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OSHA Fall Arrest System
2

OSHA Fall Arrest System

OSHA Fall Arrest System

(OP)
I have been asked to design a fall arrest system based on something a contractor has used in the past or maybe he just found this picture online. I have not been provided any information as far as why type of lanyards will be used etc. so I was going to design for the minimum 5000 lb load. However, this particular device was attached to the a web member of the wood roof trusses. I am concerned that during a fall event that the gang nail plates will not be sufficient to resist the lateral force at the ridge. On my specific project, the roof trusses at the peak have a single king post in the center, rather than two sloping web members. Using the bracket in the supplied picture, my king post gang nail plate would be put in shear, which I am uncertain how it would perform. I tried doing a google search for fall arrest systems thinking this was a picture of a proprietary system, if it is, I was unable to find it. Am I the only one that thinks this looks a little questionable? I am thinking it would be better to move the anchorage to the top chord, then if a fall occured, the bracket would be putting the truss top chord into compression like it is designed for already. I just finished reviewing the truss shop drawings and there were some revisions to be made so the trusses are not fabricated yet, so getting some of the trusses beefed up to support this falling load could still be done. Has anyone else designed some fall arrest anchorage like is shown in the picture?

RE: OSHA Fall Arrest System

Gotta think about this one.

it doesn't look right. 5000 lbs is a 'bureaucratic" rule i know, but what ever yo are hooking a man tyo hav=s to follow the "law' even if that "law" is too high. Regardless, this s=doesn't look right. A nailed in-place split angle iron wrapped around a 2x4 holding a 1-1/2 pipe welded to an angle iron nailed to the next 2x4?

RE: OSHA Fall Arrest System

I doubt this will work by the numbers. The steel is surely fine but I don't see the nailed connection or the truss web working. A truss might be designed to carry 2500 lbs total. Asking one web member to deal with 2500 lbs shear is a lot. That's a Toyota Corolla worth of weight. The load has to be assumed to act in any direction, right?

I will concede that, given wood trusses as a starting point, this is a pretty robust system, relatively speaking.
 

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: OSHA Fall Arrest System

Jeffhed:
Actually, I think the device you show in the picture is installed up-side-down. The stl. end flgs. should be on top of the wood diag. members, so that you are not counting only on the shear/bearing of the 10 nails in the side grain of the wooden member, thus significantly improving its bearing and support cap’y. Furthermore, the wire rope cable should not be bent over the stl. locating/centering loop when the cable is under load, in a fall. That is, the locating loop should be on the high side of the cross pipe, which will happen when it is properly installed. Obviously, the wooden members must be able to support the design load, and that may be in question here. The system can fail at 5001 lbs., or you can use a lanyard which has been tested and proven to apply a lesser load in a fall.

RE: OSHA Fall Arrest System

Whatever you do here, it is bound to be better than the fall arrest systems I see in use on most wood frame residential construction sites- that would be NOTHING.

RE: OSHA Fall Arrest System

Racook, let me quibble a bit. If by "bureaucratic" you meant arbitrary then I don't think it is, though I also don't think that's what you meant.

My understanding is that the 5,000# number is based off of stopping a 200# man who had fallen 6' and stopping him over a distance of 6" using energy-dissipating fall-arrest gear. Oh, and with a factor of safety of 2 embedded within it.

I agree with dhengr that it looks like it might be supposed to be installed in the other direction.

jeffhed, I'm certainly no expert on the topic but I do think that the 5,000# requirement is from OSHA. So, if you're being asked to sign off on it you may want to make sure that it satisfies that criteria...and I have my doubts as to whether the 2x chords or that nailer plate will...

But...again, I'm not sure what the regulations actually are, or if there are different ones governing residential work. Good luck with it.

RE: OSHA Fall Arrest System

The 5000 lb load was determined I feel somewhat arbitrarily however I do believe it is a FOS of 3. A person dies when they are subjected to an arresting force of around 1600 lbs. Multiply that number by 3 and boom, basically 5000lbs.

If he is going to use special shock absorbing lanyards I believe the load can be reduced a certain amount.

