Calculation of the Torsion and Warping Constants for a double-channel beam

[As-Lag]

Hello

I want to confirm the Warping and Torsion constants for a double channel (Please see jpg's). Sorry about the scale of the images.

The formulae I have used I obtained from the "P385-Torsion.indb - Sci_p385" publication.

For the Warping Constant, I used AutoCAD to calculate the "Moments of Inertia" and I_z which I used in the Warping formula.

For the Torsion, should I substitute b_e (208 mm) or 2*b (180 mm)for b in the formula and multiply t_w by 2. The cavity is 100 mm.

Or am I wrong on all counts?

Regards

 

[Agent666]

How are they joined? If they are not integrally joined then its the sum of two channels.

I went through a similar exercise in this blog post regarding helping out another ENG-tips poster with some cold formed channels and came to the conclusion that if its just two separate channels bolted together, then the values are added together. If they are welded together then use of FEM analysis of the cross section will be the only way. Most formulae are simplifications at best, refer to the post linked above for an example of this demonstrating how bad they can be. I don't believe that you can use a formula for doubly symmetric I sections if you have the gap.