5000 lb fall arrest load - Safety Factors 1

[darthsoilsguy2]

If you have add a fall protection point on a steel frame and you check it with LRFD... Does the anchor point become a live load that needs to get factored like a regular live load? Or is the 5000# OSHA number already a factored number plugged into the dead/snow/live load equations without any additional factors? I've been thinking 5000# as it's own final number and had never questioned it until a coworker asked.... but now i'm wondering if there is info out there to clarify. thanks

 

[Lomarandil]

The 5000# specified by OSHA for fall protection loads is already at an ultimate level and to be compared to the ultimate capacity of the system. It does not require an additional live load factor. It could even be argued that it may not require concurrent evaluation with other types of transient loading, depending on your situation.

----
just call me Lo.

 

[human909]

Yoyr thinking seems correct, though it probably depends on your code.

But a good start would be to go back to where the load factors come from. As I understand it load factors are probabilistic and due to uncertainties in calculating the live load.

In contrast the a 5000pound figure isn't an uncertainty it is likely a threshold requirement. (I say likely because I don't know your code.) To put in in perspective there will normally be something else in your fall protection system that will break before 5000pounds is reached. I wouldn't like to be on the person in a harness on the other end of that 5000pound load!

(I have fallen in a harness with an estimated force of 600-1000pounds and that was bad enough!)

 

[Geoff13]

The majority of fall-arrest lanyards and SRL's have a Maximum Arrest Force (MAF) of 900 lb. A few lanyards (such as 12' free-fall lanyards) will have a MAF of 1,800 lb. I believe OSHA limits the arrest force on the worker to 1,800 lb.

The 5,000 lb anchor load already includes a significant load factor. No need for you to add more.

 

[phamENG]

I don't think OSHA has jumped on the LRFD bandwagon, so it's tough to apply it directly.

One thing to point out - the 5000# rule is a prescriptive requirement for non-engineered systems. If designed by a qualified person, the system can be designed for the maximum fall arrest load with a factor of safety equal to 2. In most cases, the fall arrest load is limited to 900#, but in some instances I believe it can go as high as 1800# if the proper equipment is used.

If it is reasonable to assume that a significant amount of other load will be present on the structure at the time of the fall arrest loading, it is prudent to combine it with the other loads. For instance - if it's a rooftop tie off, it'll only be loaded when there are workers on the roof. The roof live load only occurs when workers on the roof, so it makes sense to combine them. If you're looking at a crane rail maintenance catwalk and the crane is locked out while on the catwalk - no sense in including live loads from the crane with the fall arrest load.

To what level you factor probably requires a conversation with the owner. Do they care if they have to replace the tie-off and/or supporting structure if the anchor is used? If it's a rooftop tie off, probably not. It could get expensive to tear the roof apart and replace a beam or pair of joists. So I'd use it as a roof live load, but apply it separately - 1.2D+1.6R[sub]L[/sub]+2.0FA. If it's easily accessible and the owner is ok with making it and the supporting structure "disposable" I might compare the calculated load effects to a phi=1.0, or if the situation warranted it maybe even applying anticipated strength factors (R[sub]y[/sub]=1.1 or 1.3, etc.) to predict actual failure. That would be an incredibly rare case and I can't think of a reason to do it off the top of my head, but I'm not sure I'd rule it out every time either.

Here are a couple of links from "fall protection experts" that may help some. There are also lots of threads here that cover the same topic.

[URL unfurl="true"]https://www.rigidlifelines.com/blog/entry/much-ado-aabout-safety-factors-of-two#[/url]
[URL unfurl="true"]https://simplifiedsafety.com/blog/the-myth-of-the-5000-lb-anchor-point/[/url]

 

[JLNJ]

Fall protection design loads is one of those things where if you ask three different engineers you get three different answers.

ASCE 7 has an "Extraordinary Event" load combination. I think it would be appropriate to use for a (hopefully extremely unlikely) fall protection event which lasts a split second.

If you are the EOR, you need to decide for yourself which loads to use and how to combine them. OSHA rules are not written like ASCE 7 provisions. You need to peer through engineering lenses at the OSHA regs, devine the intent, and design accordingly. You want to be conservative enough to be safe, but not so conservative that you are telling every Owner that his building is going to collapse if someone ties off from the roof.

 

[human909]

One thing to point out - the 5000# rule is a prescriptive requirement for non-engineered systems. If designed by a qualified person, the system can be designed for the maximum fall arrest load with a factor of safety equal to 2. In most cases, the fall arrest load is limited to 900#, but in some instances I believe it can go as high as 1800# if the proper equipment is used.

And it can go higher if equipment isn't used correctly. If I was working at heights and saw some 'engineered system' only rated to 900pounds I would refuse to work and if possible throw my steel karabiner rated over 10,000pounds at his head.

There is a reason why fall arrest systems across the world are almost always rated over 20kN (The exact amount varies by code and item). Even 1800pounds is relatively easy to exceed with a solid object dropped a couple of meters.

