When confronted with the choices of connecting two slings together: girth-hitch, strop-hitch, or carabiner- which is the best choice and when?

The Scoop on Sling-on-Sling Connections  

Note: The information contained in this article is gleaned from the web column of Black Diamond Equipment engineer Kolin Powick, whose blog:  Kolin's Corner or QC with KP, is an ongoing "off the record" project involving tensile pulls and drop-testing of climbing gear and systems to ascertain more objective information regarding the strengths and weaknesses of the tools that help keep climbers alive. 

Kolin's informal testing of used equipment and systems has helped to inform climbers of the limitations, considerations and most importantly; shed light on the primary safety question for those who cling to the vertical with only a rope and equipment as their life-line:  What are the weakest links in my rope system and the more likely causes of system failure?  

Sling-on-sling connections are a common necessity in order to reduce friction through rope-drag on a pitch which can be a limiting factor and source of frustration when trying to "send the proj".   

We commonly build anchors with slings and may choose to connect slings together without carabiners to minimize links or due to a lack of equipment.  

The harness and belay loop are also made of nylon and we usually girth hitch slings to either of these points as an extension for rappel and belay anchoring in the form of a "daisy chain" or longer sling (24"-48").

A 48" sewn sling girth-hitched to the belay loop for an "extended rappel".

Any connection in a climbing system could potentially be a weak point in that system so knowing which components are your more likely sources of concern in regard to system failure is key in the decision-making process. 

 

A close-up of a girth-hitch being used to join together  two pieces of equal diameter nylon webbing.  With materials of equal diameter, the loss in strength is approx. 30% in this connection using a girth-hitch.

The girth hitch is one of the most commonly used hitches in climbing for joining sling-to-sling.  Examples of these connections would include:  Belay loop to sling (or daisy chain) and sling to sling (including cam slings). 

With newer materials being introduced into the climbing world each year, and thinner and smaller being the focus of many manufacturers, what happens when we girth-hitch materials of different diameters together?  Here is where the problems begin:

When two 10mm dynex slings are girth-hitched together, the loss of strength is approx. 47%, which is enough of a strength loss to be of concern. 

And when an 11/16" sling is girth-hitched to an 10mm dynex sling, the loss in strength is approx. 54% which could be a big enough reduction in strength to become the weakest link in a climbing system.

Due to this significant difference in strength loss, it is recommended to girth-hitch only slings made of nylon.  With any other choice of materials, using carabiners would be the strongest method for sling-on-sling attachment.

Using a carabiner to attach two slings together is the strongest method in almost all situations unless the strength of the carabiner could be affected by an edge (open gate) or leverage or a whiplash affect.  Doubling up carabiners (opposed and reversed) in these exceptions will mostly negate the risk of carabiner failure.

What about using other hitches for joining slings together?   The strop hitch is very similar to the girth-hitch, but for joining nylon to nylon, this is a stronger method due to the symmetry of the strop-hitch.

The strop-hitch used to join two nylon slings together reduces the strength of this connection by only 20% which is 10% stronger than a girth hitch.

Despite the strength gain of using the strop-hitch with two nylon slings of the same diameter, the same advantage is not realized as much with smaller materials or materials of different diameters.  There is one exception gleaned from Kolin's tests:

The strop-hitch being used to join a 10mm dynex sling to a nylon sling.  The total loss of strength in this connection is approx. 46% compared to the 54% loss of strength that occurs when connecting these two materials with a girth-hitch.  Neither loss of strength is ideal, but the strop-hitch might bring the connection strength up to a "strong-enough" level for some climbing applications.

The summation of this sling-on-sling connection testing is:

1.  Joining two slings together using a hitch reduces the ultimate strength of the material up to and over 50%.   Only a nylon-on-nylon connection with a hitch seems to result in a reasonable amount of strength loss compared to other sling materials.

2.  In general, the narrower the material used, the greater the reduction in strength when joined together with any hitch.  

3.  In general, materials of different width joined together with a hitch results in a significant reduction of strength.

4.  If you must link two runners together, a carabiner is stronger.  If you must use a girth hitch then put the girth hitch on the web that has higher strength or use a strop-hitch.

5.  Better yet - use a longer runner altogether.

Many thanks to Kolin Powick and Black Diamond Equipment for their work and support in the testing of commonly used techniques and equipment in various climbing applications.