Stair Stringer Design and Configuration

[BN4537]

This is a stair I ran across in the field, two stringers, supported with a couple 2x4s screwed to the sides of them, the top is bearing on a piece of 7/16" OSB sheathing that is hanging down from a structural beam. There was no engineer involved on this project. I have contacted the contractor with my recommendations and I have not received any pushback.

However, this scenario illustrates a problem I have, which is that I currently have no objective way to determine two things:

1) How many stringers are needed for a particular stair?

2) How far can those stringers span?

Given:
IRC 301.1 that says that you have to provide a complete load path that is strong enough to support the loads involved.
Table R301.5 that says that stairs have to be able to support a 40 psf live load and a 300 lb concentrated load.
Other than that, there is no guidance in the IRC that I can find.
AWC's DCA 6 document recommends that a 3' wide stair have three cut stringers no more than 18" apart and spanning no more than 6' horizontally.
I am an inspector, not a structural engineer.

My impression is that this configuration of stair stringers as-built will not meet the loads from table R301.5, based on my experience and judgement. However, that is a (somewhat) subjective call on my part.

Does anyone have a code section I am missing, or maybe a document from an organization like the AWC or the SBC, or information about calculating stair stringer span, or rules of thumb that you have, etc. that they would be willing to share?

Objective decisions are great, but if I have to make a call I like to have as much information as possible, so anything you have will be appreciated.

Thanks!

 

[steveray]

Eating popcorn emoji.......

 

[SCBO1]

Drinking beer emoji....

7/16-inches of OSB, not enough meat for the stringers. Have'nt seen a stairway build that way usually the framer wants the sidewall support.

 

[BN4537]

It's definitely odd. If they had used the walls, I might have been more comfortable.

I played with some rafter tables thinking that rafters may be similar to stair stringers - but I didn't get any numbers that really made sense for stair stringer spans. The 300 lb point load requirement is important, and the rafter tables can't really address that consideration.

 

[SCBO1]

I would agree that the stairway is missing the center stringer and that the photo indicates it does not meet the 18-inch spacing that would require the third stringer for compliance. IMO, I would get the third stringer installed and would be ok with the offset outside stringers, due to not being in the walkline. Do you think the 2x4's are temps? Will a closet under the stairs be built later?

 

[Sifu]

I wish I had answers to this too. It has always confounded me. Codes gives zero guidance prescriptively. In the absence of code they could be considered joists however joists don't include a 300lb concentrated load, and for notched stringers we would be left with joist spans for a 2x6 at best (usually 2x4) for the minimum throat and the minimum end bearing requirements of ch. 5, and none of it considers the stresses at the incised cut of the notch or at the bottom of the stringer when bearing on the landing if it's not full bearing. The lack of prescriptive guidance is frustrating. Maybe I'll get some popcorn and wait out some better answers too.

 

[BN4537]

I would agree that the stairway is missing the center stringer and that the photo indicates it does not meet the 18-inch spacing that would require the third stringer for compliance. IMO, I would get the third stringer installed and would be ok with the offset outside stringers, due to not being in the walkline. Do you think the 2x4's are temps? Will a closet under the stairs be built later?

I think they originally only put in two stringers with no supports, then became concerned because it was way too bouncy, and installed the supports. Once they installed the supports, I don't think they intended them to be temporary.

Either that, or one of the other contractors threw them in because he didn't want it to collapse on them while they were working.

Who knows...

 

[Mr. Inspector]

I agree their should be something in the code.
I was told once in a code class that stringers are the same as joists and the treads are the same as lumber floor sheeting.
Table R503.1 can let 5/4 lumber span up to 16" for a tread.
Usually a 2x12 stringer is about 5.5" at the narrowest spot. Table 502.3.1(2) #2 SPF 2x6 joists spaced 16" can span up to 9'-4" for a 40 psf live load. Measure the stringer span horizontally, not along the stringer's length.
Also joists (stringers) must be supported by beams, ledgers or the concrete pad at the bottom as per code. Not like the OSB in the photo. I always want a metal bracket if not directly supported.
The 2x4's supporting the stringers in the photo should be directly under the stringers. But i think they should be 4x4's to hold the load.

R311.5 Construction.
R311.5.1 Attachment. Exterior landings, decks, balconies,
stairs and similar facilities shall be positively
anchored to the primary structure to resist both vertical
and lateral forces or shall be designed to be self-supporting.
Attachment shall not be accomplished by use of toenails
or nails subject to withdrawal.

