thanks in advance for you help with these questions.

Dave

[Mike Henderson]

Torsion boxes work the same way an I-beam works. Look up the theory of an I-beam and you'll get a lot of insight into torsion boxes. In essence, a torsion box consists of I-beams in both the x and y dimensions. See my comments below.

Mike


Looks like I will be building 3-4 torsion boxes in near future. Would like some information about the design of torsion boxes.

1. What is the relationship, if any, between height of lattice and strength of the box?

The height of the web has a direct relationship to the strength and stiffness of the box.

2. Does the thickness of the skins matter to any significant degree? If so, is there an optimum thickness?

The skins need to be thick enough that they don't deflect to any significant degree between the webs. And a lot of the strength of an I-beam is in the flanges. The top flange has to resist compression and the bottom flange has to resist tension in a loaded I-beam.

3. Should the lattice be composed of squares or rectangles? If rectangles whats the optimum ratio of length to width?

Squares would give you equal strength in both the x and y directions but you could use rectangles and still have a strong, workable box.

4. Is the thickness of the lattice significant? If so what is optimum?

The purpose of the web is to hold the two flanges apart. In general, thin material will work fine, as long as it doesn't collapse under load. I've used quarter inch material with success.

I understand, I think, that continuous contact between the lattice and the skins is quite important. Is this correct?

You need to glue the web to the flanges very well so that no slippage occurs between the web and the flanges (skins).

Thanks in advance for you help with these questions.

Dave

[Joe Chritz]

Search is your friend. ;)

Here is a little something from an old post that has more info then any one man can handle. They call them stressed plywood panels or something like that.

For your reading pleasure........

http://gp.com/BUILD/DocumentViewer.a...elementid=3815 (http://gp.com/BUILD/DocumentViewer.aspx?repository=BP&elementid=3815)

In general to answer your questions as I understand the answers.

The thickness doesn't matter as much as the height of the webs. The taller the webs the stronger the box. The stiffer the skins in compression and tension the stronger the box. Yes continual contact and glue and I believe the bigger the area between webs the weaker the box, however you can use boxes big enough for a brad nailer and have a very strong system.

I think I just said all the same stuff as Mike. I think I need a nap. :D

Joe

[Chris Friesen]

The height of the lattice has a huge impact on the stiffness. Doubling the height increases the stiffness by about 4x. Doubling the thickness of the lattice members only doubles stiffness.

At the same time you want to avoid buckling or rupturing the skins or the lattice members, this is where skin thickness, latice size, skin/lattice bonding, etc. all come into play.