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]

1926.502(d)(15)
Anchorages used for attachment of personal fall arrest equipment shall be independent of any anchorage being used to support or suspend platforms and capable of supporting at least 5,000 pounds (22.2 kN) per employee attached, or shall be designed, installed, and used as follows:

1926.502(d)(15)(i)
as part of a complete personal fall arrest system which maintains a safety factor of at least two; and

1926.502(d)(15)(ii)
under the supervision of a qualified person.

I'd argue that the OSHA requirement supersedes IBC and is clear.

 

[phamENG]

Be careful here. You guys are throwing around prescriptive values. That's fine, if the fall arrest system is being configured in accordance with prescriptive guidelines. If the fall arrest system is being engineered, those values can vary. That's why the 2018 IBC has a "floor" on the anchorage load. Even if your engineering analysis of the fall arrest system shows a lower reaction (which IS possible when you account for deflections and energy dissipation in the system), you still have to provide 3100lbs.

But...engineered systems can also produce reactions much greater than 5000lbs for unique configurations or situations where multiple lines are anchored together. This is uncommon and doesn't fit nicely into the prescriptive cases, but is possible with engineered design.

 

[phamENG]

Also - that 900lb MAF is the force felt by the worker falling, and if I recall correctly is the maximum for belt type harnesses and the preferred max for all, though you can technically go up to 1800 for mid-back d-ring harnesses. Somebody correct me if I'm wrong - I haven't had to touch fall arrest in a year or so.

 

[SwinnyGG]

But...engineered systems can also produce reactions much greater than 5000lbs for unique configurations or situations where multiple lines are anchored together. This is uncommon and doesn't fit nicely into the prescriptive cases, but is possible with engineered design.

I agree that in an engineered fall protection system, the maximum real world load can exceed the values published by OSHA and in IBC (I think that's the point you're making.. correct me if I'm wrong about that).

Despite the fact that forces can exceed those values, OSHA's limit is also a 'floor' on the load capability of an anchor point, which is 5,000 lb per attached device.

All that means is that any anchor for fall pro needs to support 5,000 lb - unless the engineering analysis conducted during the design of that anchor says it needs to support greater than 5,000 lb, in which case that's your new minimum.

 

[siuceric21]

SwinnyGG, thanks for the input. I see your point on the OHSA clause and certainly have read that and agree with the interpretation. My only difference is the IBC clause supersedes the OHSA not the other way around.

 

[JStephen]

I assumed the intent was that 3,100 lbs x 1.6 load factor = 5,000 lbs, which matches the OSHA requirement. That is, I'm assuming the IBC load was derived from the OSHA load.
If I remember right, both OSHA and IBC establish minimum requirements, and where both apply, either may control.
The big point I see in the above is that per OSHA, you have a default minimum load, but can use an engineered system with actual arrest load x a safety factor. But the IBC clause above requires the structure itself to be designed for that fixed load regardless.

 

[phamENG]

SwinnyGG - yes, you interpreted my statement correctly.

The 5,000 is only a floor if using prescriptive design. The components are required to meet certain minimum requirements based on OSHA regs - such as the 5,000lb for the anchorage, but the structure behind it can be less provided the system is properly analyzed for the impact load. That's where the IBC minimum comes in.

 

[phamENG]

There was another post that got no traction at all a couple months ago that brings up some interesting points:

thread507-478399

 

[Mike Mike]

JStephen - that's correct, IBC's commentary makes it clear their intent was to match OSHA, although it only matches if the controlling load combination is 1.2D + 1.6L + 0.5(Lr or S or R).

siuceric21 - I assume your local building code references both IBC and OSHA, so unfortunately your structural design must satisfy both. For better or worse, at least IBC's recent contribution is easy to follow: simply apply a 3100lb live load, no matter what material you're designing, whether ASD or LRFD. Understanding what load OSHA would like you to apply, on the other hand, is a much more difficult undertaking.

