Application of Guard Rail Load on Stairs 1

[phamENG]

Working on some shop drawing calcs for a fabricator client, and I've run into an issue that I want to take a closer look at. IBC 2015.

Everything I've done with this fabricator to date has been steel, but this is an aluminum project and includes guard rails - both for elevated platforms and stair rail systems (combined 42" guard with hand rail set at 36" above treads and connected to verticals on the guard). Unfortunately, it was bid and detailed like a steel system with 1-1/2 pipe at 4' on center...not even close to working. Before I go back and tell him he has to double his material, I need to sharpen my pencil a little. The architect classified this building as U, so I don't get to take advantage of the 20plf allowed for I-3, F, H, and S. 50plf and 200lbf point load.

When we apply live loads to a sloped surface (roof, ramp, stair, etc.), we apply it to the projected surface. So if you have a 100psf stair live load and a 3' wide stair with a 8' rise and 12'10" run, total live load would be 3'x12.8333'x100psf=3850lbf even though the actual sloped surface is about 15'2". The idea being that people or material occupy the vertical space above the surface and not the space perpendicular to it. Has anyone applied this concept to stair rails? So on that same stair, your rail has a run of 15'2", but the equivalent occupancy is only 12'10". In other words, you can't have enough people on the stair to apply the 50plf contemplated by a horizontal rail. So the rail would be designed for a load of about 42plf rather than the full 50.

Best bet is probably to just stick to the 50plf along the rake, but I'm curious to see if people are doing something different elsewhere.

 

[dik]

The live load is based on the horizontal surface. The only times I use 'slope modified' live loads is for guardrails where the horizontal load is spec'd as a load per lineal unit.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[phamENG]

dik: since we have to design the rails for 50lbf/linear foot, are you saying you would modify it for the slope of the stair (or ramp in other scenarios) and use the 42plf applied to the actual length of the rail?

 

[BridgeSmith]

I have no idea what the code would say about the proposal, but it seems reasonable to use the horizontal length with the 50 plf or the 42 plf along the sloped rail to design/check the rail. It doesn't help you for the posts, though, since the force on the posts is the same either way.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dik]

pham... I do for the horizontal load', after all, it's a plf thing; I don't know if other engineers do the same. For verical load, it is based on the horizontal projection... I also design for horizontal point loads for the sloped condition. I have an SMath program that I wrote that accomodates this with a rise and run variable... I set the rise to 0" for a horizontal guardrail or handrail. Unless you're getting into longer spans, usually the point load requirement governs.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BridgeSmith]

Unless you're getting into longer spans, usually the point load requirement governs.

You guys have it easy - under AASHTO it's 50plf and the 200lb point load.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dik]

We're nearly as good... NBCC for Guardrails:
DLv I use 3 plf
LLh 51 plf
LLv 103 plf
Ph 225 lb...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Ron]

I use the full load application. Stairs and rails are generally life safety egress, so I apply the full loading, as impractical as it is.

I apply 100 psf to the tread, reacted at the stringer. It is applied vertically. I apply the 50 plf along the slope of the rail.

I have analyzed them separately and together. When analyzed together, you get some odd stress distributions in the system since the rail is trying to torque the stringer on the rail side and rotate the whole system about the rail side stringer.

 

[dik]

I've always applied the load to the slope for horiz loading, but I've never checked with the AHJ for fear of opening up a 'can of worms'. I think the loads are adequate, even for life safety if considered either way. I find the 'big one' is the length of the cantilever at the end. Often the length approaches 2' and it means the bottom post is not adequate, specially if the backspan is short.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[phamENG]

Thanks, everyone.

BrideSmith - you have to apply them both concurrently? I wasn't aware...good to know. I don't work with AASHTO much, but everyone once in a while it comes up on small footbridges and things.

dik- I'm writing my SMath sheet now. I'll see if I can incorporate that...

