Torsion in RC T beams

Torsion calculations are based on the area enclosed within the transverse reinforcement.

If I am relying on the concrete capacity for torsion, is it mandatory that I consider area of concrete only if enclosed by stirrups.

Yes!

OP said:

If I am relying on the concrete capacity for torsion, is it mandatory that I consider area of concrete only if enclosed by stirrups.

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Below a certain demand threshold, you can use unreinforced concrete.

One needs to be cognizant of proportions with things like this. If the proportions that you've sketched are accurate, what you've got here is a stem wall loaded in out of plane flexure and cantilevered from a rectangular grade beam in torsion. As such, I would not include the stem in your torsion resistance model. The two pour sequencing is irrelevant in my opinion.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

As the others above have noted, once you're reached Tcr, then the concrete resistance to torsion is reduced to zero causing all torsion to be resisted by the reinforcing. Therefore only the reinforcing cage area can be used to determine torsion resistance.