ratline design

[lifeline38]

Hi I am doing a horizontal ratline design currently. OSHA requires the HLL system to have a safety factor of two. For the 3/8" aircraft wire we are using the safe load for the wire already has a safety factor of 5. Does this SF=5 already cover that OSHA required SF=2 or should I multiply them to have a total SF=10 for the wire? Thanks a lot.

 

[RattlinBog]

I haven't seen the word "ratline" before, but I do know what an HLL is. So you're designing a horizontal lifeline (HLL) for a fall protection system, right? Other folks on this forum may not know what ratline or HLL mean.

I haven't personally designed one, but I did specify one that was designed and provided by a vendor. I'm not sure how they handled the safety factor. Would you be able to contact the wire rope manufacturer (maybe 3M?) to get their perspective? I'm guessing this isn't the first time someone's asked this.

 

[human909]

Don't fall into the trap of multiplicative safety factors covering the same things.

As soon as you start mixing SF approaches with ASD and/or LFRD you can quickly end up with overly conservative designs.

That said, do take into account the force increase due to the acute angle of the line.

 

[Geoff13]

Also never heard of a ratline. Where does this term come from?

Do not multiply the two safety factors. 5 is greater than 2, so it meets the OSHA requirement.

Due to the shallow angles the HLL takes at arrest you may have trouble staying within the SF=5 safe load listed by the manufacturer. The Canadian code (CSA Z259.16) starts with the breaking strength and factors down, including the termination efficiency. As stated in the explanatory notes, they were aiming for a minimum SF of 2 with the factors they selected. The ANSI Z359 Codes are near identical to the Canadian Codes as the committees share many of the same experts as authors. I think the matching ANSI standard would be Z359.6.

 

[ldeem]

I used to do quite a few of these (we also used the term rat line on site). Watch your sag specification to make sure contractor follows your design. Contractors always seem to want to pull these tight, thinking that is better, but it will drive your anchor point loads way up.

I don't know where you are located but I have never heard of "aircraft wire" to spec a wire rope. Be careful about using some material the contractor has "laying around" from another job or "we use this all the time". Looking back old projects and I was using 1/2" 6x19 IPS on most projects.