Fall Protection

[PEinVA]

In this thread, thread190-97369 , they state the 5000 is an ultimate load. When checking bending with this, we should use the tensile capacity and not the yield?

(Fy=50ksi, Fu=65ksi)

Is that correct?

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[racookpe1978]

I agree that the 5000# is a fool-proof number, however, if you impose a vertical load of 900# on a horizontal beam, or a wire rope, because the small deflection angle in between the beam and the horizontal plane, the resultant force can be several times of that vertical load - 900#.

Correction: The resultant force argument is seemly only valid for horizontal lifeline using wire rope, not beams, or pipes.

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Your second comment is correct: The "load" (the person at the other end of the lanyard) is falling - by definition, so his lanyard MUST be vertical (or nearly so) then it would have to be wrapped over a rail, or steel, or pipe or something that might turn the vertical load into a side load of nearly equal amount.

The "worst case" would be a perfect inertial-less pully with no resistance: Any friction on the lanyard as the lanyard pulls down reduces the "final sideways" load on the lanyard hook.

Comes back to the [test results case] of a 900 lb load (3+ g's) on a 250 lb person's harness x 2 (factor of safety) = 1800 lb load on the hook.

 

[racookpe1978]

kslee1000 : Your very accurate concern about the increase in tension in a horizontal wire rope being suddenly pulled from below - tension in the wire rope that the lanyard is tied to - is why they also prohibit lanyards from being tied to "life lines" or "handrail lines".

You have to go back to a real support that can withstand the jerk

 

[kslee1000]

racookpe1978:

The wire rope is used in many industrial settings that have overhead cranes and runways. as long as I can remember, the suppliers claim the rope can sustain 10000# of ultimate load in tension, as well as the end fittings. Don't you think the near vertical pull of 900#/1800# will produce a tension in the rope several times higher than the applied load?

Also, I am not aware that lanyard is prohibit to hook to wire rope through the hooking device that comes with it. Can you point out where I can find info on this "prohibition"? Thanks ahead. (Instead, I am aware that the lanyard shouldn't wrap around a beam, or pipe, without using beam wrap with a D-Ring)

 

[nutte]

agree that the 5000# is a fool-proof number, however, if you impose a vertical load of 900# on a horizontal beam, or a wire rope, because the small deflection angle in between the beam and the horizontal plane, the resultant force can be several times of that vertical load - 900#.

Exactly right. That is why I asked a couple days ago about the stretch of wire rope. Apply your 900#/1800# load to the center of your line, and with trail and error, determine the elongation and cable tension that gives you the vertical reaction you're looking for. For the ones I've checked, the cable tension is in the 5000# - 6000# range. So the anchors and everything else down to the support should be checked for a service live load corresponding to the cable tension.

 

[enginerding]

There are specific requirements that state that the system as a whole must maintain a FS = 2.0. For the testing method provided in the appendix that are deemed to meet the OSHA requirements, they state that either the lanyard shall be provided with the system, or the stiffest lanyard available shall be used. I think this is why we have the 1800#/900# confusion.

OSHA limits the maximum force on a body from a body harness to 1800#. In theory, the stiffest lanyard would allow this load. However, most lanyards in use will limit the impact to 900# because most employees don't like being hit with 1800#. If the lanyard is not provided with the system, I think you should design for the 1800# as a service load - applying the 2.0 FS to that. If a 900# lanyard is supplied with the lifeline system, apply the 2.0 FS to 900#.

 

[MiketheEngineer]

And you should ALWAYS use a shock absorbing lanyard!!! Some well known companies now make a horizontal lifeline with a shock absorbing lanyard built in that limits the horizontal force to less than 2000 lbs for TWO people!!!

Well tested....

 

[kslee1000]

The lanyard and the harness are made per OSHA requirement, and verified by testing. The more critical issues are the strength of lifelines and end anchorages. Again, the suppliers claim the lifelines and the anchorages they furnish are both good for 10,000# = 2x5000# (OSHA). The most critical issue remains to be the structures that the lifelines attached to. Note that the 5000# is usually located 5'-6' above the platform, and the load is close to horizontal. Can your building columns, or pipes, survive during the pull, or prevent fall more than the max. fall distance specified by OSHA (6'). Never been easy to design a good horiz. lifeline system.

 

[mark1234]

There are other issues as well... like the workers that did not tie off...

 

[enginerding]

The way to deal with the load being close to horizontal is to install a predetermined amount of "sag" in the line when it is installed. Obviously the angle will still be low (about 7 degrees - still close to horizontal), but it will be known. Place a weight (say 10#) haning from the cable at the center, and tighten the cable until the load is a certain distance below the anchorages (we use length/30). From here it is possible to determine the initial state with pretty good certainty. Energy methods or iteration with assumed final deflected shape will get you the deflection (and thus the tension) with the men on the cable.

We will typically have tensions on the order of 7000# (service level) for two men with 900# lanyards. Design the anchorages for an ultimate tensile load of around 14000#.

 

[kslee1000]

Nutte and enginerding have laid out the end anchorge issue correctly. Beware that analysis on the cable will take some efforts to properly taking into account of initial sagging, and final pull. Also, keep an eye on OSHA specified fall distance from where the worker was standing.