Safe Rig?

[fuzzy-hair-man]

This has been a good topic, it's confirmed some of the stuff I intuitively thought about redirects, ie they can see some very high forces if you aren't careful and are basicly pulleys and act in much the same way.

Ah, but those "very high" forces you mention can never (theoretically) exceed twice the load. And even then, any higher-than-normal loads are on the deviation anchor only. Tell me - how many anchors and anchor systems do you use that do fine with a single-person load, but would be in danger of failing with twice that load? (Please think regular SRT, and not agency-response technical rescue situations.) This would mean a safety margin of only 2:1 - not very likely.

Bottom line - pick good anchors and you should be fine. Don't use a flimsy piece of cord to rig the deviation (which I've seen before). Be aware of the higher forces involved - but don't worry about them too much. After all, we're not talking tensioned highlines here...

Hmmm

But.... we said before in this thread the consequenses of a redirect/deviation failure aren't generally considered as severe as the consequences if a rebelay or main anchor were to fail this is why as discussed before in the example we could do away with the backup to the deviation/redirect. The exception I'd put is what is the rope going to come in contact with if the redirect fails? if it is a sharp edge it may end up cutting the rope .
For the same reason anchors that cannot be used as rebelays are able to be used as redirects/deviations. If we are going to be placing forces equal to or greater than that of the mainline doesn't this suggest as you said the that the anchors and rigging components need to be at least as strong as a rebelay?

The way I see it is we probably need to avoid redirects which put an angle of greater than say 45 degrees on the rope, if we do put them in we need to do so with the knowledge that they are taking a load greater than or approaching that of the main line, and select anchors appropriately.

I'd also say that any thing over 90 degrees would probably be simpler with a rebelay anyway, correct me if I'm wrong but I think you'd need 2 redirects in a Z or rather N pattern to achieve an angle of 90 degrees or over any way then you may as well make the second redirect a rebelay (its seeing > 100 percent load anyway) and do away with the upper redirect. (maybe that needs a picture, tomorrow it's dinner time )

thoughts?

[LifeOnALine]

Ah, but those "very high" forces you mention can never (theoretically) exceed twice the load...

For a deviation that's true, but what's your "load"? It's not the weight of your caver for a start - prusiking up a rope creates about 150% to 200% bodyweight in terms of rope tension, more if you're crap at it. If further down the rope you're wresting another rebelay or deviation and something pings, even a fall of a few feet can put many times your bodyweight as a peak load up into your anchors, so yeah - on average it's probably "a person and a half", but if your rig was designed only to support that, any jolt as a krab twists and you'll do the whole dead cow impersonation thing...

This wasn't/isn't a rescue topic but I'd always work out the forces using two people instead of one - it adds enough safety for that bouncy climb and it means if someone gets stuck the rig's safe enough for someone else to go out and get 'em.


A few ppl have asked about rigging a Y-hang instead of a deviation, and yeah - it means there's a knot to pass, but there's a pigtail in the original diagram to do that anyways. The best "reason" for a pulley and deviation is it removes the knot from the main line, and knots reduce strength. You don't add one unless you need one, and in this rig you don't. makes it a heck of a lot easier to rig and recover too, as there's no fiddling with a knot in mid air to adjust the lengths, or fighting a wedged Bunny knot with a screwdriver...

[NZcaver]

My numbers are coming out almost identical...

Yeah, so did mine. Thanks for confirming it!

You do realize that we are full-fledged geeks to be even having this conversation, right?

No kidding! It's great mental gymnastists, anyway. Almost as much fun as actually playing on rope, although my head sometimes hurts a little thinking about these things...

[NZcaver]

Ah, but those "very high" forces you mention can never (theoretically) exceed twice the load...

For a deviation that's true, but what's your "load"? It's not the weight of your caver for a start...

I realized as soon as I re-read that comment, that I should have amended it. Bad wording on my part - sorry. It would have been better to say "the force on the deviation anchor cannot (theoretically) exceed twice that of the force on the main anchor - assuming no failures in the system."

The best "reason" for a pulley and deviation is it removes the knot from the main line, and knots reduce strength. You don't add one unless you need one, and in this rig you don't. makes it a heck of a lot easier to rig and recover too

I agree, but with one little hair to split. Your main rope is usually knotted at the anchor point (unless you're using a Tensionless Hitch, but we'll ignore that for now). So a certain reduction in strength is already embedded in the system. Assuming you only use appropriate, well-dressed knots, adding one more in the form of a rebelay/Y-hang should not cause further significant reduction in the overall strength of the system (assuming no component failures). Or am I missing something?

[ian mckenzie]

Ropes these days are so strong I doubt the weakening effect of adding a knot puts anyone at risk. Almost every caving rig ever done involves knots! Are there any examples of a modern rope breaking at a knot (or anywhere else for that matter) under normal, or even abnormal use, excepting of course where poor riggiing resulted in abrasion?

[Clem Akins]

Actually, there was a rope that broke last year. As reported in the climbing magazine Rock & Ice (October, 2006) a climbing rope that was 4 years old, well cared for, in good condition and just inspected broke going over a 'biner when the climber took a 4-foot fall.

Detailed analysis showed that the rope had been exposed to sulphuric acid, though there was no visible sign. The only time they could imagine that the rope could have been in contact with acid was when it was, perhaps, flaked out on a parking lot. Maybe a car's battery acid had dripped there, and maybe some of it got on the rope.

I try to rig with a 10:1 safety factor, just because "stuff happens" and the consequences are so severe where we play.

Clem

[Lurah_GB]

tanks...for all keep posting