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Rope Rescue
Point Loading

Point Loading on Litter Rigs

by J. Danis

In answer to a question which arose, this article addresses point loading and carabiner to M.P.A. and/or Litter rails.

There are Two problems

One, is poor load distribution in the carabiner causing point loading - a dynamic force will cause the weaker of the two materials to crack or break.
Two, a static load can even cause cracking of the litter frame or carabiner.

If the stokes basket is of a tubular type, there is an even higher possibility of this. I discussed this at length with Robert Chisnall, of the Ontarion Rock Climbing Association, about four years ago. We are in agreement.

I am a rock climber and I always avoid dropping or linking carabiners in a manner that may cause impact hardening. The distance or length of the fall or spread is not as important as the amount of force resulting from an impact. Even linked carabiners can impact hard enough to damage them. Also, each carabiner that is added creates another component in the system that can fail. Thus, the fewer links, the safer the system.

I will refer you to the following page extracted from a manual I wrote some time back, called "Cliff and Aerial Rescue". Robert chisnall was a contributor. Also see the diagrams (following the text) demonstrating point loading (from the manual as well).

I hope all of this helps clarify things. Please feel free to share your views in the discussion area. We are always looking for a better mouse trap.

Maximum Safety for Litter Rigs

From: "Cliff and Aerial Rescue" by J. Danis and R. Chisnall, 1991

The problem of carabiners attached to litter frames

You do not ... not ever ... never clip a carabiner from the litter rig bridle over the rod or tubing of the litter frame. The most obvious reason would be that the carabiner will slide around along the litter frame causing an imbalance in the horizontal position of the litter, and therefore, an imbalance in the load distribution in the bridle.
The most important reason is this: the inner dimensions of the carabiner are rounded; the litter tubing is rounded. Therefore, you are placing very high forces on a very small area of the tubing, which has been known to collapse under these conditions.

For example: place a carabiner (I use offset steel 'D') over a piece of 20 mm tubing. You can quickly see that the area of contact is 1 - 2 mm. Ask yourself if you are truly satisfied with this. The load is better spread out over a larger area.

There is also a likelihood of the carabiner breaking or at the same time compressing/indenting the tubing of the litter, be it aluminum or steel frame, from two directions in the hook of the carabiner.

As of January, 1995, only a handful of load distributing litter rigs exist. Since these have not yet come into mainstream use, the chances of: carabiner failure, tubing failure, sling failure in the rig, or "zippering" in the bridle, is increased, since the classic litter rigs so poorly distribute the load among the connection points.

Even if the radius in the turn of the hook in the carabiner matched that of the litter tubing, the rounded inner dimension of the carabiner would still concentrate high forces on a small area of both the litter tubing and carabiner - disastrous if mainline failure occurs.

Regardless of the type or shape of carabiner used, point loading is a fact. But it can be remedied easily.

What can be done?

Always use short webbing slings wrapped around the litter frame to create a connection point (I recommend RSI brand, as they are available in ratings up to 17,000lbs).
This will eliminate point loading, indentation and impact hardening or fracturing.

Carabiners and Litter Rail Tubing

Consider that if the litter becomes snagged on the rock or some other object during a haul, the possibility of tubular indentation is increased considerably.

It can be argued that employing steel carabiners rather than aluminum ones would eliminate the danger of carabiner breakage. That is true, however, the problem of indentation of the litter rail tubing still exists.

Some litters are made of aluminum (hollow tubing) and others are made of steel (solid rod). While the steel is stronger, it is heavier and still subject to the problem of indentation and/or cracking.

So it may be wise from time to time to test for cracks in litter rails and tubing with liquid dye penetrants, if you intend to continue using carabiners on the rails of litters.

Further, if a mainline failure occurs, a dynamic load will be placed on the system parts as the backup belay comes into play. This presents the problem of "impact fracturing/hardening" at all points of steel on steel contact, and thus system failure becomes a probability. (This is also the reason you tie directly into the M.P.A. of your harness. You eliminate a piece that can fail, especially if it is a harness with a D-type steel M.P.A. - under dynamic loading, steel on steel can fail.)


 

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Copyright 1995 Search and Rescue Society of British Columbia
E-mail: J. Danis