OSHA Fall Arrest System 2

[jeffhed]

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?

 

[jeffhed]

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.

[highlight #FCE94F]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.[/highlight]

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

 

[msquared48]

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

Mike McCann, PE, SE (WA)

 

[dicksewerrat]

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.

http://WWW.amlinereast.com

 

[jeffhed]

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

 

[Duwe6]

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.

 

[Archie264]

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.

 

[jeffhed]

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

http://simplifiedsafety.com/blog/th...il&utm_term=0_be10b3c125-77253da1ae-419967977

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.

 

[Archie264]

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.

 

[jeffhed]

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-Protection--Reusable-Roof-Anchor--4000_p_945.html

 

[Duwe6]

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.