Curved Stair Stringers

[SkiisAndBikes]

I have a client that would like a curved set of stairs between the main and upper floor of their custom home. The layout of the house does not allow for any intermediate support of the stringers. The concept has the stairs curving a total of 90 degrees with open treads. The stair contractor is proposing multi-laminations of plywood be used for the stringers. I have not used plywood in edgewise bending before.

Using a design live load of 40 psf and assuming edgewise bending of the plywood, I have calculated the minimum number of laminations of 3/8" plywood based on bending, shear and deflection of an idealized stringer without the curve.

I believe the loading on the curved staircase will produce a significant amount of torque top and bottom of the stringers. Does anyone have any information/suggestions on how to calculate how much resistance a multi laminated plywood stringer would have to torque??? None of my wood reference manuals seems to contain any information specific to this type of application.

The connection top and bottom is another question .... I am thinking of bolting the stringers to 3/8" thick steel knife edge plates inserted into the end laminations that are then welded to a backer plate that is lagged to the stair opening. Any suggestions???

Thank you!
Skiis and Bikes

 

[Lion06]

There are no torsion provisions in the NDS. I believe you are supposed to avoid torsion in wood, but if you MUST, you can probably go back to mechanics and try to ensure that the combined stresses (bending plus normal stresses from warping) don't exceed the allowable bending. Do the same for shear.
Again, I'm sure others will comment, but I think you are supposed to avoid torsion in wood.
Can you talk him into a small piece of tube steel? It would be much smaller and would look nice (my idea of nice in this instance means very small compared to what most people might expect, this would usually get people thinking right away).
Don't forget to check vibrations.

 

[jike]

They build these all the time without engineering and are very stout. Who is requesting that it be engineered?

Call a manufacturer and he might be able to give you some guidance.

http://www.arcways.com/

 

[OCI]

I have personaly built a set of stairs/stringers exactly like you are describing. They worked great. It was a number of years ago but I think I used 12 lamimations of 1/4" plywood for each stringer. The treads were made from 1 1/8" cdx and were attached to the inside of the stringers leaving the full depth(12") of stringer for strength.

 

[SkiisAndBikes]

StructuralEIT - Thank you for your suggestions. I will definitely dust off the text book and satisfy myself re the combined stresses. And yes, in general I try to avoid torsion even with steel.

Jike - Thank you for your comments. The request came from the stair contractor, they have never built unsupported curved stringers and requested engineering. This is the first request I have had for calculations related to a staircase, therefore the questions.

OCI - Glad to hear you have similar experience. At this point I have 10" depth below the treads and am in the ballpark with 10 laminations of 3/8". Do you have any information on how your stringers were connected top and bottom?

SkiisAndBikes

 

[jike]

Sounds like the contractor is trying to take on something he has never done before. As I stated before, there are speciality contractors that do unsupported curved stairs all the time.

 

[csd72]

The big brown hardcover timber design handbook has a section on torsion.

 

[w16x26]

The geometry of a curved stair induces additional downward load on the outside stringer and some upward load on the inside stringer. The connections between the stringers and the treads/risers become critical. There are design examples for curved steel beams (highway bridge) in the Structural Engineering Handbook by Gaylord, Gaylord and Stallmeyer. The design proceedure would be the same for a curved stair.