RE: OSHA Fall Arrest System

OSHA = 5K lbs without engineering calc's. It is indeed a 'rule of thumb' and this number is not

RE: OSHA Fall Arrest System

OSHA = 5K lbs without engineering calc's. It is indeed a 'rule of thumb' and this number is not an 'engineering ' number. This is the 'Withstand' rating, thus the anchor is allowed to fail at 5100 lbs. For design purposes, use something like 1.5 to 2kips for the engineered value. Besides the fact that we die at something around 1.6 kips, all these systems are required to have some king of shock arrestor or deceleration device that limits the instantaneous loading to 900(?) lbs. Thus your anchor(s) will only see the resultant force from that 900 lbs.

RE: OSHA Fall Arrest System


As for the OSHA 5000 lb fall arrest load, it caused serious argument/debate in agricultural industry. Basically, it is determined within agricultural industry that it will be mandatory for all grain storage silos to have fall arrest system installed in future. As I understand, after a series of meetings/conferences, it is decided/determined that agricultural industry will setup its own standard of a 1800 lb fall arrest load (most probably it will appear in near future ASABE Standard), and seems like agricultural industry will not adopt OSHA 5000 lb fall arrest load in silo fall arrest system design.

RE: OSHA Fall Arrest System

I have seen written in OSHA that the 5000 lbs is for non-engineered applications, which I find odd because determining the capacity usually requires some engineering....unless its based purely on trial and error and testing. My personal opinion on the matter is that providing a design for 5000 lbs also adds robustness to a permanent anchor which will see wear and corrosion and may not be readily accessible for annual inspections (if the building manager keeps up with the annual inspection requirement at all). For that reason I would never design or specify a permanent anchor for less than 5000 lbs. Note that Canadian codes require that the anchor must be designed so that there is no permanent deformation at 2500lbs (yield) and no failure at 5000lbs (ultimate).

If it is engineered, in Canada, I believe the requirement is to design to 2xMAF (maximum arrest force). The typical lanyard will reduce the MAF to 900 lbs. In your configuration, I would feel comfortable designing to 1800 lbs and specifying that the anchor point must be used with a lanyard that reduces the MAF to 900lbs.

I understand your concerns about the lateral force on the gusset plate, but looking at your picture it looks like the web is bearing on the adjacent web....so the gusset plate may not be acting alone in resisting the fall arrest force.

RE: OSHA Fall Arrest System


Quote (Archie264 (Structural) )

31 Oct 14 10:23
Racook, let me quibble a bit. If by "bureaucratic" you meant arbitrary then I don't think it is, though I also don't think that's what you meant.

My understanding is that the 5,000# number is based off of stopping a 200# man who had fallen 6' and stopping him over a distance of 6" using energy-dissipating fall-arrest gear. Oh, and with a factor of safety of 2 embedded within it.

Yes, your summary - and the additional comments that followed from everybody else! - are why I used the "bureaucratic" term in my very-poorly typed reply....

RE: OSHA Fall Arrest System

2
Well, as Daffy Duck said, it’s not the fall that gets you, it’s the sudden stop.

Duwe6, I wouldn’t be too quick to think that the number doesn’t have calculations behind it. OSHA may claim that the 5,000# is arbitrary but it strangely correlates to the 200#/6’/6” parameters that I’m told they also require. Maybe they claim it’s arbitrary so no one will key on to the implicit factor of safety being only 2.

From physics class a falling object obeys the following:

a = g

v = gt

d = .5gt^2

therefore t = sqrt(2d/g)

So, @ d = 6’, t = sqrt[(2)(6)/32.2] = .610467 sec

v @ t = .610467 sec = (32.2)(.610467) = 19.657 ft/sec

using that…

E = .5mv^2 = (.5)(200/32.2)(19.657 ^2) = 1,200 ft-lbs

Setting energy = work:

E = Fd

Solving for d = 6” = .5’:

F = 1,200/.5 = 2,400#

With a safety factor of 2 that becomes 4,800# ~ 5,000#

But, if da dadburn gob’ment says it’s arbitrary I believe ‘em. Just like IRS audits…



RE: OSHA Fall Arrest System

Irrespective of the fastener design for fall arrest, the thing that gets me in the picture is the worn hole through the OSB sheathing above. How much damage was done to the rope restraint during construction?

The systems I have dealt with in the past are permanent and through the roof with a sealed penetration, not temporary systems as this one obviously is, although it is still nailed in place.

