Ladder Safety Systems

[JSgam]

Hey all. Attached is a ladder safety system used commonly on longer length ladders. Most all manufacturer's I've spoken with rely on not exceeding the ultimate strength of the rungs, and bending is expected under these loads, reaching up to 3,600lbs (split amongst 2-3 rungs). Every manufacturer will state the supporting structure needs to be able to support the loads, so I'm assuming any ladder using these just needs to make sure the ultimate stress of the rungs is not exceeded. For steel ladders, the rungs are usually plug slot welded at the ends. Also, these systems are typically installed onto existing ladders. For new ladders, I'd just used thick enough rungs, but on existing ladders they can have 3/4" round bar rungs, and assuming it's A36 steel there isn't a ton of capacity in those.

My questions are:
1) For analysis, is it as simple as comparing the ultimate stress of the rung material to an applied stress of M/Z? or is an FEA program more useful here when exceeding the nominal moment FyZ (from AISC 15th edition section F11)?
2) For the welds at the ends, is it acceptable to use the applicable allowable strengths from Section J of AISC, or is FEA more appropriate when deformation is expected?
3) Any additional thoughts on how to put an analysis for this on paper is appreciated.

 

[Andrew ONeill]

I don't have direct experience, and I'm a mechanical engineer, not structural. I cna't answer Q1 and 2, but here are my thoughts on Q3.
It would be worth checking if the rating of the system is based on Testing vs Calculations. Actual loads via testing are likely to be significantly different the theory, especially once plastic deformation starts to occur.

In my part of the world (Australia), our loading standard covers these types of systems, the only trick being a definition between fall arrest, vs fall restraint, but from memory the required design load is around 25 kN or there abouts for a single person fall (this number is based on historical testing of people falling into harnesses), without any further information in my world, I'd just be using that load to size things, using the normal design process (like any other structure member, not going to design it to reach UTS)

If the design basis is to size it so it plastically deforms, I would definitely be doing physical testing, as virtually all the hand calcs in design on the basic assumption of "small deflections, and no yielding".

Other part is risk.. this is a human safety device, if the job is to retrofit to existing ladders, and there is pressure to "make it pass" I'd look long and hard, last thing you want is a call in 15 years time because the ladder broke and someone is fatally injured.

 

[JSgam]

I don't have direct experience, and I'm a mechanical engineer, not structural. I cna't answer Q1 and 2, but here are my thoughts on Q3.
It would be worth checking if the rating of the system is based on Testing vs Calculations. Actual loads via testing are likely to be significantly different the theory, especially once plastic deformation starts to occur.

In my part of the world (Australia), our loading standard covers these types of systems, the only trick being a definition between fall arrest, vs fall restraint, but from memory the required design load is around 25 kN or there abouts for a single person fall (this number is based on historical testing of people falling into harnesses), without any further information in my world, I'd just be using that load to size things, using the normal design process (like any other structure member, not going to design it to reach UTS)

If the design basis is to size it so it plastically deforms, I would definitely be doing physical testing, as virtually all the hand calcs in design on the basic assumption of "small deflections, and no yielding".

Other part is risk.. this is a human safety device, if the job is to retrofit to existing ladders, and there is pressure to "make it pass" I'd look long and hard, last thing you want is a call in 15 years time because the ladder broke and someone is fatally injured.

Thanks for your input. Your last sentence is essentially why I come to the forum. For some background, yes the load rating is based on testing, and most manufacturers have informed me that the rated loads are for "expected bending, but not breaking", which I'm assuming means rupture does not occur.

For entirely new ladder designs, I'm keeping everything within the elastic range. But a lot of these systems go onto existing ladders that seem quite thing and flimsy for these loadings (e.g., 3/8" plate siderails, 3/4" rungs). As much as I'd like to prove it on paper for these types of ladders, I'm thinking a more detailed FEA would be required each time (given every ladder geometry is different).

