FALL ARREST LOAD 3

[siuceric21]

Wanting to get clarification on the proper loading regarding fall arrest anchor points my structure should be designed to.

My take is the standard 5,000 lbs. I have seen this debated on other threads but nobody as noted the 2018 IBC clause 1607.10.4 that I believe provides clarity.

"1607.10.4 Fall arrest and lifeline anchorages. In addition to any other applicable live loads, fall arrest and lifeline anchorages and structural elements that support these anchorages shall be designed for a live load of not less than 3,100 pounds (13.8 kN) for each attached lifeline, in every direction that a fall arrest load can be applied."

Previous versions of IBC (2015), OHSA, ANSI do mention a 900 lb Maximum fall arrest forces (MAF) & can see where that would have been allowed as I do not believe IBC touches on the topic, BUT IBC 2018 seems to take the or clause out with the statement noted above.

Thanks in advance for any input.

 

[SwinnyGG]

In LRFD, the design live load is given as 1.6 LL = 5000 lb. Solving for LL yields LL = 3125 lb. Not sure why they rounded down to 3100 lb. Sig figs, I guess.

I'd agree that's a reasonable interpretation of the history, and explains (at least probably) why the IBC limit is what it is.. but that's based on an assumption.

The real point I'm attempting to make is that if you get into this exact argument with an OSHA inspector while you're standing in the field and a crew is slinging steel, the odds are extremely high that you're going to lose the argument even if you're right.

At that point, it's way more expensive to quick-turn a new design that makes your OSHA inspector happy than it is to just use the "right" load in the initial design.

doesn't mean that the OSHA inspector has seen it before. And what they haven't seen they probably don't understand, and what they don't understand they won't accept.

This is exactly how the story played out in the anecdote I keep using.

The engineer had designed the anchorages according to the anticipated real load, along with a safety factor.

The OSHA inspector's comment was "you didn't design the harnesses, the cables, the carabiners, or the SRLs. That means you didn't design 'a system' which means that part of the code doesn't apply to you. I'll accept your design when it's installed according to a design that uses a 5,000 lb live load, apply whatever safety factors you want"

 

[bones206]

I'd agree that's a reasonable interpretation of the history, and explains (at least probably) why the IBC limit is what it is.. but that's based on an assumption.

Not really an assumption. As Mike Mike stated above, the IBC commentary is pretty explicit. I copied the commentary from the other referenced thread on this topic:

IBC 2018 1607.10.4 Commentary
"Lifeline anchorages, also known as fall arrest anchorages, are called on to resist impact loads when a suspended worker on the face of a building experiences a fall. Because the loads are highly variable depending on the weight of the worker, the fall distance and the energy-absorbing characteristics of the fall arrest system, and because the lifeline is the last defense against a fall, OSHA requires that lifeline anchorages be capable of sustaining without failure an ultimate load of 5,000 pounds (22.2 kN) per person. Using a design live load of 3,100 pounds (13.8 kN), when combined with a live load factor of 1.6, results in a total factored load of 4,960 pounds (22.1 kN), which matches OSHA’s requirements for lifeline anchorages within an acceptable margin of error. The load is used to design the lifeline anchorage and the structural elements that support the anchorage."

 

[JStephen]

In the past, OSHA tended to prescribe required strength or loads without an indication of how they were used or what allowable stresses or safety factors were required. IE, if the support held 5,000 lbs and failed at 5,001 lbs, you were good, and I think this assumption is being followed by IBC in this case.

 

[SwinnyGG]

Not really an assumption. As Mike Mike stated above, the IBC commentary is pretty explicit. I copied the commentary from the other referenced thread on this topic:

The assumption isn't about IBC's code phrase - it's about OSHA's.

I apologize if I'm coming across a certain way; ultimately I agree with the positions being taken by you and others in this thread that the IBC value is rational.

My point continues to be that just because I understand it, and you understand it, doesn't mean an OSHA inspector is going to understand it and approve it.

 

[CrabbyT]

The real point I'm attempting to make is that if you get into this exact argument with an OSHA inspector while you're standing in the field and a crew is slinging steel, the odds are extremely high that you're going to lose the argument even if you're right.

