double figure 8 question

Discuss vertical caving, equipment, & techniques. Also visit the NSS Vertical Section.

Moderator: Tim White

Re: double figure 8 question

Postby GroundquestMSA » Mar 13, 2014 5:00 pm

trogman wrote:
Anonymous_Coward wrote:
I do not agree that a (normal non-equalizing) double figure eight knot would fail in the event of one anchor failure. If that was true, then the whole operation of setting two bolts and using this double loop knot would be for no reason.


Not entirely for no reason. Yes, part of the reason for two bolts is so that in case one fails, you will have a second one. The other (and in my mind, the more important) reason is that it distributes the load, so that each anchor is supporting <100% of the load. This makes it less likely that either anchor will fail in the first place.

Trogman :helmet:


It wouldn't be the first time a time-honored operation was shown to be pointless...

I don't know much about the knot in question, but if you feel the need to distribute a load between two anchors, doesn't that imply that one or both anchors are potentially inadequate? And if one or both of the anchors are potentially inadequate then rigging them with a knot that fails when an anchor fails wouldn't (as Andy says) make any sense.
User avatar
GroundquestMSA
NSS Hall Of Fame Poster
 
Posts: 1343
Joined: May 5, 2011 1:32 pm
  

Re: double figure 8 question

Postby caver.adam » Mar 13, 2014 9:12 pm

And sometimes you use two anchors to redirect your rope away from an obstacle or sharp edge.
User avatar
caver.adam
NSS Hall Of Fame Poster
 
Posts: 345
Joined: Feb 8, 2012 12:26 pm
Primary Grotto Affiliation: Louisville Grotto
  

Re: double figure 8 question

Postby trogman » Mar 14, 2014 7:43 am

GroundquestMSA wrote:
trogman wrote:
Anonymous_Coward wrote:
I do not agree that a (normal non-equalizing) double figure eight knot would fail in the event of one anchor failure. If that was true, then the whole operation of setting two bolts and using this double loop knot would be for no reason.


Not entirely for no reason. Yes, part of the reason for two bolts is so that in case one fails, you will have a second one. The other (and in my mind, the more important) reason is that it distributes the load, so that each anchor is supporting <100% of the load. This makes it less likely that either anchor will fail in the first place.

Trogman :helmet:


It wouldn't be the first time a time-honored operation was shown to be pointless...

I don't know much about the knot in question, but if you feel the need to distribute a load between two anchors, doesn't that imply that one or both anchors are potentially inadequate? And if one or both of the anchors are potentially inadequate then rigging them with a knot that fails when an anchor fails wouldn't (as Andy says) make any sense.


Not implying that the anchors are potentially inadequate, but just adding a little insurance to the equation. It is always best to improve your odds of survival. I don't expect to have an accident when I travel in a car, but having a seat belt and air bags is a nice bit of reassurance.

According to this website, http://www.ropebook.com/information/vector-forces, the ideal angle to use for your rig is 45°. Thus rigged, each anchor is holding 54% of the load, making the odds of anchor failure much less than rigging to a single anchor. Let's say you are using a single anchor, and you and your buddy are climbing tandem. Suppose the total weight load between the two of you and your gear, etc., is 500 lbs. With a single anchor, the one anchor is supporting the entire 500 lbs. But if you distribute that with a two-anchor rig, and the angle at 45°, then each anchor is supporting only 270 lbs. This makes it much less likely that either anchor will fail.

Trogman :helmet:
User avatar
trogman
NSS Hall Of Fame Poster
 
Posts: 911
Joined: May 2, 2008 8:35 am
Location: North Alabama
Name: Stephen Brewer
Primary Grotto Affiliation: Gadsden Grotto
  

Re: double figure 8 question

Postby William Tucker » Mar 14, 2014 12:22 pm

I agree that this is a very interesting theoretical discussion and all the more so when I learn something useful, which I have. Now, back to reality.

A cut loop scenario is hard to imagine so that is not a big worry, for me. Even with bolts, an unlikely anchor failure would leave a carabiner in the loop making even that scenario of complete failure unlikely. So, I doubt that I will change my practice of using a double 8 in double bolt situations unless it is advantageous to offset the drop to avoid a rub point. I like the load distribution. What surprised me was that I had always thought the knot was stable when one loop was disconnected and it was a surprise to find that it can come apart if one of the loops is allowed to work back through the knot. It might be difficult to do under load; but, I don't care. If I can do it unloaded, which I can, I don't want to depend on it not happening when loaded and repeatedly cycled.