Mike McCann, PE, SE (WA)


RE: OSHA Fall Arrest System

(OP)
I understand the 5000 lbs is a minimum design force unless your are using other fall equipment that limits the force imparted to the person to 900 lbs or 1800 lbs. Then you can design the anchorage for 900X2=1800 lbs or 1800X2=3600 lbs. However, this information has not been provided to me, so that's why I am going with the 5000 lb force.

Msquared,
This will NOT be a permanent fixture to be used multiple times, it is a sacrificial anchorage that will be for construction only and then will be left in place. Looking online I have seen the systems you mention that bolt to the truss top chord and stick through the roof. I was thinking along those same lines by maybe taking the pictured anchorage device and moving it to the top chord so the fall will direct forces through the top chord of the truss. I am still not sure how that would affect the truss gang nail plates at the ridge. If there is sheathing nailed to the truss, maybe it doesn't affect the gang nail plates at all. But since this could save someones life, I want to be sure.

RE: OSHA Fall Arrest System

I was a truss designer in a previous life. Here's my take on your concerns regarding the connection plates:

1) The likely loading will be a force parallel to the truss chord. The top chord peak splices are generally fairly week because designers count on the two top chords butting together in compression. The plate there mostly just tension ties the chords down to the king post. Heel connections tend to be very strong however. A probable load path would be the top chord dragging the load down into the heel joint.

2) I was under the impression that the OSHA load needed to be applied in any direction. In this case, your main concern would be a load applied perpendicular to the truss top chord. 5 kip is a big load for that. The issue may have less to do with the strength of the plate than the tendency of the plate to tear a chunk of wood out of the chords in tension perpendicular to grain style.

3) I don't see sheathing helping much.

4) I like your idea of attaching to the top chord, for exactly the reasons that you outlined. It's the most capable place for the expected load.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: OSHA Fall Arrest System

After I re-read the calculation of Archie264's post, I found that before we discuss this issue we need clarify one thing, i.e. what this 5000 lb means? If this 5000 lb means the ultimate strength of the fall arrest system, i.e. to design the fall arrest system fail at just over 5000 lb(say 5001 lb), then I would say the agricultural industry standard is not very far from 5000 lb standard because the agricultural industry requirement of 1800 lb is the required design force, i.e. with a safety factor of 2 (same safety factor as Archie264's post), the fall arrest system can be design to fail at just over 3600 lb(say 3601 lb), Apparently, 5000 lb in Archie264's post has different meaning with jeffhed (Structural) (OP)'s meaning, in which, jeffhed (Structural) (OP)’s 5000 lb means a minimum design force, i.e. with a same safety factor of 2, the fall arrest system has to be designed to fail at just over 10000 lb(say 10001 lb).

I think it is critical to clarify what this 5000 lb exactly means.

RE: OSHA Fall Arrest System

What is the height of the structure? I typically do not see fall arrest systems until the building is 3+ stories. Below that, contractors can use portable ladders for access and windows can be washed from the ground.

RE: OSHA Fall Arrest System

Chris, I don't know; I'm sure no expert - or even novice - on the topic. But, as it was told to me OSHA requires the 5,000# capacity for the anchorage of the fall arrest system. I suppose then on top of that are the safety factors embedded within various proprietary anchorage systems' published capacities. I suppose the philosophy is that if the anchorage can take the 5,000# force then I suppose there is a degree of confidence that the anchorage could support a 200#/6'/6" fall arrest system. That's all supposition on my part and not one on which anyone should bet his life. The parameters of any life safety system are best verified for one's self.

RE: OSHA Fall Arrest System

(OP)
KootK
Yes, the force is supposed to be in any direction. I was thinking of providing anchorage on each side, but that doesn't mean that guys will tie off to the correct side. The best would be to locate the anchorage at the ridge with the attachment protruding through the ridge. That way I could provide sufficient anchorage for all directions. The top chords resist the fall force parallel to the roof and the king post directs forces through the rest of the truss. I could design a large plate that could tie into the king post and both top chords and anchor it to the trusses with Simpson SDS screws.

chrislaope,
I think the 5000 lb force includes the factor of safety, and I believe it is ultimate. However, regardless of the level of force, there is still the question of what happens to the truss.

steellion,
This building is 3 and 4 stories tall. That is why the fall arrest anchorage is needed.

RE: OSHA Fall Arrest System

(OP)
This is the info from OSHA's website on fall protection system anchorage. I have highlighted the anchorage load requirements.