So what you're saying is... nobody wins in a dairy challenge (YouTube Kenny Roger's Jackass, if you haven't seen it)

Here's my rebuttal, and this is how I'd win the dairy challenge. OSHA doesn't actually say anything about what the design load should be; they specify what the connection capacity should be.

1910.140(c)(13)
Anchorages, except window cleaners' belt anchors covered by paragraph (e) of this section, must be:
1910.140(c)(13)(i)
Capable of supporting at least 5,000 pounds (22.2 kN) for each employee attached

That is, with respect to connection capacities,

φF_n ≥ 5000 lb for LRFD; or
F_n/Ω ≥ 5000 lb for ASD

If using LRFD, F_u reverts back to the 1.6 LL and my previous logic from a few posts above works out the same way. That is,

φF_n ≥ F_u
5000 ≥ F_u
5000 ≥ 1.6 LL
5000 / 1.6 ≥ LL
3125 lb ≥ LL

Use LL = 3125 lb.

 

[SwinnyGG]

If using LRFD, Fu reverts back to the 1.6 LL and my previous logic from a few posts above works out the same way.

IF your OSHA inspector either already understands LFRD factors OR is willing to listen to you explain them in detail, sure. You're correct.

Based on my own experience, in my area, this does not and will not happen. Granted, this is location dependent- maybe OSHA inspectors in the areas you and others contributing to this thread operate in are more knowledgeable and/or more willing to listen to an argument in favor of something they initially think is suspect. If all of that is true, no problem. If it isn't true, however, you're exposing yourself to some level of financial risk.

At this point I think my point is made, and I'll quit clapping back at everyone, I promise

 

[phamENG]

SwinnyGG - do you mind sharing what part of NC you live/work in? I spent some time at an industrial facility in eastern NC, and 60% of my tasks were related to fall protection. Granted, I never had the...privilege?...of going to toe to toe with one of the OSHA inspectors. I'd be curious if there's any overlap in the areas where we work - might help me guard against such confrontations in the future.

 

[SwinnyGG]

I'm currently working at a site in Warsaw - it's a little half-horse town right on 40 about half way between Raleigh and Wilmington.

Although really, Denver is home; I'm working on this project out of my employer's Denver division, for a client based in Boulder. There's just a lot more pigs here than in that part of CO, which is a prerequisite for what we're doing.

 

[phamENG]

Good to know. I'm out of Hampton Roads, VA, so Warsaw is a bit of a hike. That's not to say I wouldn't take a project down there, so I'll keep that in mind if I get any industrial work that way in the future.

If I remember correctly, a friend of a friend has a large pig operation down that way.

 

[Mike Mike]

siuseric - to summarize, most the time, in most places, you would be fine to design your structure for a 3100 lb live load, but sometimes, in some places, inspectors who don't understand the very regulations they're charged with enforcing will cost you and your client a lot of time and money, so it's probably better to design for a 5000 lb live load all the time just in case. the extra steel we have to throw at it is just the toll humanity pays for having an entrenched bureaucracy with stipulations so old no one knows where they came from in the first place.

In the past, OSHA tended to prescribe required strength or loads without an indication of how they were used or what allowable stresses or safety factors were required. IE, if the support held 5,000 lbs and failed at 5,001 lbs, you were good

JStephen - that's fascinating. Would you happen to recall where you saw this? Combined with swinnyGG's OSHA inspector quote above, this hints that OSHA's required factor of safety is one.

 

[phamENG]

this hints that OSHA's required factor of safety is one.

I disagree there, since OSHA regs clearly indicate the FoS is a minimum of 2 for designed anchorages.

A few other data points to try to run this down:

1910.140(d) Personal fall arrest systems- the maximum arresting force on the employee has to be limited to 1800 pounds. That's an experiential limit - you have to guarantee that the maximum force will be 1800 pounds or less on the system (otherwise you're just moving the point of death or severe injury from the ground to the end of the tether).

With the exception of the anchor points, all hardware in the system with a minimum strength of 5,000 pounds have wording that suggests this is an ultimate failure load - "Lanyards and vertical lifelines must have a minimum breaking strength of 5,000 pounds." "D-rings, snaphooks, and carabiners must be capable of sustaining a minimum tensile load of 5,000 pounds." There are a few others. BUT - the D-rings, snaphooks, and carabiners only have to sustain a 3600 pound proof load test "without cracking, breaking, or incurring permanent deformation."

