https://www.homebuildingandrepairs.com/codes/index.html Click on this link to learn more about stair safety, building codes and design. I will provide my interpretation of a building code that might help you design your next set of winder stairs or curved stairs. Keep in mind to always check with your local building and safety departments if you have them to verify whether or not you can use the information in this video.
Page 60 in 2012 International Building Code Book
Reference number - R311.7.4