Great question, Colin. In general, I recommend that the contractor consider a pre-manufactured lifeline system. These have been tested, tried & true, and will provide the most reliability. We try to avoid designing custom systems since there is greater chance for error in the design, the component selection, the installation, or all three. However, sometimes custom systems are necessary, due to the desire to have more than 2 users per span (typical limit of most manuf. systems), or the need to reduce the fall clearance. Designing a custom system is a specialty field and should not be approached without having an experienced SME in custom fall protection perform a review/ensure everything was done correctly.
To design a custom HLL, follow ANSI Z359.6 Design of Active Fall Protection Systems. This is a relatively new standard but is quite comprehensive. It can be a little difficult to follow, so feel free to reach out if you have questions or need further clarification on the content.
General process
1. Determine and confirm the available fall clearance, along with the likelihood of swing falls/etc (how far will you allow the user to walk away from the perfectly straight fall path?). These are absolutely critical component of fall protection design and cannot be ignored or left up to the field. If you are not familiar with this, ask an expert for assistance.
2. Determine the connecting system (for example DBA Sala Nano Lok Edge). This will allow you to determine your design loads and check your required fall clearance, including the effects of swing falls and/or anchorage points below the D-ring.
3. Determine # of users per system and per span. For example 4 users on the entire lifeline/2 users per span, would lead to 2 load cases
- LC1 (strength or max tension in line) 2 users in one span/already fallen + 2 users in the loaded span
- LC2 (max sag or max required fall clearance) 2 users in loaded span only
4. Determine your design load - The 5000 lb that you referenced is just a general value/not used when actually designing a complete system. Per OSHA, you determine the actual force and design to a FS=2. First need to identify the MAF for the SRL being used (be careful, it's no longer 900 lb, most SRLs these days are leading edge rated and as a result have higher MAF). For example, the DBI Sala Nano Lok Edge had MAF of 1350 lb the last time I designed for this. Always check the actual systems that may be used and the most current product specifications. For multiple users per span, you'll need to decide if you are using lumping factors or the sequential fall method, to determine the total load in the span.
5. Determine cable materials - wire rope material and construction greatly effects the resultant stretch and tension. Need to be very specific about what is used. Many sites prefer aircraft cable, which is difficult to find specs for. Other wire rope types are have more widely published specs for modulus of elasticity, wgt/ft, metallic area, etc. Just be sure to use the exact specs.
6. Run analysis - as Reuben mentioned in his reply, the analysis of multi-span lifelines is relatively complex. The most reliable way to analyze these is to do a multi-part spreadsheet that converges the resultant tensions/elongations etc and cross checks several ways. It is in iterative calculation, that converges once the total change in length of the entire cable (all spans), equals the change in length of the loaded span due to load. I'd be happy to run you through some of these examples sometime. The spreadsheet should then also include both load cases (max tension vs max sag).
7. Check required vs available fall clearance
8. Create deliverable - Reference both OSHA and ANSI for items and noted that need to be listed on the drawing. There is a lot here to make sure that the system is properly implemented and used in the field.
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Angela Hunter P.E., C.Eng, M.ASCE
Teaneck NJ
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Original Message:
Sent: 01-19-2023 03:33 PM
From: Colin O'Hearn
Subject: Life Line Design
I am working to help out another demolition contractor on a job in Maryland. They installed twin 5/8" wire rope cables from wind brace to wind brace. They wrapped the cable around the first wind brace went over the top of the next wind brace then finally wrapped around the third wind brace. They used a come-a-long so the cable is fairly taut. From the 5/8" cable they plan on hanging 1/2" wire rope chokers which would connect to yo-yo's or other fall arrest systems. I know that OSHA requires a 5,000 pound load when designing these type systems. Has anyone else ever designed something similar? Any help would be greatly appreciated.
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Colin O'Hearn P.E., M.ASCE
Owner
Kinetic Demolition & Engineering, LLC
Framingham MA
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