The prescriptive version of the anchorage requires it to be capable of supporting at least 5,000 pounds for each employee attached. So I would agree with what JStephen said - if it works at 5,000lbs but fails at 5,001lbs, you would have met the OSHA requirement. You don't need to apply ANOTHER safety factor to the 5,000lbs. In a correct arrangement for a fall arrest system (proper positioning of the anchor point relative to the worker, no obstructions, etc.), a 5,000 lb load on the anchor will already be: severely injuring or killing the worker, permanently deforming the hardware in the fall arrest system, and approaching the breaking strength of the lanyards.

Now, if this is a lifeline system, those loads may approach 5,000 due to the statics of cable support systems, but you still don't need to factor it up again.

 

[bones206]

But once you start applying omega and phi factors on the capacity side of the equation, aren't you applying an additional overall safety factor?

 

[phamENG]

bones - yes. And that's part of the problem with the new IBC requirement. For the case of a worker falling straight down:

The peak load that the worker can experience is 1800lbs. So assuming the system works as intended, that's your design load using Ω = 2.0. (If yield is 36ksi, and with FoS=2 you have a net tensile stress of 18ksi at 1800lbs, then it follows that at 3600lbs you'll be right at the cusp of "no permanent deformation".) If you're doing concrete, then you'll have to work out a good implementation with LRFD. (Keep in mind that even though your design load is 1,800lbs, you should still use hardware that meets the various ratings stated above.)

But now the IBC has come in and put this new load in the mix that is sort of based on the OSHA load but, it seems to me, has been haphazardly and improperly applied. And what is worse, it will likely give designers with little to no experience in fall protection the false sense of security that they can design anchorages for 3100lbs without giving the system more thought. This may be okay for individual D-ring anchors, but what if somebody tries to use it on a horizontal life line system (which, by the way, can be used for multiple works while the IBC language suggests that you only need 3100 per lifeline when that meaning likely refers to lanyards or workers as the OSHA language sets the anchorage requirement per worker) that puts closer to 8 or 9 kips at the wall under the most severe condition?

If they want to make the 3100lbs the absolute minimum, that's fine. But perhaps language more along the lines of: "Fall arrest and lifeline anchorages and structural elements that support these anchorages shall be designed for the live load determined by the analysis of a worker weighing 330lbs falling a distance of 6 feet, but not less than 3,100 pounds for each worker attached to the anchorage, in every direction that a fall arrest load can be applied."

 

[Once20036]

I`m with JStephen above. The new IBC load will control the design of these systems and eliminates some of the options that OSHA allows.

1) As I understand it, the 5000# load already includes a FS=2.0. Therefore, I`ve always used it as an LRFD/factored load, and would "derate" the 5000# by a factor of 1.6 for ASD calcs.

https://www.eng-tips.com/viewthread.cfm?qid=374163

This is in line with the IBC guidance.

2) OSHA 1910 and OSHA 1926 have similar language. They say, "Capable of supporting 5000#... OR... part of a complete system than maintains a factor of safety of 2."
- You can install an assembly of pieces that aren't a complete system, and design for 5000# LRFD (2500# FS=2)
- You can install a complete system that complies with "Class A requirements". Stopping distance is limited to 24", max average arresting force is limited to 1800#, design force is 3600# LRFD (1800# FS=2)
- You can install a complete system that complies with "Class B requirements". Stopping distance is limited to 54", max average arresting force is limited to 900#, design force is 1800# LRFD (900# FS=2)
We've used this understanding in the past to limit the loads from new fall protection points added to an existing structure.

3) This is no longer an option. The design load is 5000# LRFD, per IBC, without any additional conversation.

 

[hotmailbox]

From a safety consultant's drawings of roof tie-back anchors.

 

[Jody Behm]

An anchor point is an essential piece of a fall protection system.
A fall arrest system is your first life defence against injuries from falls.
Choosing the right anchor point for your system is crucial to keeping yourself and your employees safe from falling.
This provides an extra layer of safety and also avoids injuries from falling objects.
All anchor points in positioning systems should be able to support 3,000 pounds. For engineered anchor points, an anchorage should be able to support double the maximum anticipated impact load.