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Curved Stair Stringer

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I looking for some tips for modeling a curved beam. I have an circular
custom stair. It is supported at the 1st floor (slab on ground) and 2nd
floor. It looks something like a slotted hole in plan. From the first
floor it does a 1/4 turn. Then there is a straight run, then another 1/4
turn back to the 2nd floor. I am supporting it with a tube at the second
floor and anchoring it to the slab at the first floor. The tube will give
me torsional resistance. The tube is about 16 ft. long and spans to 2
perpendicular beams, one at each end. The beams then span to columns
buried in the wall. Looking at this in plan view the 2nd floor beams
would form an 'H' shape. 
I want to get feedback for modeling the stringers in a computer frame
analysis. I guess my first impulse is to use short chord elements to
simulate a curved member. I started doing this with 15 degree angles for
the chords.  I would then uniformly load these members and have a fixed
joint at the tube column at the 2nd floor. I guess the connection to the
floor slab at the first floor would be pinned.  I would use a moment
frame from the vertical legs of the 'H' shape to give me lateral
stability. Any other suggestions for modeling this?
I am wondering what would be the best section for a stringer.  Since it
is a curved member it will have torsion.  Should I be using a closed
section like a tube?  A tube might be hard to bend on the radius.  Should
I make a closed section from welded curved plates?  Should I just use a
single thick plate for a stringer?
Thanks for your help.


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