Great Article on Stairs by a P.E.

[jar546]

I hope you enjoy this article. Here is a snippet and the link to the full article.

It’s amazing that something as simple and as common as wood-framed stair stringers do not yet have specific prescriptive code construction provisions. Despite the commonality of wood-framed stair stringers, they still suffer from structural performance issues which result in scenarios that range from cracked drywall to severe injuries. While the International Code Council (ICC) recently moved to address lateral residential deck failures by bolstering the prescriptive requirements of Section R502.2.2 in the 2009 International Residential Code (IRC), there still remains a void in an area where a few simple changes could make a dramatic difference.

STRUCTURE magazine | Wood-framed Stair Stringer Design and Construction

www.structuremag.org

PDF Here: https://www.structuremag.org/wp-content/uploads/C-StrucDesign-Fournier-March131.pdf

 

[Paul Sweet]

God article. Thanks for posting it.

Stringers are often nailed to walls at the side(s) of stairs, which greatly reduces the span. The typical notched 2 x 12 should be good for a free-standing stair a half-story high. I agree that the connection at the top is rarely done correctly.

 

[Beniah Naylor]

Really annoys me that there is no span table for stringers. The only way I can question an over spanned set of stairs with any objective criteria is with the DCA6, which is probably too restrictive, but it's the best I have been able to find.

 

[wwhitney]

Really annoys me that there is no span table for stringers.

The article discusses the "horizontal plane" analysis method, which you could apply to transform the arrangement into a near-equivalent horizontal joist arrangement, and then apply the floor joist span tables. As the article mentions in passing:

The joist depth is the uncut throat depth of the stair stingers.
The joist spacing is the stair width divided by the number of stringers.
The joist span is the horizontal (projected) distance between the top and bottom supports.

The article does that to an example stair and come up with 1.5" x 5" joists spanning 12' 3" at 12" o.c. for cut stringers of the example geometry made from #1/#2 SPF 2x12s.

Now 2021 IRC Table R502.3.1(2), for 40 psf live and 10 psf dead, gives an allowable span of 10' 3" for SPF #2 2x6s @ 12" o.c. So you can conclude that the stair stringers are over span, as the horizontal span is longer than 10' 3", but the member depth is less than a 2x6.

Cheers, Wayne

 

[bill1952]

Thanks for posting.

I will simply note if reducing injuries is the goal, he reports 10,000 instances from NEISS. That's compared to well over a million injuries on stairs. You could reduce injuries by way more than 10,000 from structural failure by decreasing max rise, increasing min tread, and requiring true graspable handrails.

 

[steveray]

Thanks for posting.

I will simply note if reducing injuries is the goal, he reports 10,000 instances from NEISS. That's compared to well over a million injuries on stairs. You could reduce injuries by way more than 10,000 from structural failure by decreasing max rise, increasing min tread, and requiring true graspable handrails.

Or outlawing stairs....

 

[Mr. Inspector]

I noticed the NC addition Appendix M to the 2009 IRC on wood decks has specifications on how to make stringers.

https://www.thebuildingcodeforum.com/forum/attachments/pages-from-2012_nc_residential_code-pdf.10442/

 

[Sifu]

On a few occasions I have used the methods outlined by wwhitney, but it is stretchy. I have also found myself referring to the AWC and NC code for guidance, but that is stretchy too. I don't pretend to understand the physics outlined by the article, nor can I pretend to take the time to try, but I agree 100% that it is an overlooked area, and in one of the worst places. Stairs are playgrounds for kids, and endure higher loads from jumping, running and taking them two at a time. Always been a a bit of a confounding deal for me. Mostly I see prefab stairs now. I wonder what design criteria is used on those.

 

[Mr. Inspector]

According to the code all stairways should have sealed drawings since it's not in the code book. But usually I don't even see stairway construction even on sealed drawings for houses.

 

[Inspector Gadget]

I had a situation like this, and what I did - essentially - was run it through floor/joist calculations and state that either the designer (a) alter the drawings or (b) provide calculations (ie: hire an engineer) to verify it met the intent of Code.
Guy altered the plan to something that didn't give me the heebie-jeebies.
I *assume* the US codes have intent statements?

 

[Beniah Naylor]

The article discusses the "horizontal plane" analysis method, which you could apply to transform the arrangement into a near-equivalent horizontal joist arrangement, and then apply the floor joist span tables. As the article mentions in passing:

The joist depth is the uncut throat depth of the stair stingers.
The joist spacing is the stair width divided by the number of stringers.
The joist span is the horizontal (projected) distance between the top and bottom supports.

The article does that to an example stair and come up with 1.5" x 5" joists spanning 12' 3" at 12" o.c. for cut stringers of the example geometry made from #1/#2 SPF 2x12s.

Now 2021 IRC Table R502.3.1(2), for 40 psf live and 10 psf dead, gives an allowable span of 10' 3" for SPF #2 2x6s @ 12" o.c. So you can conclude that the stair stringers are over span, as the horizontal span is longer than 10' 3", but the member depth is less than a 2x6.

Cheers, Wayne

What scares me about this method is that it is far less restrictive than the DCA6, and I don't know how to do the calculations to prove that using the floor joist span table is reasonable. I have also looked at maybe using the rafter tables, but it's kind of the same thing.

 

[jar546]

According to the code all stairways should have sealed drawings since it's not in the code book. But usually I don't even see stairway construction even on sealed drawings for houses.

I've noticed for the past 2 decades that most stairs are factory-built and delivered onsite so shop drawings are required.

 

[Beniah Naylor]

I've noticed for the past 2 decades that most stairs are factory-built and delivered onsite so shop drawings are required.

That would be nice...

We see that on commercial, at least. Usually. Fortunately I have never had to fail a pre-fab set of stairs that weren't right. At least not yet...

 

[wwhitney]

and I don't know how to do the calculations to prove that using the floor joist span table is reasonable. I have also looked at maybe using the rafter tables, but it's kind of the same thing.

The rafter span table uses the same "horizontal plane" approximation, so that idea is already part of the prescriptive code. While the method discussed for stair stringers isn't exact, I don't think it's too far off, and it's the only option available in the prescriptive code.

Cheers, Wayne

 

[Genduct]

Seems like we are focused on just looking at the amount of lumber left over after we cut the stringer and forget that the Thread and Riser make the 2 or better 3 stringers a real Structure or Frame. It is not just the single equivalent of a roof rafter.

Notching the bottom and hangers for the top should also "Lock it In" and add to the strength

I have always been surprised there is not more"bounce" as you might think possible.
Certainly, the 5/4 rabbited side stringer, prefab stairs are reinforced with the thread/ riser assembly

 

[rktect 1]

I know I was thinking about the stairs when I had two guys, probably each weighing 250 plus pounds, bring my 800 pound gun safe through my garage and down my carpenter built stairs on a two wheel dolly. I was seriously relieved when they got to the bottom the correct way along the slope of the nosing and not vertically from first floor to basement. There was no way I was going to be on those steps at the same time and in fact was a bit leery to even go on them after they were already at the bottom.