Curved Stair Stringers from Built-Up Plywood

[Whodapookie]

I have a curved stair that is free standing (not connected to a wall) and was wondering how others designed the stringer. I know how to do this if I were using steel, but the requirements are to be wood. The proposal is to use 10 strips of 1/4" plywood (glued & nailed) to make the beam. I have reviewed the APA documents Plywood Design Specs, Supplements 2 & 5, etc. but nothing has values for 1/4" plywood nor how to account for curved beams. I know the recommended radius is 5' for 1/4" Plywood, but I could not find a reduction value for radii less than recommended (R= 28" for inside stringer).

Any help would be appreciated...

 

[SperlingPE]

Glulam?

 

[Whodapookie]

Glulam may work structurally, but not in the time schedule for fabrication of the stairs. Built up plywood beams have been used for years according to people in the stair industry, but how are they designed? I'm starting to think they are using the overall stiffness of the completed stair to work as a unit. I contacted the tech support at APA and they did not have any mystical equations to take into account the torsion component.

 

[skier1578]

We recently were asked to do this (after it had been built of course) and after some thought and trial - we passed. The resulting torque was just out hand for what our client was proposing.

 

[OkieDokie]

Whodapookie, As a carpenter I was always curious about a stair case like you are asking about I have had people tell me they have built this types of stair case (out of wood) for 30 years and yet when press to tell me where they are, so I can go see them, they can not remember the exact location, the building has been torn down, ect..ect... I have had one guy tell me he built one at a office building and went to look at it and it was a steel stair case wrapped in wood. If he was the carpenter that wrapped it, did a great job.

A stair case like you are asking about as always fascinated me. I have seen picture of them but never on how they were constructed. I can see how this could work if using a glulam but not laminating 1/4" plywood together for stringers. If you figure this out please make a post. It would be interesting to see. Take lots of picture too. GOOD LUCK.

 

[jike]

You might want to check out the videos on this website to see how they put together the factory built curved stairs.

http://www.arcways.com/video.htm