Curved plywood stair stringer

[shacked]

My client is removing the existing bearing walls below the sides of a curved staircase. See the attached pdf plan view with dimensions.
Although the loads are relatively light, DL = 80plf & LL = 120plf I opted to use curved steel stringers, but the Architect came back with, "I would rather use curved plywood stringers." Not sure why, probably trying to save the owner money.

I am a little apprehensive about having some yahoos constructing this in the field. I can just see these guys squirting Elmers glue on the plywood then trying to clamp it together...with something like duct tape.

Has anyone had any experience with specifying a curved plywood beam? Or any input would greatly be appreciated.

 

[BAretired]

A little apprehensive? I'd avoid it like the plague.

BA

 

[EdStainless]

If it were my building I would take either material as long as it was shop fabricated.
I wouldn't trust field built plywood stringers.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[hokie66]

Agree with Ed. A factory built wood staircase is the way to go.

 

[Canuck65]

I have done curved beams using plywood before. Both for curved stair stringers and also bending in the other direction for curved roof rafters to make eyebrow dormers. With some conservative design decisions, I can confirm that the end result was rock solid. We did some informal testing as I was initially tentative with the results and the testing confirmed they were capable of supporting significantly more than the design loads.

I would start by reading the various supplements related to Plywood Design Specifications produced by the APA. I started my design for simplicity sake assuming the stringer was straight, then added from there to account for the curve. I was very conservative, many thin plies bonded together. You need to stagger the joints, account for the joints in your analysis, etc... The plies were adhered together with structural wood adhesive (there are standards for the type of adhesives for wood)and lots of structural screw fasteners. Only the screws were accounted for in the design, not the glue. Everything was shop fabricated by a very good stair builder then brought to site and erected.

All the above being said, my preference would be for curved steel stringers clad in wood. Simpler and less time consuming to design and fabricate.

 

[jike]

Curved plywood stair stringers, curved handrails, etc. are used all the time!

I would specify a "shop fabricated" stairs and then list pre-qualified fabricators. I suggest that you contact a company like "Arcways" or others whose specific business has been to build and install curved stairs, curved handrails, "Juliet" balconies, etc. for many, many years. Do NOT use someone who is inexperienced in this type of construction. Request a list previous projects, photos and visit one of their nearby projects. Check CSI to see if they have a specific spec. for this specialty.

PS Indicate required radius on your drawing.

 

[shacked]

Ha, like the plague is exactly how I feel about it, but after hearing what Canuk & Jike said this may be something to consider, IF "and thats a big IF" they are fabricated in a reputable shop.

I think that the Architects reason for using plywood is to cut cost for the homeowner, but the way I see it steel stringers may be comparably priced considering the labor intensive process of fabricating the plywood stringers.

Thanks guys, that helps.

 

[hokie66]

A bit further, rereading your posts. You don't want to just replace the bearing walls with stringers. You should replace the whole thing. Trying to marry in the existing treads and risers with new stringers will be a nightmare.

 

[jike]

I may have mis-read your post. If this is indeed an "existing" condition, then you would need to replace the entire stairs and anchor the ends properly. Chances are very likely that the "existing" stairs (previously supported by bearing walls) are not built as needed to be free standing.

The information that I previously provided is still applicable.

If your Architect wants you to design the structural details of the stairs, you can suggest that he use a "Performance" Spec where the fabricator can provide everything needed including the structural calculations. Sometimes it is better to engage an expert to do the specific design rather then to take on something that you are unsure of.

I hope this helps!

 

[shacked]

No, I agree totally about replacing the whole staircase. The problem is that the Architect has not yet given me any information about the stair design. No run or rise, no guardrail/handrail details...nothing.
Thanks for your help

 

[SteelPE]

Sorry I didn't read the whole post, but I wouldn't hesitate at all for a curved wood stair as long as it was shop fabricated. Recently I have been working with a large mill working client who does items like you described all the time (I designed their steel building).

I love walking through their building as seeing the different stairs ready for shipment.