 

[Lomarandil]

There's a difference between the fall protection system rating, and the maximum arresting force.

A good fall protection harness and lanyard will reduce the peak force applied to the user during a fall (when worn and tied off according to instructions) to no more than 900/1800lbf. (other MAFs exist, but those are the common thresholds.). They do this by spreading the required total deceleration over a longer time period and smoothing out the peak. As somebody else mentioned, even 900lbf is pretty darn uncomfortable.

That's not to say that the harness & lanyard are rated for that 900/1800lbf. They're much stronger than that. (Life safety FOS, margin for improper use, etc).

----
just call me Lo.

 

[human909]

Agree. The nominal value is meant to be simple. 22kN, 5000lbs. Don't mess with that.

Here are a couple of links from "fall protection experts" that may help some. There are also lots of threads here that cover the same topic.

https://www.rigidlifelines.com/blog/entry/much-ado-aabout-safety-factors-of-two#

https://simplifiedsafety.com/blog/the-myth-of-the-5000-lb-anchor-point/

I'm not excited by that thought process. Something seems a bit amiss when the connection I'm attached to is only rated for 1800pounds when my equipment is rated for 5000pounds.

 

[r13]

The explanation below provides (quite interesting) insight to the OSHA 5000# per person anchorage force requirement. Also note that the force includes a safety factor of 2.

There are various theories as to why OSHA settled on 5,000 pounds. One is that 5,000 pounds is twice the total forces incurred in the free fall of an average worker. A macabre explanation is that the weight was determined by dropping dogs from heights during testing (many years ago). Nevertheless, the regulations have been set forth for decades. In fact, when testing systems under OSHA, you are required to use a weight of 220 pounds (plus or minus 3 pounds) at a free fall of 6 feet [29 CFR 1926 Subpart M Appendix C].
The system would fail the force test if it records greater than 2,520 pounds of force during this test. If the system passes, then, a safety factor of two applied to 2,520 pounds would be approximately 5,000 pounds. However, 5,000 pounds is where many people stop reading. The "or" clause in the above standard often gets missed or ignored. But why does it matter?

It matters because the forces incurred during a fall of a 220-pound worker who is utilizing a fall arrest system could be between 900 and 1,800 pounds, not 2,500 pounds. Be mindful that fall arrest systems would have some type of deceleration device that engages after free fall and reduces the force. If we look at the "or" clause in the standard and apply the safety factor of two, it means that in this scenario our anchor point may only need to support 1,800 pounds of force.

 

[human909]

Most of the type a structural engineer isn't designing the fall arrest system nor are they enforcing the use of the fall arrest system. So deviating from the nominated required anchorage load is risky and will likely not be appreciated by the actual users of the anchorage.

 

[Tomfh]

For comparison purposes the Australian values are 1.5kN (3400lbs) for one person anchorages and 2.1kN (4700lbs) for two person Anchorages.

I remember our OHS/anchor consultant emphasising the 1.5kn figure when considering Anchorages. He said when doing ad hoc anchorages on site that you need to ensure the anchorage can carry the static load of a car. I’ve used that rule or thumb myself a few times when working up high. One time I even tied the line to my car on the other side of the building as it was easier than anything else nearby.

 

[Mike Mike]

I agree with Lomarandil, JLNJ, and Geoff.

According to IBC 2018, the live load you should apply is 3,100, because the 1.6 LRFD live load factor takes you up to around 5,000:

"1607.10.4 Fall arrest and lifeline anchorages. In addition to
any other applicable live loads, fall arrest and lifeline anchor-ages
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."

Commentary states:

"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."

However, in my opinion ICC has dropped the ball here. The word "ultimate" unfortunately has two (or maybe even more) meanings in structural engineering. It's used to refer to both the combined LRFD load, and the strength determined by testing. For example, Simpson says their 3/8" Titen HDs are tested to an ultimate load of 2,390lbs in CMU, but the allowable load is only 480lbs for use in ASD design (safety factor of 5). The 2,390lb ultimate load has nothing to do with LRFD, and I would argue OSHA's 5,000lb ultimate load has nothing to do with LRFD either. ICC has misinterpreted OSHA's use of the word to mean LRFD factored load, and has assumed the controlling load combination is 1.2D + 1.6L + 0.5(Lr or S or R).

Question: Setting aside the IBC for a sec and looking at OSHA, let's say Darth is designing steel bolts with a phi factor of 0.75 and an Ry of 1.1 (just throwing some numbers out there), doesn't Darth already have a safety factor of 1/0.75 * 1.1 = 1.47 built into the capacity side alone? If OSHA limits loads applied to human beings to 1,800lbs, isn't Darth's LRFD factored design load (2.0/1.47) * 1,800lbs = 2,450lbs? Why are we applying the full OSHA required factor of 2 to the load side, plus applying additional safety factors on the capacity side?