FALL ARREST LOAD (ASCE and OSHA) 6

[nameSrp]

I have a question about fall arrest load as per ASCE and OSHA.

Why does the fall arrest load need to be 13.8 kN? Is it too much for a human's weight?
Can I use only 1 or 2 kN for a human's weight? (I have to design a fall arrest system on a metal sheet roof top, and it seems impossible to use 13.8 kN as the fall arrest load)

Thank you in advance.

 

[JStephen]

The dynamic load from stopping a falling mass in a relatively short distance is higher than the weight of the mass, thus the increase.
I assume that load is 5,000 lbs per OSHA divided by 1.67 to convert from ultimate load to service-level load.
Under OSHA, where you have a fall protection system designed in such a way that it is known to generate lower forces, that loading can be reduced.
Not sure if that's incorporated into ASCE 7 or not.
It is not necessarily a requirement that the anchorage or attached structure be undamaged by the load.
There should be a number of other threads on the topic.

Also see:

https://www.structuremag.org/?p=21620

 

[human909]

Sure you can use 1kN. As long as you the designer is willing to take the first (and last) fall on that 1kN restraint.

 

[GeorgeTheCivilEngineer]

12kN in the UK. 6kN being the most a human body should be taking (and is set out in the fall arrest standards) with a factor of safety of 2.

That is a vital factor of safety as often people fall as part of a wider issue.

If the 12kN / 13.8kN load is too large then you need to consider a system of anchors or using an alternative fall from height protection system (edge protection / work restraint / nets or air bags etc)

 

[Geoff13]

The article linked by JStephen is excellent. I'll be keeping that link handy.

In particular the "Hierarchy of Anchorage Design" says everything I was thinking about saying, and more. You can design for lower values, but make sure you are trained in fall arrest system design and understand the risks / controls associated with lower design loads.

Every country seems to have their own rules about fall arrest design and required anchor loads, so research what rules apply where this anchor will be used.

 

[dik]

There seems to be a link failure on JStephan's link.

Can you re-post a link to the article?

Thanks...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JStephen]

I think that part of the website is just temporarily down, check it later.
I may have the printed version of the article, not sure about that.

 

[Geoff13]

If the site doesn't come back, I downloaded the article in PDF format and will post.

 

[bhiggins]

Don't skimp on fall arrest safety, just not worth it.

 

[JStephen]

Link above is working again.

 

[bones206]

There was a rebuttal letter published a couple months ago that is very good as well:

https://www.structuremag.org/?p=23372

This paragraph in particular influenced my standard of care for design of fall protection anchorage:

Finally, although the designer could attempt to design some anchorages for 5,000 pounds and other anchorages for smaller forces, it is important to remember that workers typically cannot look at an anchorage years after a building is constructed and reverse-engineer the design to determine the original design criteria. Consequently, it generally makes sense to design anchorages for all purposes for which they may be used over their lifetime. In short, if it looks like an anchorage to which one could connect a lifeline or rope descent system line, it may very well be used for that purpose, no matter what the designer intended at the time of design.

For these reasons, unless the anchorages are only being used during construction of a building, we believe that most anchorages likely need to be designed for a minimum factored load of 5,000 pounds.

 

[JLNJ]

I would consider using the extraordinary event load combination.

In most existing (and many new) buildings we are trying to make things work reasonably and not find a reason to needlessly spend the Owner's money.

 

[dik]

got it... thanks JStephen

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[MacGruber22]

We design these systems as part of major facade remediation and roofing replacement projects. The major hurdle is convincing owners to have a permanent system installed. You seem to have made it past that difficult part, though it is not clear to me what your role is - EOR, component designer, or both? Regardless, the time worth spending is on optimizing the anchor layout for the intended use, not arguing over whether the anchors need to be designed for 5000 lbs and what that precisely means. The $$ per anchor isn't going to change much so the parsing of the code language is not worth it. I don't know that person from 3M, but something tells me she never has actually had to take responsibility with her seal on a roof anchor system design as EOR. Erring on the side of non-conservatism when faced with vague code standards is inviting liability when a fall event occurs. I have no interest in this when most of the designs I do involve post-installed adhesive anchors, many times in overhead installations in balconies and eyebrow slabs.