 

[tbz]

The 2 single stringers might be perfectly fine in size, as standard stairs are only built with 2 stringers on the sides and then we have mono stringers and so on and so forth, however the securing of the flight structurally to the base and upper-level floor or header is absolutely missing.

Based on the framing it looks like the stair builder expected there to be a beam or header in the doorway for them to secure into, and which is not there.

The simple and proper way to handle it, is simply say there is something not right or safe with it and you need to have an engineer provide documentation that it complies for the structure of the flight and the mountings and I need a sealed set of drawings.

Don't start providing solutions for bad situations.

Thats my personal view.

 

[TheCommish]

Are they temporary stairs, I have done similar, while the custom stair shop fabs the fancy oak ones

 

[e hilton]

The almost-double joists above ... hung with L-brackets rather than U-hangers … acceptable? And the picture is a little fuzzy, but it looks like all the nails for the hangers are overdriven.

 

[BN4537]

So, do the people writing the IRC assume that if you are building stairs, you just need an engineer to be involved, since they provide no prescriptive guidance on the structural aspects of the stairs? Not trying to be snarky, just wondering what the rationale is. I have heard that this issue has come up before at code hearings, but I have never been to one.

There are paragraphs on the specific diameter of the handrail for the stairs, but nothing for the structural except for performance criteria?

Even if we wanted to make people use an engineer (which would be a lot easier for us), the State of Kansas has laws that are designed to protect a contractor's right to build single-family dwellings without an engineer's guidance, and those laws would make such a blanket mandate tricky to set up.

 

[Sifu]

As Rick says, it has been said to treat them as joists, but I think that has holes in it and is really just an expedient answer because of the lack of prescriptive guidance. Using the 2x6 span chart ignores the incised cut which causes the member to be weaker than an uncut member under uniform load on a flat plane, the required concentrated load and the heavier dynamic load exerted by the user. Then consider that field cut stringers are almost always over cut to make the full cut. Few framers will stop short and hand cut the remainder of the notch. This results in less than the 2x6.

We get engineered design from applicants by choice and habit, rather than by rule. If not though, in the absence of prescriptive guidance, we are justified in requesting an engineered design or judgement, though I never have. Right or wrong will use the following very loose criteria as I observe and use the steps:
- "determine" if they meet deflection criteria, (how much do they bounce?)
- look for support at the top, midspan and bottom,
- observe the stringers for cracks/damage, especially at the incised cuts

We have code citations for all of these things if needed so I am not just saying they aren't right, rather I try to give them specific prescriptive comments. Given the information and picture provided I am guessing they are not temporary since they have risers. A quick overview of my possible comments would be along these lines: The stringers are supported mid-span so I imagine they do not over deflect but not sure with what since they are supported from the side, adequate support would need to be determined. The top is not supported adequately IMHO. Given the use of two stringers and a cantilevered tread (not sure what the material is) I would say each "joist" is supporting > 16" and the span chart used would be for 19 or 24" spacing for either 2x4 or 2x6 (depending on throat and if over-cut), the tread, viewed as a floor sheathing should have the edges supported. There are far better ways to have done these stairs that would not result in comments and it would only have incurred the cost of an additional stringer and by adding a header or a tread and riser.

 

[bill1952]

As far as engineers and the Kansas example, buy the stair from a stair fabricator is one option.

On the example, have the builder load up each tread with tube sand to 80 psf and then have one of his employees (injury will be more costly than if owner) run up and down stairs.

I got used to requiring load testing so seems simple to me. Someday I may talk about testing rolling a W18 roof beam about 30 degrees. Ugly.

 

[e hilton]

On the example, have the builder load up each tread with tube sand to 80 psf and then have one of his employees

That’s not realistic. Assume the treads are 12”x36” … 3 sf x 80 psf = 240 lbs on each tread. You’re never going to have a large person on every tread at the same time.

 

[bill1952]

60 psf then? 40? I was looking for a design factor of 2. Just an option. Hire an RDP if you prefer.

I find very few residential stairs with 12" deep treads, though I wish there was more. There would be billions of dollars saved.

 

[tbz]

You can read into requiring RDP's for anything you like, not the norm, but simplifies the conflict.

Hey, I see issues here with the structure I don't feel good about, none of which follow standard installations we see all the time, as thus I have concerns, but if you want to leave it, I need an engineer's review and sign off it meets code. I can't tell you what to fix, because I am not sure what is there is compliant.

You not saying its not compliant, what you are saying it is not the norm and you can't verify any compliance, thus the engineer.

