stair guardrail loads 2

[struct_eeyore]

Hi all. I'm working on repairing an old exterior wood stair, and am struggling to come up with a proper way to resolve the 250 lb @ 42" lateral loads at the guard posts.
The stringers are 2X12 - with the treads cut in, they are 5" deep in cross section, so there isn't much of a couple that can be created.
Does anyone know a way this can be done?

 

[struct_eeyore]

CEC17 - my question isn't really about the mechanics. The problem is that the 10500 lb-in force is monstrous compared to the 5" of depth of the stringer I'm left to work with, not including the edge/end distances. If I throw in some blocking with a tension tie, assuming 3" couple distance, I'm looking at 3500lb couple that needs to be resolved. Also, I don't entirely agree that the load will be shard - it's a point load, and assuming the guard is just nailed/pinned, there wont be much continuity across the post to alleviate the situation.

The more I think about it, the more I'm inclined to rebuild with new posts embedded in concrete at grade level.

 

[phamENG]

Short answer - you can't. At least not the way that thing's currently built.

For metal guardrails with rigid connections, I agree with cec17. Load sharing helps a lot. A wooden rail with toe nailed connections spanning 8', and those posts with less than stiff connections? Load sharing isn't something I'd count on.

Best solution for these is to put solid blocking between stringers at the post locations. Then, make your top through bolt go all the way through to a tension tie fastened to that blocking. Make sure you have a tension load path between pieces of blocking if you have more than 2 stringers and you can't use a riser.

 

[JStephen]

Also confirm that it needs to meet that if an existing stairway.
If the original stairway wasn't designed for that load, you're not "repairing", you're "modifying".

 

[lexpatrie]

It's possible there's something that is being applied here as "code" that doesn't strictly apply.

Code calls for a guard at a free edge (or something similar) and that's where the 50 plf, 200 pound point load (I don't think it's 250 pounds as you state), and the 4" sphere passage shows up. (21: sphere passage for a roof). The 4" thing applies to open risers on stairs, but the other items, I'm not absolutely certain.

The intent is to prevent someone from falling into free space from a height exceeding (what was it? 18"? 3'? I forget. Anyway, that's what I'm wondering. The 4" sphere is to keep a crawling infant from becoming trapped between the infill, the treads, etc.

On a stair, the objective is to prevent a fall, and it's a handrail. So the guard load you are applying may not actually be required? Now, those loads might be the same thing, I don't know offhand... maybe someone else does.

OSHA has other requirements, but that's for worker safety so like the toe guard isn't in the IBC/IRC. They may have requirements on stairs that are more "rational" for this situation?

Guards, Handrails, and the IRC, Matthewson, Journal of Light Construction, Oct 9, 2021.

 

[phamENG]

lex - handrail and guardrail are technically two different things, but stairs are required to have both. In some circumstances, they can be the same thing doing double duty. In other cases, the guard rail goes up to the required height and the handrail is attached at a lower level for accessibility and comfort (short people apparently have trouble using a 42" guard as a handrail).

 

[lexpatrie]

Yeah that kind of matches what that article said, I thought maybe a stair was a different situation.

Also, it looks like the "fall" is 30", and the height for residential is 3'-0", not the 3'-6" (42") in the building code, if that does any good for the OP. Photo looks like it's maybe a restaurant or something.

Guardrails vs. Handrails, Matthewson, Journal of Light Construction, July 11, 2019.

 

[Eng16080]

For something so common as a handrail post on a set of stairs, it's a little surprising how many of these in the wild aren't even close to meeting code. You're best bet is probably what phamENG wrote above: solid blocking and hold-down connectors with threaded rods, although I doubt you'll even get that to work by code (despite making it much better). I would assume the stringer has zero capacity to resist the moment due to an outward (or inward) force. You're asking it to resist a torsional moment, which it won't have much capacity for. I bet the post moves easily with a little force applied at the top if you were to push on it.

I recently did a lot of research concerning a guardrail post to deck connection detail and had a terrible time getting things to work by code.

A few other comments:
[ol 1]
[li]If this is per IRC (residential), I think the concentrated force should be 200 lbs, not 250[/li]
[li]The moment arm is likely a little more than 42". This is perhaps nit-picking a little, but I would calculate it as the distance to the upper (tension) bolt for an outward applied force. That will make your moment a little bit higher.[/li]
[li]Depending on the type of lumber and your design assumptions (like whether you should use 1.6 or 1.0 for the load duration factor, CD), I think it's possible that the 4x4 post doesn't even work (assuming it's a 4x4).[/li]
[li]If you really have a 3500 lb force couple, I wish you luck in getting a connection to work![/li]
[/ol]

 

[SWComposites]

just extend the center post down to a footing in the ground.

 

[JStephen]

"it's a little surprising how many of these in the wild aren't even close to meeting code"
Just to comment on this item- I normally am dealing with OSHA requirements- but under the old rules, a stair handrail/guardrail was REQUIRED to be between 30" and 34". So if you meticulously followed OSHA requirements of the time, that stairway no longer meets current OSHA rules (although generally grandfathered in). So it's not necessarily people just building stairs willy-nilly.
The handrail in the original photo appears to be a residence, so presumably doesn't fall under OSHA, and depending on the details, may not have been governed by ANY building code when built.

