Triloading 101It’s important to understand the strength ratings of your various equipment. Carabiners are rated for a couple of different types of loading, and without exception the pull-load (coincident with the spine of the carabiner) is the highest rating. Loads in other directions (pulling on the gate, for instance) are usually substantially lower. Simply put, this means you should have your loads pulling down the spine of the carabiner, and not at off-angles.
With lifting equipment, strength ratings are usually given in a more comprehensive manner, with both safe working loads, and ultimate break strength. Climbing gear only gives you the UBS, sadly. Likewise, in lifting, there are load angle charts that can be used to determine if you are loading in a safe way. Climbing doesn’t offer these, but with a bit of personal speculation, I believe they can be partially adapted.
Here’s an example of what I would consider “acceptable” loading for a lowline. This sort of tri-loading is going to reduce the break strength of the carabiner, but not to an unsafe degree (for a lowline!), in my opinion. As with anything, use your discretion. I accept this as safe because the loads on the line (with user on line) are in the range of 3-5Kn (and major axis rating is 20+Kn).
Here’s an example of tri-loading that I wouldn’t trust. There is just not enough safety factor involved here. On an industrial lift sling, this would reduce the break strength to about 50%. On a carabiner not designed for tri-loading, it might even be 25% of major axis after repeated use in this manner (aluminum can fatigue in weird ways). Scary.
The best solution involves an extra carabiner and rappel ring (per anchor).
A couple more shots of a lowline “continuous anchor sling” held together with an extra carabiner, with a threaded rap ring.
Lastly, here’s an example of a tri-load calculation chart that is included with an industrial lift sling (crane hauling sling)…