Ron - I agree, and that's where I went initially, but figured I'd explore it to see what other people were doing. I think I've found a compromise, though. I'm going to suggest a mechanical fastening detail for the post to stringer detail so we don't have to get into weld affected zone reductions...that results in a modest increase in material whether I do it on the rake or not.

 

[dik]

I occasionally come up with questions to see how others handle things, but for the most part just do what I think is right...

Incorporating is easy... a chunk of the program determines moments and reactions based on length of members (cont, cant, simple span) with the cant being horizontal at all times. I use approximations for the moments and reactions for continuous members and reactions for cantilevers. The length is increased by an amplification factor based on rise and run... if you set rise=0, then it's horizontal. It also checks if there is a backspan for the cantilever.

Loads under NBCC are not concurrent...

attached... didn't attach... below...
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Mike Mike]

phamENG - Interesting thoughts. I've done a lot of these calcs for fabricator clients and I always designed for 50plf over the full 15'2", for a total of about 760 lbs. It never occurred to me I might be allowed to take a slope factor bonus. I suppose the IBC language to the effect of "applied in any direction" swayed me toward applying a full 760 lbs.

Ron - I don't have the IBC in front of me but I think I remember reading these 2 live loads don't act concurrently.

Dik - Are you saying you would apply a total of 50plf x 12'10" = 640 lbs?

Bridgesmith - Why would the load on the post be the same either way?

 

[dik]

If that was the slope length and the load was horizontal... it is not a bonus; it increases the effect of the horizontal load on the posts slightly by a factor of 1/cos(angle), where the angle is the arctan of (rise/run). The span is increased by that amount with a subsequent increase in moment from the UDL loading. My latest work has been designing connections for a welding company and in the last 1-1/2 years I've worked on nearly 500 projects. I write SMath programs for any connections that I might do 4 or 5 of.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/raw/upload/v1630253202/tips/NGuardrail_HSSc_Template_zd1oft.sm[/url]

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BridgeSmith]

Bridgesmith - Why would the load on the post be the same either way?

Because whether you apply the 42plf over the slope length or 50plf over the projected (horizontal) length, the posts are still 4' apart, so it's still a 200 lb load on the post (50plf X 4' horizontal or 42plf X 4.76' along the slope).

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dik]

In my case the 51plf horizontal live load would be applied over the sloped length, not the horizontal component of it or the horiz component of the length. If the sloped length is 10', then the total load applied is 10'* 51plf = 510 lbs and the effects of this load are resisted by the posts, or whatever. I didn't realise I had confused anyone (I do sometimes, but not usually intentionally, unless for humour). If continuous spans, the the post load would be 510 lbs horiz. If simple span, the reaction would be half, 255 lbs. The load on the post increases with increased slope... by a factor of 1/cos(arctan(rise/run))

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Ron]

@MikeMike....you don't apply the two lateral loads (200 lbf point load and 50 plf distributed load) concurrently, but you do apply the tread load and rail loads concurrently. You can't get a lateral load on the rail without someone standing on the treads.

 

[BridgeSmith]

...you don't apply the two lateral loads (200 lbf point load and 50 plf distributed load) concurrently

As mentioned before, the AASHTO bridge design spec does have us apply those loads concurrently...but also in any direction (I don't know if they would be added to the 75psf "pedestrian load" when applied vertically).

Of course, that wouldn't apply to the case under consideration, or presumably to railings anywhere but bridges, but I found the differences interesting.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[phamENG]

Rod - me, too. I'd be curious to see what the justification is for that. Maybe a whole bunch of people leaning on the rail, and then a bike crashes into the rail in the middle of it? I could see wanting to make sure those people don't suddenly fall off a bridge in that situation, and it's unlikely to occur in a hotel, office building, etc.

 

[BridgeSmith]

phamENG, it's probably because pedestrian railing loads are such a small part of design, and such a small component of the loading for us, they thought they make it simple by doing it that way. Not sure it really does, through.

Rod Smith, P.E., The artist formerly known as HotRod10