 

[Greenalleycat]

Andrew, I'm in NZ but our fall arrest standards are joint with Aussie. Loading is 15kN for single user, 21kN for dual user - on anchors at least. I imagine it is the same for ladders. We do a lot of fall arrest work and it is a PITA to engineer because all of the tested systems rely on incredibly light supporting structures....nothing that would ever stand up to calculations. Particularly as the actual design load is 6kN with a FOS of 2-2.5 depending on your local code (AS/NZS is technically 2 at the moment but international practice is 2.5).

I imagine the same applies to ladders. If it's stainless steel it will be ductile as hell, I can't imagine you would ever break it with fall arrest loads.

 

[human909]

I don't have direct experience, and I'm a mechanical engineer, not structural. I cna't answer Q1 and 2, but here are my thoughts on Q3.
It would be worth checking if the rating of the system is based on Testing vs Calculations. Actual loads via testing are likely to be significantly different the theory, especially once plastic deformation starts to occur.

In my part of the world (Australia), our loading standard covers these types of systems, the only trick being a definition between fall arrest, vs fall restraint, but from memory the required design load is around 25 kN or there abouts for a single person fall (this number is based on historical testing of people falling into harnesses), without any further information in my world, I'd just be using that load to size things, using the normal design process (like any other structure member, not going to design it to reach UTS)

If the design basis is to size it so it plastically deforms, I would definitely be doing physical testing, as virtually all the hand calcs in design on the basic assumption of "small deflections, and no yielding".

Other part is risk.. this is a human safety device, if the job is to retrofit to existing ladders, and there is pressure to "make it pass" I'd look long and hard, last thing you want is a call in 15 years time because the ladder broke and someone is fatally injured.

I see this as poor advice.

For starters 25kN is not the measured fall load of ANY person in a harness. For starters think about the damage that would do to your body! I've taken more falls in harnesses than I can count and

Also 25kN is a required ultimate load. Plenty of fall arrest systems achieve this with plastic deformation.

We are talking about 3/4" bars. There is significant capacity in these well over 3600lb or even 25kN if you are spreading the load over three bars.

Unless your welds are poor then you will comfortable achieve 25kN.

 

[Ron247]

For new ladders, I'd just used thick enough rungs, but on existing ladders they can have 3/4" round bar rungs, and assuming it's A36 steel there isn't a ton of capacity in those.

Are the rungs you see smooth or do they have "lugs" present for traction. I know a lot of tank suppliers use 3/4" A706 rebar for rungs. It comes in Grade 60 and 80 I think. It is the only rebar that is reliably weldable. If you do any analysis, you have to decide which end condition applies to the rungs. Are you modeling it simple span or fix-fix. What are you thinking the plug weld is doing to the ends? The size the side-rail material affects this decision too. I have no idea what the industry standard is.

Testing to some degree is highly advisable due to the expectations of systems like this.

 

[JSgam]

Are the rungs you see smooth or do they have "lugs" present for traction. I know a lot of tank suppliers use 3/4" A706 rebar for rungs. It comes in Grade 60 and 80 I think. It is the only rebar that is reliably weldable. If you do any analysis, you have to decide which end condition applies to the rungs. Are you modeling it simple span or fix-fix. What are you thinking the plug weld is doing to the ends? The size the side-rail material affects this decision too. I have no idea what the industry standard is.

Testing to some degree is highly advisable due to the expectations of systems like this.

Thanks for your input!

1) I've seen various rung types. Smooth round bar, rebar rungs, square bar rungs, you name it.
2) Regarding grade of material, the hard part is making rung material assumptions when there are no existing drawings provided, so most of the time for existing unknown materials A36 is assumed.
3/4) First, treated at simple span, but second treating it as fix-fix to help the rungs out if the siderails can take it. Part of my question is how exactly would a plug weld behave when the rung reaches plastic.
5) There isn't an industry standard. From my experience working in walking-working surfaces and fall protection all these years, people don't really give ladders any attention when it's initially attached to a structure, and then they slap one of these systems on it. I'm trying to get the industry there as a matter of fact, which is why I was reaching out to you all with opinions.
6) Testing for fall arrest loads I typically frown upon and would like to be more confident in modeling/calculating these items out first.