Personal Fall Arrest Systems

These consist of an anchorage, connectors, and a body belt or body harness and may include a deceleration device, lifeline, or suitable combinations. If a personal fall arrest system is used for fall protection, it must do the following:

Limit maximum arresting force on an employee to 900 pounds (4 kilonewtons) when used with a body belt;
Limit maximum arresting force on an employee to 1,800 pounds (8 kilonewtons) when used with a body harness;
Be rigged so that an employee can neither free fall more than 6 feet (1.8 meters) nor contact any lower level;
Bring an employee to a complete stop and limit maximum deceleration distance an employee travels to 3.5 feet (1.07 meters); and
Have sufficient strength to withstand twice the potential impact energy of an employee free falling a distance of 6 feet (1.8 meters) or the free fall distance permitted by the system, whichever is less.

As of January 1, 1998, the use of a body belt for fall arrest is prohibited.

Personal fall arrest systems must be inspected prior to each use for wear damage, and other deterioration. Defective components must be removed from service. Dee-rings and snaphooks must have a minimum tensile strength of 5,000 pounds (22.2 kilonewtons). Dee-rings and snaphooks shall be proof-tested to a minimum tensile load of 3,600 pounds (16 kilonewtons) without cracking, breaking, or suffering permanent deformation.

Snaphooks shall be sized to be compatible with the member to which they will be connected, or shall be of a locking configuration.

Unless the snaphook is a locking type and designed for the following connections, they shall not be engaged (a) directly to webbing, rope or wire rope; (b) to each other; (c) to a dee-ring to which another snaphook or other connecter is attached; (d) to a horizontal lifeline; or (e) to any object incompatible in shape or dimension relative to the snaphook, thereby causing the connected object to depress the snaphook keeper and release unintentionally.

OSHA considers a hook to be compatible when the diameter of the dee-ring to which the snaphook is attached is greater than the inside length of the snaphook when measured from the bottom (hinged end) of the snaphook keeper to the inside curve of the top of the snaphook. Thus, no matter how the dee-ring is positioned or moved (rolls) with the snaphook attached, the dee-ring cannot touch the outside of the keeper, thus depressing it open. As of January 1, 1998, the use of nonlocking snaphooks is prohibited.

On suspended scaffolds or similar work platforms with horizontal lifelines that may become vertical lifelines, the devices used to connect to a horizontal lifeline shall be capable of locking in both directions on the lifeline.

Horizontal lifelines shall be designed, installed, and used under the supervision of a qualified person, as part of a complete personal fall arrest system that maintains a safety factor of at least two. Lifelines shall be protected against being cut or abraded.

Self-retracting lifelines and lanyards that automatically limit free fall distance to 2 feet (0.6 l meters) or less shall be capable of sustaining a minimum tensile load of 3,000 pounds (13.3 kilonewtons) applied to the device with the lifeline or lanyard in the fully extended position.

Self-retracting lifelines and lanyards that do not limit free fall distance to 2 feet (0.61 meters) or less, ripstitch lanyards, and tearing and deforming lanyards shall be capable of sustaining a minimum tensile load of 5,000 pounds (22.2 kilonewtons) applied to the device with the lifeline or lanyard in the fully extended position.

Ropes and straps (webbing) used in lanyards, lifelines, and strength components of body belts and body harnesses shall be made of synthetic fibers.

Anchorages shall be designed, installed, and used under the supervision of a qualified person, as part of a complete personal fall arrest system that maintains a safety factor of at least two, i.e., capable of supporting at least twice the weight expected to be imposed upon it. Anchorages used to attach personal fall arrest systems shall be independent of any anchorage being used to support or suspend platforms and must be capable of supporting at least 5,000 pounds (22.2 kilonewtons) per person attached.

Lanyards and vertical lifelines must have a minimum breaking strength of 5,000 pounds (22.2 kilonewtons).

RE: OSHA Fall Arrest System

Interesting that OSHA is only concerned about employees. Consultants like me are unimportant.

Mike McCann, PE, SE (WA)


RE: OSHA Fall Arrest System

If the anchorage will be left in place, it becomes a permanent anchorage. I'd have them take it down and give it to you.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: OSHA Fall Arrest System

(OP)
Mike,
Consultants must be required to furnish their own anchorage system. Or a parachute.

