Fall Arrest Rescue Anchor Capacity

Fall Arrest Anchor Point Capacity for Retrieval, Arrest, or Restraint?

One of our customers, who works for a local electric utility, recently wondered:

     What is the required capacity for an anchor point used solely for rescue (retrieval) as opposed to fall arrest or fall restraint?

It seems like a simple question, and there should be a fairly simple answer. But once you start digging, you discover that the question can be fairly complicated.

Let’s assume that we’re talking about an anchor point on a tripod used to retreive entrants from a vault. Since this utility is located in Oregon, we can get some guidance from Oregon OSHA IM-89-04 which addressed standards for the design and use of tripods. This states: “When tripods, davit arms and similar lowering equipment are used for lowering people, or for rescue and similar purposes, they are considered temporary elevated work platforms (scaffolds) and must meet the following requirements.”

They go on to say in (A) (4) that: “Tripods and davits used for raising or lowering personnel shall be capable of supporting a 5,000 pound dead load. (437-02-125 (4) or 437-03-1926.104).”

So for tripods and davits arms that’s pretty clear. And it makes sense to have a lot of capacity on these devices because the anchor may be used (intentionally or not) for fall arrest. That is, an entrant may be descending a ladder attached to lifeline intended to be used only for retrieval, but the entrant slips and falls off the ladder and the fall is arrested by the retrieval system. So in this case 5,000 pounds of capacity makes sense.

But what about an anchor that was only going to be used for pulling someone up and out of a manhole, but not on a davit arm and not in Oregon?  This really isn’t addressed by OSHA (or the NFPA) since they don’t set these types of rescue requirements. But it is addressed by ANSI Z359.1-2007 for both engineered (certified) rescue anchors and improvised (non-certified) rescue anchors.

A certified rescue anchor (i.e., a manufactured anchor with documentation and installed by a qualified person) must have a static strength that is five times the applied load. In a system that was strictly used for vertical retrieval (no freefall potential) the applied load would be determined by the weight of the person being retreived and the geometry of the rope rescue system. For example, if you use a pulley as a change of direction you would increase the force on the anchor because the two vectors (one leading to the load, the other to the mechanical advantage) create a resultant vector on the anchor which can be up to twice the load.

Fall Arrest Anchor

Determining the forces that may be applied on a rescue anchor requires a solid understanding of the physics involved.

Determining the forces and ensuring the system, including the anchor, can withstand five times the applied load would be the job of the qualified person.

When looking at a non-certified rescue anchor (i.e., running an anchor strap around a piece of structural steel) ANSI would require it to be able to support 3,000 lbs.

When considering non-certified anchors, remember that the problem isn’t the capacity of the connectors (since they would be rated to 5,000 lbs. or more). Instead, the problem is assessing the strength of the structure(s) serving as the anchor foundation. It can be very challenging to visually assess a structure and provide a meaningful estimate of the strength of that structure.

In a practical sense, all this talk about capacities of improvised anchors is sort of academic anyway since estimates of structure capacity are just that, estimates. In a technical rescue you always select the strongest anchor you can.  If you’re uncertain about the integrity of the anchor, you rig a backup or multipoint anchor.

Jim Johnson July 9, 2014

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