Fixed Ladder Design

phamENG · Mar 16, 2023

Looking at a stainless steel fixed ladder design. The fabricator wants to use plug welds for the rung to rail connection (punch a hole in the rail, put the rung halfway in, and fill the hole with weld. Putting aside the fact that this isn't a great application of a plug weld to begin with (though I know it's commonly used), with a 1" diameter rail we violate the maximum hole size for the 3/8" thick rail anyway. My preference would be to tell them to do a fillet weld all around - 3/16" would suffice. But the EOR spec'd CJP welds for everything. Is there a prequalified CJP for this application? I started to think of a bevel-groove weld 'all around', but you can't put a backer in there or backgouge it...

Another thing - this is the first ladder I've had to design with a central fall arrest rail on it. The connection to the rungs has to support 3600lbs right at the ladder center line (this coming from the manufacturer's literature based on limits for engineered fall protection - max force in the system to be 1800lbs and a factor of safety equal to 2). That's a lot on a ladder rung. Minimum OSHA rung diameter is 0.75". This is stainless and at 18" wide I need 1" diameter anyway for strength. EOR also spec'd 1" rungs. But that 3600lb load pushes me up to a 2in ladder rung for strength. Does that sound right to anybody? Seems like a lot, and possibly uncomfortable for the worker climbing it. I can't find any maximum sizes in OSHA or the IBC. Anyone else have any experience with this? Did you just spec massive rungs or did you do something else?

straub46 · Mar 16, 2023

For the first part, the typical detail I've seen is the plug weld inside the hole, and an all-around fillet on the opposite side (where the rung goes into the rail). Maybe see if the EOR is ok with that? (it also helps seal the ladder in exterior applications)

For the second, I've designed a few ladders with a fall arrest rail. The typical steel ladder I've used has 3/4" rungs at 18" wide. I've pushed the fall system over basically as close to one rail as possible, and even then my rungs had to be bumped up in size to 7/8", and that was with a 2x load of only 2700 lb. With a mid-rail system, I wouldn't be surprised if stainless steel needed almost 2" dia. for strength, especially for a 3600 lb load.

On a side note, will a centered fall arrest system interfere with exiting the ladder at the top?

Go Bucks!

phamENG · Mar 16, 2023

Thanks, straub. In theory, a CJP weld should be just fine, and it's what the EOR spec'd. So I'm going to put it back on the EOR to verify prequalification and check that it's okay. Either design the thing or get out of my way.

Interesting thought. All the products have images with the rail in the center (makes sense as it keeps everything in line with the worker's body, keeps it out of the way, and essentially eliminates any swing) - but I did find one that shows it mounted to the rail. Again, EOR spec'd 1" dia. rungs. So I'll tell them what they need to do to make their design work. It has to mount to the rail or within 0.75" of the rung to rail connection.

straub46 said:
On a side note, will a centered fall arrest system interfere with exiting the ladder at the top?

It does seem that way. But it's how most of them show it in their literature. I guess you get to the top, move off of the ladder around the rail, turn around and unclip, and then go on your way? Most of my experience actually using ladders is either in older industrial sites with cages, on ships under construction with nothing, or in historic buildings with woefully inadequate ladders and no fall protection whatsoever. So I haven't used these myself.

straub46 · Mar 16, 2023

The system I've used in the past is by DBI-Sala, and here's a snippet of their installation literature

You can see they show it as an option to install on the rungs close to the rail. The smaller moment keeps the rail size down, and it seems like it would be more out of the way for anyone climbing/exiting the ladder. I've never climbed on one with this installed though either, I've been limited to cages as well. In fact, I would still probably install a cage unless the owner/EOR was going to throw a fit about it. I feel it's a worker comfort thing, where at least the cage makes it feel like there's something there, even if the fall arrest system is doing the heavy lifting.

Go Bucks!

phamENG · Mar 16, 2023

Thanks for sharing that. That's one of the allowable installers per the specs, so if the contractor can't get their selected system to work they have an option.

Problem with cages is that they're no longer OSHA compliant. They can't be used on new installs and they have to be phased out by 2036.

HTURKAK · Mar 16, 2023

My points acc. to past experience ,

- The use of plug welds for the rung to rail connection is OK for bar rails ( plt 63X9 , or 73X9 acc. to stair ht ) and fillet weld for channel rails ,

-Ladder rung design is based on 200-pound concentrated live load and Ladder connections are designed for a maximum total load of 1000 pounds on one side rail.

Refer to PIP STF05501 ( Process Industry Practice )
I have written STF05501 and search the web and found the subject document but i am not sure that , this violates the copyright . ( I have old version than this doc.)

https://kupdf.net/download/pip-stf-05501-2012_5ca332bce2b6f50b53488dba_pdf

Not to know is bad;
not to wish to know is worse.

NIGERIAN PROVERB

phamENG · Mar 16, 2023

Thanks, HTURKAK. My comment about plug welds come from minimum and maximum hole size according AISC. A 1" hole it too large to be classified as a plug weld in a 3/8" plate. Perhaps it would work, but it's not an AWS prequalified weld. And this ladder has to be designed for concurrent 300lb point loads for every 10 feet of ladder as well as fall arrest loads resulting from the connection of the fall arrest system rail to the treads.

I'm sure that document provides some good background, but that standard is not directly applicable (this is governed by project specs written by the EOR, OSHA, and the IBC). Also, I'm not big on clicking on "something-something-pdf.net" websites. (See dik's thread about ransomware...)

SwinnyGG · Mar 16, 2023

You could always reduce the size of the hole in the rail to the maximum size to classify the weld as a plug weld. You'd have to put a fillet weld on the inside between rung and rail anyway, just to protect against corrosion. Although by that point you're getting roughly the same result as a fillet weld against flat plate anyway and just doing a lot more work to get there.