 

[JSgam]

I see this as poor advice.

For starters 25kN is not the measured fall load of ANY person in a harness. For starters think about the damage that would do to your body! I've taken more falls in harnesses than I can count and

Also 25kN is a required ultimate load. Plenty of fall arrest systems achieve this with plastic deformation.

We are talking about 3/4" bars. There is significant capacity in these well over 3600lb or even 25kN if you are spreading the load over three bars.

Unless your welds are poor then you will comfortable achieve 25kN.

Thanks. Assuming the 3,600lbf is evenly spread over the three rungs, that's 1,200lbf on a 3/4" A36 rung, putting it past yielding if following the flexure limit state per AISC 360 section F11 (15th ed.). I'm curious how the rest of the structure behaves once those rungs pass yielding, and proving it on paper. To prove the rung doesn't rupture, I'm assuming I could just compare the applied stress to its ultimate and call it a day for the rung, but is that an entirely accurate assessment?

 

[JStephen]

We are currently seeing two situations:
-One manufacturer of rail-type systems refers to an OSHA requirement that the ladder be able to handle 500 lbs dropping 18 inches, with no specific load given. We actually did testing to confirm ladder adequacy with our typical details.
-One manufacturer of cable-type systems furnishes online installation instructions that give specific loads to be used. For these loads, we have been beefing up stringers and top two rungs, and using smaller support spacing. I believe this same manufacturer also can furnish rung stiffeners that slip over smaller rungs to boost strength, but we have avoided that on new ladder installation. Some of the hardware can also attach to the top three rungs instead of top two

Historically, I would say there are a LOT of ladders in use where no consideration whatsoever was given to fall-protection loads from these systems.

There are some design issues not clearly addressed- for example, the extent to which stringers are laterally supported against column buckling or lateral buckling by the rungs.

 

[JSgam]

We are currently seeing two situations:
-One manufacturer of rail-type systems refers to an OSHA requirement that the ladder be able to handle 500 lbs dropping 18 inches, with no specific load given. We actually did testing to confirm ladder adequacy with our typical details.
-One manufacturer of cable-type systems furnishes online installation instructions that give specific loads to be used. For these loads, we have been beefing up stringers and top two rungs, and using smaller support spacing. I believe this same manufacturer also can furnish rung stiffeners that slip over smaller rungs to boost strength, but we have avoided that on new ladder installation. Some of the hardware can also attach to the top three rungs instead of top two

Historically, I would say there are a LOT of ladders in use where no consideration whatsoever was given to fall-protection loads from these systems.

There are some design issues not clearly addressed- for example, the extent to which stringers are laterally supported against column buckling or lateral buckling by the rungs.

Thanks JStephen.
-How did the testing go per your typical details? I take it you recommend I do testing as well for different geometries?
-Yeah there's a few things to juggle on my end regarding choosing systems that have two versus three attachment points at the top rungs. We also avoid the rung stiffeners on existing ladders.

I agree with your historical remark. I see it almost every week.

Yeah it's hard to determine whether the rungs actually provide support in that manner. If anything, they've gotta provide SOME support in against buckling in the stringer right?

 

[GeorgeTheCivilEngineer]

If there's a lot of ladders then it could be economic to undertake load testing on the actual ladders/fall arrest system in question.

If it snaps like a twig then the Client has a very visceral sense of why it's not acceptable.

 

[human909]

Regarding buckling.... Many if not most ladders I've seen or engineered are "hung". So buckling under compression isn't really an issue.

