Fall protection system from elevated platform - cable analysis

[ARS97]

The company I work for has started to make a few customized platforms for the oil & gas industry. The platform, when fully assembled, supports a centrifuge and provides access to the centrifuge. The handrail surrounding the platform is removable for shipping purposes, so during initial setup there is not any fall protection. We're planning on providing a basic fall protection system utilizing some cable and attachment lugs. OSHA requires a 5,000 lb force to be applied to this system, so it needs to be robust.

There are pre-manufactured fall protection systems, but it still seems as if I'll have to do some type of analysis on the system to calculate the forces transferred to the structure.

Another interesting piece of information is that the PM wants to place this cable system at feet level around the perimeter of the platform, not up high like most fall protection systems require. So, the person will fall a greater distance before being "caught".

Cable analysis isn't something I'm terribly familiar with. I understand how to find the tension in a catenary cable (subjected to a distributed load along the cable, such as selfweight), but a concentrated load of 5,000 lbs is another ballgame. I have Staadpro, which apparently has some ability to handle cable analysis, but I've never really had to use it.

I'm sure the initial installation tension is critical to the analysis. I think we can do some testing here in our fab shop to measure this tension. We'll purchase the cable, string it up between points, and measure the sag. From this, I can estimate the initial tension in this catenary condition. However, the application of the 5,000 lb load eliminates the catenary shape and this is where I need to start my research.

Any thoughts or recommended references?

 

[CANPRO]

Once the 5000 lbs force is applied, the cable will basically form two straight lines from each end connection to the application of the load. From there you just need to the resolve the forces of the triangles.

The initial sag/tension will let you know what length of cable you have and the geometry of the triangle once the load is applied.

Be careful of the pre-manufactured anchors for fall arrest systems, they may only be rated for 5000 lbs...but if you're using it to fasten your static line the force will be much higher than that.

As for the static line being on the floor...as long as you (or the worker using it) considers this distance in their fall calculation it shouldn't be a problem. That is unless your local OSHA or client requires it to be overhead. I've seen somewhere (can't remember which jobsite/company) that limited the max possible free fall to 6 ft.

 

[theonlynamenottaken]

There is a few facets of horizontal lifeline systems that are not straight forward. The 5000 lb anchorage strength stipulation, fall factors and maximum arresting force are a few of the key ideas to HLL's. In short, my advice is to buy the book, "Introduction to Fall Protection" by J. Nigel Ellis. To give you a bit more info;

-The single-point anchorage strength is required to be 5000 lbs by OSHA, but as CANEIT pointed out, HLL's can easily result in higher forces at the termination of a HLL
-Be very careful about adding up the total height of a fall. Its easy to think you'll cover someone falling from a structural standpoint just to see that they're feet would actually contact a lower level floor surface. For your situation, for instance: Needed clearance to level below = [initial sag in HLL] + [height of d-ring attachment on body harness from initial standing elevation] + [length of fall arrest lanyard] + [length of fall arrest lanyard ripout/extension] + [elastic stretch in HLL] + [~20" of clearance]
-The maximum allowed arresting force on a human through a body harness = 1800 lbs (OSHA requirement)
-When you put an HLL at foot level you usually creat the possibility of a swing-fall
-Considering the last two points you really, really should try to keep the HLL at chest height or overhead
-If you end up with astronomically high HLL cable tensions then you can use a shock absorber. Most of these are devices that "tearout" and allow the HLL to lengthen under the shock of a fall, thereby lowering the cable tension. Just remember you have to add that extra cable length into your fall clearance calcs.
-So far we've just been talking one user per HLL. Get's more fun if you're putting several workers on one line...


Again, buy the book. This should give you an idea of what's involved, but its just not as straight forward as us structural engineers like to imagine.

 

[ARS97]

Thanks guys.....I'll do some more research on the topic. Lots of good info provided above........