 

[bill1952]

Odd the 2x4 verticals sit on what looks like a slab and plate is beside, not under, 2x4s. And don't appear to be pressure treated. Doesn't any load bearing wood on concrete have to be treated?

 

[BN4537]

Although the concrete would have to be in direct contact with the earth, and theoretically there is a plastic barrier between the slab and the earth...

 

[bill1952]

Although the concrete would have to be in direct contact with the earth, and theoretically there is a plastic barrier between the slab and the earth...

I did not realize an under slab on ground vapor barrier changed the requirement for wood on the slab. Thank you. Still seems wrong verticals don't sit on the plates.

 

[Mr. Inspector]

Although the concrete would have to be in direct contact with the earth, and theoretically there is a plastic barrier between the slab and the earth...

not always in existing or unheated buildings

Not sure by having the stairway being built by a stir fabricator automatically complies to code and they don't build the support.

The Wood council wood deck design has a detail drawings and text design that show the stringer to not have less then 5" wood left at the notch on three 2x12 stringers and the max span as 6' horizontally. But this is for treated southern pine outside. This would be shorter than what the joist tables say for southern pine 2x6's. A solid 2x12 stringer can span 13'-3" with a max spacing of 18" o. c. There is a table on tread sizes too.
It also shows a metal slop joist hanger at the top of the stringer.

 

[BN4537]

R317.1 Location Required

2. Wood framing members, including columns, that rest directly on concrete or masonry exterior foundation walls and are less than 8 inches (203 mm) from the exposed ground.

3. Sills and sleepers on a concrete or masonry slab that is in direct contact with the ground unless separated from such slab by an impervious moisture barrier.

R317.1.2 Ground Contact

All wood in contact with the ground, embedded in concrete in direct contact with the ground or embedded in concrete exposed to the weather that supports permanent structures intended for human occupancy shall be approved pressure-preservative-treated wood suitable for ground contact use, except that untreated wood used entirely below groundwater level or continuously submerged in fresh water shall not be required to be pressure-preservative treated.

The intent of the code is to prevent moisture from wicking from the exposed earth through the concrete into the wood and rotting it out. My interpretation is that the slab is separated from the ground by the plastic, which acts as a barrier to moisture, therefore the concrete is not in direct contact with the ground, and therefore does not have to be treated.

Rick18071, I think they technically could use untreated wood on a foundation wall the way it is written, although the second code section references concrete exposed to weather. However, since the plate is usually not "imbedded", that section does not apply.

 

[e hilton]

Odd the 2x4 verticals sit on what looks like a slab and plate is beside, not under, 2x4s.

Not odd at all. Put on your builder hat. You need the vertical 2x4 to support the stringers ... how are you going to anchor the bottom end? Simple … tapcon a piece of 2x flat on the floor, then toenail the verticals to it. Or not really toenail … sidenail i guess.

 

[Genduct]

This is a stair I ran across in the field, two stringers, supported with a couple 2x4s screwed to the sides of them, the top is bearing on a piece of 7/16" OSB sheathing that is hanging down from a structural beam. There was no engineer involved on this project. I have contacted the contractor with my recommendations and I have not received any pushback.

However, this scenario illustrates a problem I have, which is that I currently have no objective way to determine two things:

1) How many stringers are needed for a particular stair?

2) How far can those stringers span?

Given:
IRC 301.1 that says that you have to provide a complete load path that is strong enough to support the loads involved.
Table R301.5 that says that stairs have to be able to support a 40 psf live load and a 300 lb concentrated load.
Other than that, there is no guidance in the IRC that I can find.
AWC's DCA 6 document recommends that a 3' wide stair have three cut stringers no more than 18" apart and spanning no more than 6' horizontally.
I am an inspector, not a structural engineer.

My impression is that this configuration of stair stringers as-built will not meet the loads from table R301.5, based on my experience and judgement. However, that is a (somewhat) subjective call on my part.

Does anyone have a code section I am missing, or maybe a document from an organization like the AWC or the SBC, or information about calculating stair stringer span, or rules of thumb that you have, etc. that they would be willing to share?

Objective decisions are great, but if I have to make a call I like to have as much information as possible, so anything you have will be appreciated.

Thanks!




View attachment 8531

SO the contractor discovered he made the stairway opening to small, so instead of having the stair stringer end at the girt where the floor joist are installed with strap hangers, he had to shorten the run of the stairs by one thread and "HUNG" the stair stringer from a piece of OSB WOW!

The contractor needs to attach some 2x8 from that frame wall to what looks like a lolly column to proved a real termination for those stair stringers