## Torsion Constant - J

## Torsion Constant - J

(OP)

Hey everyone...quick question:

I have a cross section of a solid that is, well, quite nasty looking, but I need to calculate the Torsion Constant J (not polar moment of inertia). Is there a software package that would allow me to do this? I have been using SolidWorks for my modeling and other structural properties, but I can't seem to make it spit out the Torsion Constant.

Any help would be greatly appreciated...thanks all.

Jon

I have a cross section of a solid that is, well, quite nasty looking, but I need to calculate the Torsion Constant J (not polar moment of inertia). Is there a software package that would allow me to do this? I have been using SolidWorks for my modeling and other structural properties, but I can't seem to make it spit out the Torsion Constant.

Any help would be greatly appreciated...thanks all.

Jon

## RE: Torsion Constant - J

## RE: Torsion Constant - J

corus

## RE: Torsion Constant - J

corus

## RE: Torsion Constant - J

If anyone has any input on this or, again, knows where I could find something that calculates the Torsional Constant J, that would be most helpful. I should have mentioned that I also have access to CosmosWorks...

Jon

## RE: Torsion Constant - J

Basically, I have a cross section of an aircraft with the skin and the stringers attached to the skin. I have a solid model of this, but I need to find the Torsional Constant at a give cross section...cute, eh? This is why I'm trying to find software, etc. that would assist me with this...

If I was able to find the Torsional J for all the stringers and skin seperately, is it a simply summation to get the torsional J constant?

Jon

## RE: Torsion Constant - J

For the "torsion_properties" sheet you should refer to Roark (since this is the reference used). Roark uses a particular form for the torsional calcs which include this constant, so refer to this. The convention can be somewhat mixed for a "torsion constant". It's down to a description of the parameter J I suppose wherever it's used. Usually the torsion constant and the polar moment of inertia are the same thing - J - but it does differ. The torsional modulus is r/J (cf. section modulus in bending problems Z = y/I).

The "Section_by_nodes" sheet looks like it is indeed the "polar moment of inertia", but you'd need to go throught the calcs on the sheet to verify that it is truly the polar moment and not just the sum of the Ixx/Iyy second moment of areas.

Cheers,

-- drej --

## RE: Torsion Constant - J

For torsional resistance constant R and Polar Moment of Inertia J are approximately the same for closed sections with them identical for circular sections.

The general equation for closed tubular sections is:

R = 4(A)^2/(sum of (mean perimeter/associated thickness))

Where A is the area enclosed within the mean perimeter.

HTH

VOD

## RE: Torsion Constant - J

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

## RE: Torsion Constant - J

## RE: Torsion Constant - J

That general equation is for thin tube type structures that should apply to an aircraft cross-section. If the cross-section is not tube in nature, such as a thick cross-section relative to the thickness of the elements and with reentrant corners, I doubt this formula will be accurate.

Regards

VOD

## RE: Torsion Constant - J

At the very least designing appropriate end conditions is an eye opener, if you haven't done it before.

The advantage of building these simple models is that you can also investigate joint efficiencies very quickly - which given that a badly detailed joint can be only 50% efficient is well worth doing.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

## RE: Torsion Constant - J

Aren't these the same things? IE, sum of R^2*dA = sum of (x^2 + y^2)*dA = sum of x^2*dA + sum of y^2*dA

## RE: Torsion Constant - J

## RE: Torsion Constant - J

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

## RE: Torsion Constant - J

see Roark, 7th ed, pg381

then there is a table of torsional constants depending on the shape and constraint conditions. remember this is for prizamatic and open sections. it would appear that there isn't a general calculation approach for determining this, other than for open, thin walled sections (being 1/3 bt^3).

closed sections, and particularly sections with stiffeners and multi-cells, need a different approach, because the shear flow is different in the various segments of the cross-section; refer bruhn.

## RE: Torsion Constant - J

I notice in "Formulas For Stress and Strain", they use J for the polar moment of inertia and K for the torsional constant, which would avoid some of the confusion above.

## RE: Torsion Constant - J