Regarding the excessive concern about a single 3/4" round bar being unable to sustain a 1200lb load. You should have a closer look at reality. Steel is quite ductile. If you are concerned about the peak bending moments then look at the plastic behaviour not the elastic behaviour. My quick analysis just now confirms the previous gut feel assertion that 3/4" round bar can sustain 1200lb in a simply supported fashion. Sure there will be plastic deformation, but that is the nature of steel. Plastic deformation is entirely acceptable in fall arrest systems. In fact it often form an integral part of the system as the loads are fundamentally dynamic.

If there's a lot of ladders then it could be economic to undertake load testing on the actual ladders/fall arrest system in question.

If it snaps like a twig then the Client has a very visceral sense of why it's not acceptable.

Really?

Make a decision based on engineering knowledge. Controlled testing quite expensive and time consuming. The fall arrest suppliers have done that. The engineer's job is simply to ensure that the fixtures will not suffer complete failure for the required loads. (Deformation is acceptable.)

Of course this is very much my engineering approach. If another engineer wishes to take a more cautious approach then they are welcome to do so.

Fall arrest loads frequently come up in this forum and part of it is because they are so god damn high. But you need to understand WHY they are that high:

• They are brief instantaneous loads and not static loads
• They are ultimate loads (aka they already have all the load magnification factors and material reduction factors included.) Specifically from a manufacturing perspective if the item typically fails completely at 26kN and has a 3sigma deviation of 1.5kN then it is generally considered suitable

My comments and knowledge has been heavily shaped by my climbing experience. I've relied for decades on fall arrest equipment in my recreation. I've taken falls, seen tests and delved deeply into the physics of it all. There are already SIGNIFICANT safety margins built into the ultimate loads required. Don't go overboard in trying to meet them.

 

[GeorgeTheCivilEngineer]

Make a decision based on engineering knowledge. Controlled testing quite expensive and time consuming. The fall arrest suppliers have done that. The engineer's job is simply to ensure that the fixtures will not suffer complete failure for the required loads. (Deformation is acceptable.)

Conducting a test is applying engineering knowledge. I just think when you're getting into the realms of FEA on a ladder, rigging up a test could be time better spent...

Any code of practise or safety regulation I'm familiar with allows testing as a means to demonstrate suitability.

 

[bones206]

@JSgam thanks for bringing this topic up for discussion. Definitely something that deserves more attention.

From a high-level point of view, I think the critical aspects to the ladders being used for fall arrest are the rung-to-stringer connections and ladder-to-structure connections.

From an analysis perspective, I'd assume the rungs have fixed end connections. Then the rung weld could be checked as a circular fillet weld with an applied shear and out-of-plane moment on the weld group. Theoretically you could apply some dynamic load increase factors, but I would conservatively neglect those and use a typical AISC strength check. The design forces on the weld group should correspond to the plastic moment of the rung increased by some factor to account for dynamic loading and material overstrength.

I'd assume the rungs would have sufficient ductility to strain in bending after yielding. You could check that using energy methods and maybe use a limiting ductility ratio of 20 and limiting support rotation of 10 degrees (from typical blast design criteria). Basically you are checking if the kinetic energy of the fall can be fully absorbed as strain energy in the rungs without exceeding those deformation limits.

As for the ladder-to-structure connection, that seems like a case-by-case siutation.

 

[human909]

Conducting a test is applying engineering knowledge. I just think when you're getting into the realms of FEA on a ladder, rigging up a test could be time better spent...

How so? These caculations can be done in under an hour, testing of a ladder far from it. Destructively testing of a ladder tells provides you information about that single ladder. What does it tell you about the next ladder if it isn't identical. Are we entering into the Calvin&Hobbes world of structural engineering?

As already pointed out numerous times, for these loads we are about talking plastic deformation, the ladder would be expected not to be serviceable after testing.

Any code of practise or safety regulation I'm familiar with allows testing as a means to demonstrate suitability.

This is a structural engineering forum discussing structural engineering. In general we don't test our structures to ultimate capacity. For multiple reasons I should have to explain.