RE: OSHA Fall Arrest System

Ah Archie, led astray by simple physics. Those calc's are exactly right if the entire arrest system is rigid, including the 'dummy' in the harness. Even the old, outlawed rope lanyards had significant elastic stretch, as does the harness webbing. And, believe it or not, the motions you make when the harness and lanyard pull tight also have been proven to absorb some of the energy. It is possible to put 5K into an anchorage, but it is practically impossible using the current harnesses and lanyards. Nothing wrong with designing for "Withstand 5,000#", as I really prefer a safety factor larger than '2' [when brand new; less when worn and sun-faded]. And it still comes back to a structural connection with a very minor load of between 1 and 2Kips.

Check out what rock climbers use [and don't use]. Their harnesses do not have hardware and webbing sufficient to hoist my 5000# pickup, and its been working for 50-years.

RE: OSHA Fall Arrest System

Duwe6

The force imparted in the scenario I described was 2,400#, not 5,000#.

Of course rock climbers don’t have anchorage standards. Who’s going to certify such a thing that would have to work in unspecified strata at an unspecified elevation of an unspecified mountain? Further, people willing to climb rocks for entertainment are not particularly risk-adverse. Their version of "working for 50-years" likely includes an untold number of deaths in pursuit of self-aggrandizement. I don’t see their pursuit as directly relevant to occupational safety equipment.

As for wiggling and squirming: yes, I’m aware of the phenomenon. The biggest mistake rookie bass fishermen make is thinking they can land an 8# bass on 8# line. But it further illustrates my point in that with wiggling and squirming the stopping distance is the stretch of the stopping system, likely shorter than the 6” tear-away arresting gear designed to spread the arrest over a distance sufficient to dissipate the energy in a tolerable manner, as with cable arresting gear on an aircraft carrier. (Try stopping that same plane with a static cable and see what results.) In short, the shorter the stopping distance the greater the force. The “falling” from wiggling is much shorter than the 6’ freefall described earlier but the distance it is arrested is much shorter too.

Again, this isn’t my field or even my area of interest, particularly. But I do think it’s worth having a sense of the magnitude of the forces at play.

RE: OSHA Fall Arrest System

(OP)
My local OSHA office finally got back to me and provided me with this link

http://simplifiedsafety.com/blog/the-myth-of-the-5...

At this point I have moved the anchorage point to the roof ridge and made a couple of other modifications from what the contractor provided me. I am designing the anchorage force for 2X1800 or 3600 lbs as this article states is acceptable. Thank you everyone for your input on this matter.

RE: OSHA Fall Arrest System

The author of that article belittles the premise behind how the force is calculated without offering up what stopping distance he uses. That’s rather unprofessional, in my opinion. If he wants to play that game then the stopping force could be very small indeed…so long as the height fallen will accommodated it…

Just get that OSHA guy’s reference to that link in writing, which, most likely you have since he sent you a link.

RE: OSHA Fall Arrest System

(OP)
Archie264,
Yes. It was an internal email that was forwarded out in the OSHA office to address the same questions I have. The biggest thing that the OSHA guy told me is the or in the clause. It must be designed for 2x the load or 5000 lbs. He stated that if I designed the anchorage for the 3600 lbs, I may need to add some notes on there about maximum worker weights, etc. But after looking at some of the proprietary anchorage out there that state they are good for fall protection, this anchorage I am working on is much more robust. There are fall protection systems out there that are attached to the top of the roof sheathing. Try to get that to work. At least what I am designing is tied into some meat. Here is a link for some anchorage I am referring to.

http://www.aikencolon.com/Safewaze-PPE-Fall-Protec...

RE: OSHA Fall Arrest System

And please keep in mind that OSHA is ]not using 5K in the industry-standard engineering sense. They are just demanding that the anchorage withstand 5K; it can fail at 5.05K and be deemed adequate. Thus, really OSHA is requiring a 1.3 or 2kip anchorage, with an [added] Safety Factor of 4 or 2½. They actually expect us to design w/o a Safety Factor.
And ignoring fall distance is ignorant, as ignoring swing caused by the anchorage being out of plumb from the axis of your fall. You will probably live, but the damage you'll take from either running out of distance before you are fully decelerated, or getting decelerated vertically and accelerated horizontally [hooked up waaay off center] and then smacking into steel protrusions, and you'll never walk the same, look very nice, and will always know when the barometric pressure is changing.

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