Refer to phamENG's link above for the IBC code commentary section and other thoughts

 

[SwinnyGG]

Understanding what load OSHA would like you to apply, on the other hand, is a much more difficult undertaking.

It really isn't... unless you're designing the entire system, that load is 5,000 lb per anchored device.

I typically hate to use anecdotal evidence to support a claim.. but I've had this exact argument in the past with an OSHA inspector in the field on an engineered fall protection system on a sloped roof, which the engineer designed for less than 5,000 lb because someone falling doesn't fall straight down, they slide down the roof.

My opinion was at that time, and still is, that the engineer applied the code provisions correctly and with an appropriate level of consideration for the real world circumstances; however the OSHA inspector was not swayed by calculations on paper except for the fact that those calculations verified the anchor points were good for a value less than 5,000 lb. The job was shut down for a couple of days while new anchors were hastily designed and fabricated, which the engineer wound up doing for free.

Code provisions are what they are... but if I were the one making engineering decisions I'd do the guys in the field a favor and make sure that my anchors calc'ed out with a 5,000 lb load.

My only difference is the IBC clause supersedes the OHSA not the other way around.

Respectfully, I disagree.

IBC applies to whatever is accessible by the public. OSHA applies to anything used by employed staff in the course of their duties. In some cases that's clearly murky water, but in the case of fall protection it's very clear. Unless you expect members of the public to show up to a chemical plant or whatever with fall pro on and ready to use.

Whether you're designing temporary anchors for construction, or permanent anchors for use by facilities personnel, OSHA controls. Look at it this way - if you're designing temp anchors, your building inspector isn't going to care about them but OSHA sure as heck is. If you're designing permanent anchors, your building inspector might care about them one time for 5 seconds; OSHA inspectors are going to care about them for their entire design life.

 

[phamENG]

SwinnyGG - I think the issue is where the "line" gets drawn between OSHA and the structure. OSHA is clear about minimum component strengths. Anchors have to be designed (and often times tested) to resist a minimum of 5000# per attached worker. Regardless of the calculations, that's a hard and fast rule. The issue comes into the design of the structure itself. That 5000 lbs is required at the point of least redundancy and usually point of least energy dissipation. Once you get into the structure - concrete wall, steel beam, whatever - there's no OSHA requirement. As far as the as-written OSHA regs go, the load path essentially stops at the anchorage. After that point, it's up to the designer to ensure the structure can take it.

That's where the IBC provision and Mike Mike's issues come in. There's a semantic variation between OSHA and the IBC and related building codes that makes interpretation and application difficult - especially when using LRFD for design. The OSHA codes are, as best we can tell, based on simple safety factors. LRFD is not. The "safety factor" is broken out into more statistically accurate factors that are intended to account for variability of loads (Load Factors) and variability in material strength, workmanship, etc. (Resistance Factors). This is what Mike Mike means about what to apply (I think). If you plug that 5000 lbs into an LRFD analysis, you end up designing for up to 8000lbs because the highest live load factor is 1.6. But the 5000 isn't a factored LRFD load - as well as any of us can tell, that requirement predates the widespread use of LRFD in the building codes. So the IBC using an adjusted version and shoehorning it into the code probably confuses things more than it helps.

There's a similar problem for ASD, since the safety factors for designing the building are generally different from what OSHA requires for the fall arrest system components. So direct application of component strength capacities can lead to misinterpretation there, too.

 

[SwinnyGG]

I'm familiar with LFRD factors and what they mean...

I honestly think this is a case of analysis paralysis.

If I pay an engineer to design a building and as part of my use case I tell them a certain column in a certain location is going to be subject to a 5,000 lb live load attached in a certain location, they're going to take that live load and factor is appropriately using whatever design methodology they are using.

