Step 1. Create a stair model. If you dont have 1001 Bit tools, you can model it by
native modelling procedure.
Step 2-3. Create line parallel to red axis, The stair should be also parallel to the red axis and must be converted into component. Create also an arc prior to your spiral stair dimension
Step 4. Activate your Shape Bender Tools. Shape bender Tools is available for download at SKETCHUCATION. http://sketchucation.com/ Just register.
Step 5. Select your stair component
Step 6. Select the line
Step 7. Select the arc
Step 8. The stair component will follow the arc. It will take first the inner side. Press up arrow on your keyboard to take the other side of the arc
Step 9. The hit enter
Step 10. Tweak and clean if necessary...Done.