Warping Constant formula for channels

[TXTank]

I am trying to find a good formula for Warping Constant for C channel sections. This formula will be used in conjunction with AISC 360-10 section F2. In another thread (thread723-206373) I saw a reference to xcalcs.com where a formula is given along with sample results. However, I have yet to be able to get that formula to work in Excel and match the example output. Here is a link to the xcalcs page in question:
xcalcs:channels
To see formulas on that page, click on "Show Formulae" under Options on the lower left of the page. Click on "Show Results" to see their sample output.

In my Excel spreadsheet, the equation is as follows:
CW=POWER((h-t),2)*POWER((b-a/2),3)*t*(1/6 - 1/(8+4*(h-t)*a/(b-a/2)/t))
That yields a value of 52.39977 when using the English units with the following inputs:
h=7.87, t=0.449, b=2.95, a=0.244

If I tweak the formula and add an extra set of parentheses around the (b-a/2)/t term:
CW=POWER((h-t),2)*POWER((b-a/2),3)*t*(1/6 - 1/(8+4*(h-t)*a/((b-a/2)/t)))
I get 32.08804. Still not as close to their value of 36.9 as I'd like.

Can anyone get their formula to work?

I've also found formulas for channel section warping constants elsewhere:

http://www.cisc-icca.ca/files/technical/techdocs/updates/torsionprop.pdf

http://www.steel-insdag.org/TeachingMaterial/Chapter17.pdf

Naturally, they yield different results. The ultimate question is, which formula should I use? I'll also be needing Cw for I-Beams, but that one seems less controversial.

 

[BAretired]

According to my source, "Cold-formed Steel Structures" by W. Yu the expression for a cold-formed channel agrees with your second reference when t_f = t_w if the dimension 'b' is taken from the middle of the web to the end of the flange.

Comparing this with your first reference:

t_f = 12.7 t_w = 7.2 b = 70.4 h = 292.3

Cw = 29.1e9 which agrees with your first reference for C310x31 (C12x20.7 Imperial)

BA

 

[prex]

TXTank, sorry, there was a factor missing in the displayed formula: it is now corrected (and thank you for pointing out that).
Your first reference (CISC) is exactly equivalent, and the second one (Insdag) is too, if you (correctly) take b as b-t_w/2
Note also that when a tabulated value is provided for C_W, this one is shown in xcalcs, so you won't find an exact correspondence with the formula.

prex
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[TXTank]

Yes, that works much better with the new formula.
Thanks!