I do not know exactly what you are designing for, but be careful and think through the system that is needed (if that is your role)! Do not forget about the more realistic loading that can occur - swingstage scaffolding. If the anchors *could* be used for both fall restraint lines and as a direct tie back for swing stages, then you need to adhere to the OSHA safety factors for them. If it is a masonry building that would eventually use swing stages for facade remediation, the maximum weights need to be carefully considered along with the width of swing drops, etc. The building owner needs to be provided an operations plan that shows how the system is to be used and all of the limitations.

https://www.osha.gov/etools/scaffolding/suspended/two-point[/URL]]Each suspension rope, including connecting hardware, must be capable of supporting, without failure, at least 6 times the maximum intended load applied to that rope while the scaffold is operating at the greater of either [29 CFR 1926.451(a)(4)]:
The rated load of the hoist.
Or 2 times the stall load of the hoist.

It is not necessarily a requirement that the anchorage or attached structure be undamaged by the load.

Again, this is all well and good coming from people who are not the engineer responsible for designing and overseeing the testing, install, and certification of these systems, often times in older buildings. It is also not a good idea given that it leaves you with little standard by which to certify the in-situ load-carrying capacity. You need to connect the dots between the required load and capacity - much easier to do this with straightforward capacity calculations. Not having a standardized calculation requires knowing the failure resistance via testing independent of the required proof load testing during installation (which is a headache in of itself).

It is not worth trying to dart around the code language. Meet your loading requirement no less than 5000 lbs ultimate and focus on a useful and efficient layout.

-Mac

 

[NorthCivil]

I've seen some super dodgy anchors that claim to offer 15kN resistance, when fixed into 0.55mm steel long run roofing sheet.

They claim to have done a bunch of testing but wont show any evidence.

Still, they find engineers around here to sign off on them.

stainless steel plates rivetted to the roof sheet with about 8 rivets. I would never tie off to one of those.

 

[human909]

I've seen some super dodgy anchors that claim to offer 15kN resistance, when fixed into 0.55mm steel long run roofing sheet.

Could you elaborate on your engineering concerns? Or are you simply uncomfortable with the idea, uncomfortable with heights, or uncomfortable trusting somebody else's engineering in this case.

I ask because I can see how it can be made to work suitably. I say this as an engineer and also a user of fall arrest equipment.

This only needs 8 rivets into 0.42mm sheet.

https://sayfa.com.au/products/anchors/

 

[dik]

Neat... do they have one for 22KN?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[bones206]

This is a fun watch:

https://www.youtube.com/watch?v=4Fc6LPB01kE&ab_channel=AswanMurwana

I watched a guy pass out from the heat and slide off the edge of a roof. He was tied off to a lifeline and the set screw clamp anchorage device held (barely) but really tore up the standing seam roofing we had just put on the day before. He was a big fella though

 

[MacGruber22]

Bones - Yikes. I hate the stiffness of that "base plate". Not sure if it is helping to absorb energy or hurting by causing serious second order forces on the fasteners.

-Mac

 

[human909]

Bones - Yikes. I hate the stiffness of that "base plate". Not sure if it is helping to absorb energy or hurting by causing serious second order forces on the fasteners.

Absolutely helping.

When it comes to catching falls it is 90% about energy absorption. You can readily exceed 22kN with an 80kg mass if you don't have suitable energy absorption. With suitable absorption under 3kN is realistic. That said, the plate that I linked has built in energy absorption without adding much to the moment applied to the plate.

(Due to an attachment getting caught I have taken a fall onto a fixed line without and absorber observing what broke would indicate the forces exceeded 4kN and could have been higher had energy not been absorb in the breakage. 4kN+ might not sound like much but my body was on the other end of that 4kN!)

Neat... do they have one for 22KN?

A quick perusal of what they offer and the answer seems to be no. All 15kN as they are made for the Australian market, I guess gravity is weaker down here.