Also... I'm not sure that each individual rung needs to be able to handle the full fall-arrest load. Every system I've ever seen mounts to multiple rungs; I would think you could reasonably distribute those loads among rungs which will actually carry the fall arrest system. I have no code passage to back that up, but I'm also not aware of a code passage that specifically dictates that the fall arrest load cannot be distributed among the mounting points.

phamENG · Mar 16, 2023

SwinnyGG - I'm constrained by the EOR's specs. They want the rung to penetrate the rail, so it has to be the same size (or, really, slightly larger) than the rung. I'm really not worried about that at this point. I'm recommending fillet welds but it's their building design and they can do as they please.

As far as reality goes yes, I'm sure the load gets spread out a bit. But again, I have to design it to the specs. If the fall arrest system calls for attachments to the ladder that can handle 3600lbs, then that's what I'll design them to. After all, they won't pay me enough to do a detailed analysis of the relative stiffness of the trolley rail, the ladder rungs, and the connection between them to figure out just how much load goes to the next attachment when the trolley is locked right on one of the rung connections. So they get a conservative design. If it's mounted to the side, it works. So that's what I'll tell my client they have to do if they want the design to work.

SwinnyGG · Mar 16, 2023

Ah - fair enough, I missed that detail. Apologies.

Maybe you could taper the ends of each rung, and use a countersunk penetration in the rail to reduce the size of the hole.. although that probably doubles the cost of fabricating the ladder.

phamENG · Mar 16, 2023

No worries. I may not have mentioned it. And yeah, that would get expensive.

This was mostly a reality check for me. The sizes were getting big, and while I checked and rechecked the math it was right, I knew I'd run into push back. Probably still will, but at least I've found a slightly more economical way forward.

Thanks, everyone!

Smoulder · Mar 16, 2023

They mount them on regular ladders in Aust. Is 1800lbs just a code rqmt? Never get near that with retracting systems. Theres no fall distance.

phamENG · Mar 16, 2023

We have a weird mix of prescriptive requirements and engineered options. The idea is that in any fall protection arrangement, the maximum force on the worker is 1800lbs. Through detailed design, this can be reduced. But it usually requires not only a holistic and detailed design of the entire system (from harness down to final anchorage), it also requires buy in and cooperation from the end user. For a lot of these, the end user today may not be the end user by the time the building is built, and then they may change 6 times over the life of the building. So rather than rely on the institutional integrity of a fall protection program, we have minimum, prescriptive requirements that can and often are used.

But then the language regarding those loads isn't clear and not really in line with current building codes, so what does "resist a load of 3600 lbs without breaking" mean? Is that the service/ASD load, or an ultimate/LRFD/LSD load? Nobody really knows. Then the IBC comes in with their own fall arrest loading, but it doesn't really match anything OSHA requires. There's lots of threads on here with everyone arguing back and forth about how to interpret the various loads. So a lot of people just go super conservative and design everything for 5000lbs as a service load.

SwinnyGG · Mar 16, 2023

I've been in a similar situation, where I spent a bunch of time dealing with distributing loads and reducing load factors because there was a retractable in the system and then had the inspector tell me 'what if the user clips the retractable to a 10ft lanyard instead of directly to the harness, now you're at OSHA maximum loads which your system can't handle'

So yes, conservatism rules the day even though it makes everything three times more expensive and half as practical

JStephen · Mar 16, 2023

See the load calculation on Page 4 of this pdf:

https://prod-edam.honeywell.com/con...-HIS-MF-i284-glideloc-kits.pdf?download=false

This could vary per manufacturer.
"What if the user uses a 10' lanyard", etc.- that is covered by the user instructions, and no amount of engineering will protect against every conceivable abuse.
On the cages- they are not prohibited or phased out, they just no longer qualify as OSHA fall protection.

SwinnyGG · Mar 16, 2023

JStephen said:
"What if the user uses a 10' lanyard", etc.- that is covered by the user instructions, and no amount of engineering will protect against every conceivable abuse.

I agree.. but as I've said in other similar threads, as a contractor if the inspector digs in his heels you will lose every time.

In my experience OSHA inspectors are rarely rational.

JLNJ · Mar 16, 2023

Maybe you can require that the system distribute the 3600# load out to three rungs. 1200# a rung seems doable against phi*fy.

How about stainless rebar? It has pretty high yield and has the deformations you want for climbing.

JSgam · Aug 8, 2023

Just throwing this out there, I would rarely ever put a cage on a ladder that has a ladder safety system. This could impede any rescue operations.

JSgam · Aug 8, 2023

phamENG said:
We have a weird mix of prescriptive requirements and engineered options. The idea is that in any fall protection arrangement, the maximum force on the worker is 1800lbs. Through detailed design, this can be reduced. But it usually requires not only a holistic and detailed design of the entire system (from harness down to final anchorage), it also requires buy in and cooperation from the end user. For a lot of these, the end user today may not be the end user by the time the building is built, and then they may change 6 times over the life of the building. So rather than rely on the institutional integrity of a fall protection program, we have minimum, prescriptive requirements that can and often are used.

Just throwing this out there as well, the system components, such as the energy absorbers in the system, already reduce the average arresting force. These energy absorbers MAY have a super brief maximum arresting force of 1,800 lbs, but it's pretty rare; at least with systems that have an Average Arresting Force of 900 lbs, they'll usualyl have a Max Arresting Force of 1,350 lbs. Designing the harness, and the connecting components (e.g., lanyard, SRD, ladder safety system shuttle/lifeline) is not necessary when the manufacturer tells you the loads. What IS necessary is finding out if the anchorage can support these forces within the required standards.

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