I am not arguing that the 5,000 lb load is a 'real' value that is correct and reasonable for arresting a real world fall- only that the code describes a reasonably clear requirement to support a load of that magnitude. I think the OSHA section is most definitely behind the times. In a perfect world IBC/OSHA would agree, and their sections would describe the loads in ways that align with current best practices. Unfortunately that's not reality, and until the code changes, we don't get to use a different value.

I understand your point regarding the fact that OSHA and the ICC draw the 'line' in different places. My response to that is that this is just the same as any other case where IBC minimums are superseded by some other project requirement.

For instance.. if you're working on designing a roofing system with a live load of 30 psf, but your owner says 'I need to land a helicopter up there' are you suddenly fraught with concern over whether or not 30 psf is the right loading for the roof? No, you aren't. You already know that that IBC section has been superseded by whatever section covers loading from landing helicopters, either within IBC or somewhere else. You're going to do whatever research you need to do to find applicable codes or loads, and design your roof to accept said rich owner's helicopter.

This situation is no different. OSHA is telling you you've got a 5,000 lb live load per attached device. That load exceeds what is prescribed by IBC (and I agree, the ICC's approach on adding this in didn't help anybody) so it now controls your design.

 

[CrabbyT]

ASCE 7-16 prescribes a 3,100 lb load for fall arrest as well. With LRFD factors, 1.6*3100 = 4960 lb ≈ 5000 lb.

I see this as helpful, as OSHA's requirement was unclear as to whether or not the 5000 lb load was to be considered factored or unfactored (F_u vs φF_u).

 

[bones206]

I'm trying to dumb this down so I can understand it. Tell me if this makes sense:

If you are designing with ASD load combos, use 1.0L = 1.0 x 5,000 lbs (because OSHA 5,000 > ASCE/IBC 3,100).

If you are designing with LRFD combos, use 1.6L = 1.6 x 3,100 lbs per IBC (because OSHA 5,000 ≅ ASCE/IBC 1.6 x 3,100)

 

[CrabbyT]

I'm trying to dumb this down so I can understand it. Tell me if this makes sense:

That makes sense to me. Whichever system you use, the factored load should be 5,000 lb.

 

[SwinnyGG]

I seem to be firmly in the minority.. But I think assuming that the OSHA load requirement is a factored load is a bridge too far. That requirement (at least to my understanding) well predates the structural field moving to LFRD as the primary approach to determine design limits.

And again, anecdotally, an OSHA inspector is not going to know or care about LFRD load factors. He or she is going to want to see 5,000 lb as the live load on which any additional factoring is based. I.e. if you're in LFRD, you're using an 8,000 lb load, not a 3,100 lb load.. that's a big difference.

 

[phamENG]

Anchorages, except window cleaners' belt anchors covered by paragraph (e) of this section, must be:

(i) Capable of supporting at least 5,000 pounds (22.2kN) for each employee attached; or

(ii) Designed, installed, and used, under the supervision of a qualified person, as part of a complete personal fall protection system that maintains a safety factor of at least two.

(Emphasis mine)

OSHA's regs allow you to go either way. Install everything prescriptively with the 5,000lb load, or be an engineer and design it for the actual loads with FoS=2. Though I will say that even when I design fall protection systems that result in anchorage loads less than 5,000lbs, all of the hardware has that as a minimum rating.

SwinnyGG's anecdote is a good cautionary tale, though. Just because the regs allow you to do it that way 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. Then you're faced with a) redesigning and installing or b) fighting your way through the bureaucracy to get an opinion letter from higher up the chain - which could take a long time while the facility is unable to operate.

 

[phamENG]

For construction, the companion fall protection reg (which says the same thing) is in 1926.502(d)(15)

 

[CrabbyT]

That requirement (at least to my understanding) well predates the structural field moving to LFRD as the primary approach to determine design limits.

The 5000 lb requirement was probably created in a time where ASD was the norm and gravity loads went unfactored (or rather, factored with a multiplier of 1.0). Therefore, I would argue that the design load (i.e. the factored load) was intended to be 5,000 lb.

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.