There are times when rolling a stair stringer out of a rectangular tube is not possible. Usually that occurs when you are working with larger tubing and a tighter plan view radius for the stringer.
In these instances, the cross section of the tube will suffer too much deformation to be used easily in the fabrication of the staircase. In many instances, stuffing the tube with smaller tubes or beams can help to prevent or lessen the amount of deformation of the tube cross-section. However, there are times where even that might not stop this from happening.
What options are available in these situations? Of course there is an alternate solution. Make the stringer out of plates. In this instance, four plates are rolled individually. One for each wall of the desired tube stringer, with minor calculation differences to adjust for the slightly different radius and pitch. After each plate is rolled separately, they will have to be welded together to create a stringer tube.
The two vertical plates are rolled the easy way that creates the front and the back of the tube stringer. You need to remember to adjust the radii that the front and back plates will be rolled into by the size of the tube you trying to create. There also will be two horizontal plates rolled the hard way that creates the top and bottom of the tube stringer. Both of these plates would be rolled into the same radii so there is no need for adjustment. All of these operations need to be performed for the Inside Stringer and Outside Stringer. Remember to use the correct pitch for the appropriate plates depending on which stringer they are part of.
Although this solution does create more work for the bender/roller (e.g., a total of 8 pieces to be rolled for the Inside and Outside Stringers for each level of stairs) and for the fabricator who needs to weld all of the pieces together before installing the stringer into the staircase, it will solve the problem of not being able to roll stair stringers out of a bigger rectangular tube into a smaller radius.