 

[JLNJ]

Some thoughts:

- The non-continuous top rail doesn't look so great. Probably does not conform to the IBC.

- Load sharing can be useful, but there is usually an end post which doesn't have a neighbor post with which to share the load. Heck, sometimes it's the opposite, where the rail extends (i.e. cantilevers) beyond the post, essentially magnifying your point load.

- The graphic posted by lexpatrie is for a residential stair, not a commercial one. Commercial requirements are more strict.

- If this is a commercial installation, you might have other issues, such as a lack of handrail extensions, lack of a guardrail of sufficient height, and open risers. Can't store things under a stair, either, and sometimes stairs are required to be covered so that you are not having to exit through a foot of snow.

 

[milkshakelake]

Agreed with Eng16080, I don't think you're using the correct force. Not that it will matter that much anyway, if your design ratio is like 400%. But there's also a linear load you need to consider, which might control for the long members.

Old stairs like these weren't designed. They were just built by contractors based on experience. It's very hard to get something like this to work. Maybe take a different approach to this whole thing? Like, only design your specific repair to code, not the whole staircase, because it might be able to be grandfathered in. In my jurisdiction, items like this are grandfathered and don't need to meet modern codes unless they're altered. So that's the tricky part with the alteration, but it can be navigated. Otherwise, you need a full replacement.

 

[phamENG]

OP is in Florida. An exterior wood stair in that environment... I'd be shocked it that thing is more than 10 years old.

That was not built to the code in effect at the time, and there's nothing to Grandfather.

 

[struct_eeyore]

@Pham,

Believe it or not, this stair is at least 30, and most likely 40 y.o. It's scary as hell to walk up, but has otherwise kept intact even thru Ian (the hurricane 2 years ago)

 

[lexpatrie]

I don't know where the 250 is coming from, unless that's 50 plf x the span of the top rail @ 5'? The 42" does suggest this is IBC versus IRC.

If I take the photo as "on point" the bottom end guard isn't of concern, it's below the 30" "fall height" so it may not need to resist that load. At the mid-way up the stair, that post looks like it's a 4x4 that's been cut partly and has 2" depth, so bending won't work there for the load as there's not enough section, so the remainder of the connection/load path isn't relevant because the piece itself isn't strong enough. I believe the current IRC doesn't allow a notched 4x4. Even if it's not IRC, the notched 4x4 won't work for bending stress.

As to connecting something that's actually strong enough to resist the load, I could see a 2x8 block under the stair and a holdown into it. The IRC at least seems to have been altered so the strength requirement applies in the downward and outward directions only. It seems tractable.

Strong Rail-Post Connections for Wooden Decks, by Joseph Loferski and Frank Woeste, P.E., with Dustin Albright and Ricky Caudill, Journal of Light Construction, Feb 2005.

Should be able to track the various articles from there.

I've started a FAQ entry in the Wood design and engineering forum, if anybody has any suggested articles, feel free to send some feedback.

 

[JLNJ]

250# is the ADA grab rail force. The IBC specifically allows 200 for stair handrails. So you have the 200# to design for even when there is no "fall" per se.

 

[lexpatrie]

Would ADA apply to a stair like that? That just doesn't scream accessible entrance to me. I'm used to ramps or the LALU elevators, or equivalent features on the ground floor that are accessible.

 

[LittleInch]

What is going on at the base of that mid point post? Looks like someone just hammered in a few planks?

What are you repairing exactly?

Why is it "scary as hell to walk up"??

Shouldn't you be walking away from this job?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Eng16080]

If I take the photo as "on point" the bottom end guard isn't of concern, it's below the 30" "fall height" so it may not need to resist that load.

I don't think this meets the intent of the code. If it did, seemingly it would be acceptable to not have a stair railing below a 30" fall height, and I don't think that's the case. Certainly, an occupant would expect something which looks like a railing/handrail to act like one and be designed to resist the same forces as one. Of course, I agree that falling 30" will probably have a better outcome than 6 ft, or whatever.

The IRC at least seems to have been altered so the strength requirement applies in the downward and outward directions only.

Is this in the newest version of IRC? Where I work, we're always well behind with the codes, and in the 2018 version, it still requires a 200 lb "Guards and handrails" load "applied in any direction at any point along the top."

250# is the ADA grab rail force

Also curious where this is coming from. I haven't been aware of this force as a design requirement for stairs, if it is.

 

[lexpatrie]

I wasn't so much advocating ignoring it, it was more that based on the photo, that lowest post isn't of concern to the OP. I believe a rise of 30" or less doesn't require the guard, (again, IRC), but if it's over 30" it requires a continuous handrail top to bottom. I think it's subject to building official interpretation, if they want it designed as a guard, so be it.

According to that Matthewson article, it's the 2021 IRC.

As to the 250 lb. I don't know that one. For residential, the 50 plf doesn't apply. That's not doing anybody a whole lot of favor, because the 200 pound point load is pretty rigorous already.

2021 International Building Code.

The 250 lb load I know about is for grab bars, like in a shower or bathroom.

Those are rather typically not outdoors and even if they were, there's usually a wall they attach to, and the moment is based on a more reasonable downward force at say 6" from the face of the wall.