**Regarding fall safety equipment. Most of the items should not be used after being subjected to loads anywhere remotely near the ultimate load, so PROOF testing to ultimate capacity is not an option. However for other items I recognise that it is an option and is in fact required. EG dedicated anchor points in my locality need to be tested regularly, I believe biannually.


Oh and I have presumed during this discussion that the JSgam has been talking about is Lad Saf or a similar system. You could probably ask the manufacturer, though ultimately I'd expect the certification of the item they are fixed to (eg the ladder) would need to be done by suitable person (eg the engineer or other competent person.)

You can even see similar ladders in the video.

We need to remember that probability of the design load of fall arrest systems being reached during use for its intended purpose is vanishingly close to ZERO. These design loads are typically values like 3600lb, 5000lb, 15kN, 22kN, or 25kN depending on your locality and requirement. Actual loads during a fall are generally limited to 3-6kN.
Now I am not saying don't design to the required loads. I'm just saying that don't go overboard. Of course the choices made are up to the individual engineer. But designing a ladder rung strong enough to hold a medium sized car without deformation seems a little overboard to me.

 

[human909]

From a high-level point of view, I think the critical aspects to the ladders being used for fall arrest are the rung-to-stringer connections and ladder-to-structure connections.

From an analysis perspective...

Good summary bones. Though I wouldn't be trying to include too many additional factors like dynamic factors etc as these have already been included in the load rating.

 

[Brad805]

Every ladder I have been on in a plant seems to follow practices outlined in PIP STF05501 Fixed Ladders and Cages. Attachement to the structure is usually more a concern than the plug welded rungs when I have went up them.

 

[JStephen]

I believe the system shown in the video is an older style.
See the manufacturer's instructions here: .https://multimedia.3m.com/mws/media/1522189O/ifu-5908282-lad-saf-flexible-cable-system.pdf
A single-user system applies a dynamic load of 2,700 lbs. This is applied 4.7" from the near edge of the rung.

On the issue of the ladder being "hung"- note that the bracket goes on the top two rungs, so that force is applied at the top of the ladder.

 

[GeorgeTheCivilEngineer]

How so? These caculations can be done in under an hour, testing of a ladder far from it. Destructively testing of a ladder tells provides you information about that single ladder. What does it tell you about the next ladder if it isn't identical. Are we entering into the Calvin&Hobbes world of structural engineering?

As already pointed out numerous times, for these loads we are about talking plastic deformation, the ladder would be expected not to be serviceable after testing.


This is a structural engineering forum discussing structural engineering. In general we don't test our structures to ultimate capacity. For multiple reasons I should have to explain.

**Regarding fall safety equipment. Most of the items should not be used after being subjected to loads anywhere remotely near the ultimate load, so PROOF testing to ultimate capacity is not an option. However for other items I recognise that it is an option and is in fact required. EG dedicated anchor points in my locality need to be tested regularly, I believe biannually.


Oh and I have presumed during this discussion that the JSgam has been talking about is Lad Saf or a similar system. You could probably ask the manufacturer, though ultimately I'd expect the certification of the item they are fixed to (eg the ladder) would need to be done by suitable person (eg the engineer or other competent person.)

You can even see similar ladders in the video.

We need to remember that probability of the design load of fall arrest systems being reached during use for its intended purpose is vanishingly close to ZERO. These design loads are typically values like 3600lb, 5000lb, 15kN, 22kN, or 25kN depending on your locality and requirement. Actual loads during a fall are generally limited to 3-6kN.
Now I am not saying don't design to the required loads. I'm just saying that don't go overboard. Of course the choices made are up to the individual engineer. But designing a ladder rung strong enough to hold a medium sized car without deformation seems a little overboard to me.

We're not talking about design, though, we're talking about assessment. We can make assumptions about the construction of the ladder but unless we have the original specification the assessment is going to be based, at some level, on assumption.

Anyway, I was only proposing it as an alternative if the number and type of ladders involved made it an economic proposal. I wasn't suggesting it was preferable.