I did a paper in grad school which on the theory of torsion on structural shapes such as I and T beams and tubes... Developing warping constant values is not an easy thing to do it involves a lot of calculus and assumptions.
However, if your 'flanges' (for lack of a better term) are sufficiently thin, say bf/tf > 10... I don't believe warping needs to be considered (don't quote me on that though- you'll need to check literature to confirm that). You can essentially treat it the same as a thin closed tube (ie calculate a Jeff value: sum(bt^3/3)).
If your 'flanges' are thick... then your in trouble (just kidding)...
In either case you will want to search through some papers on this subject... i'm sure this has been researched... some key words you will want to use: warping function, sectorial area and restrained warping.
Strictly speaking for slender 'flanges' saying Cw = 0 implies warping exists and the section provides no resistance, therefore angle of twist and warping normal stress tend towards infinity. I think its more appropriate to say warping is neglected.
I believe warping can be neglected because all 'legs' (as long as they are sufficiently thin) of the cruciform pass through the shear center... where as with I beams (flanges) or angles (legs), which are offset from the shear center, can develop significant warping.
I developed expressions for torsion of thin-open sections, I'd be happy to share it with anyone who is interested.
Look at TORSIONAL SECTION PROPERTIES OF STEEL SHAPES. It's easy to derive the CW of a cruciform from that of a T shape, should be (b3t3+d3w3)/144 (d being section depth).
Note however what the paper says on the small value of CW for angles and Tees, confirming what was said above. mh819, I'm interested in your expressions. You can use an email from one of the sites below to communicate.