I do have a few new personal rules regarding this knot from this discussion, though:
*Do not cut a loop
*Do not use one of the loops to isolate a worn or damaged section of rope
*Do not leave one of the loops disconnected or allow it to become disconnected
*Do not presume that if the loops start out unequalized, they will stay that way

These rules don't change my practice much; but, it is good to know.
User avatar
William Tucker
Frequent Poster
 
Posts: 52
Joined: Mar 7, 2007 7:03 pm
Location: Wolfforth, TX
Name: William Tucker
NSS #: 55566RL
Primary Grotto Affiliation: Lubbock Area Grotto
  

Re: double figure 8 question

Postby Anonymous_Coward » Mar 15, 2014 10:30 am

You are correct trogman. Thanks for pointing that out.

I agree that it is more important to do things that reduce the chance of anchor failure than to do things that may save you after anchor failure. I mentioned that briefly in the second paragraph but then ended up getting tunnel vision about bolts popping out of the wall near the end of my post. :)
Andy Armstrong
American Carbide Council
User avatar
Anonymous_Coward
NSS Hall Of Fame Poster
 
Posts: 892
Joined: Feb 3, 2006 1:40 pm
Location: Inside the Beehive
NSS #: 45993RL FE
Primary Grotto Affiliation: Paha Sapa Grotto
  

Re: double figure 8 question

Postby Anonymous_Coward » Mar 15, 2014 10:48 am

GQMSA, having two bolts at the top of a pitch is standard practice for most folks. That's not to say that each individual bolt is inadequate to hold an SRT load. The bolts I use most frequently can hold about 1,200-1,500 lbs. each. That's pretty good, really. What we are really saying by doubling bolts is that once the bolt goes into the wall, it cannot be thoroughly inspected for anchor quality ever again. You can tighten the nut down, re-position the hanger, make sure the bolt doesn't wiggle, but that's about it. When you encounter bolts in a cave, many times you have absolutely no idea who set them or how long ago. For me to use this unknown technology, the uncertainty justifies having two anchors. I can inspect a tree, big rock, or formation and make a judgement call. I have no idea what is going on inside the rock, be it corrosion, unlike metals, irregular drill hole, voids in the rock, etc. As a courtesy to all that come after, we rig with two bolts even when we know that one is perfectly fine. (We know this because we bought it, set it, etc.) One exception would be one-bolt rebelays, since they are protected from above.

Besides peace-of-mind, rescue readiness is another great reason to set two bolts. Bolts can be time-consuming to rig, so having them in place before an emergency happens can really save the day.

Just pointing out that rigging with two bolts and two-loop knots doesn't always imply that the anchors are sketchy.
Last edited by Anonymous_Coward on Mar 15, 2014 10:56 am, edited 1 time in total.
Andy Armstrong
American Carbide Council
User avatar
Anonymous_Coward
NSS Hall Of Fame Poster
 
Posts: 892
Joined: Feb 3, 2006 1:40 pm
Location: Inside the Beehive
NSS #: 45993RL FE
Primary Grotto Affiliation: Paha Sapa Grotto
  

Re: double figure 8 question

Postby GroundquestMSA » Mar 15, 2014 10:55 am

Got it. Does that mean that rigging with two seperate 8s (with minimal slack between them) is a bad idea? That's how I rigged the only two-anchor pit I've ever encountered, but it occurs to me that this accomplishes little unless the backup anchor happens to be stronger than the one bearing your weight.
User avatar
GroundquestMSA
NSS Hall Of Fame Poster
 
Posts: 1343
Joined: May 5, 2011 1:32 pm
  

Re: double figure 8 question

Postby Anonymous_Coward » Mar 15, 2014 11:05 am

It's not terrible, but not ideal either. If you want to rig in that manner, you should make the lower or second knot a butterfly as Pete Johnson mentioned early in the thread. This makes a Y-hang where one side of the Y is a single line and the other side is two lines (the loop of the butterfly). The butterfly is better suited to loading in three directions than the single eight. In any event, sharing load between anchors is better than having one be the primary and the other the back up. To accomplish this, not only reduce slack, but adjust the knot to achieve tension on both anchors when the main line is loaded in the proper direction.

In addition to better tri-loading ability, the eight/butterfly rig has the advantage of being more recognizable/familiar to cavers than what you rigged. This means that it is easily inspect-able and less likely to get re-rigged by your buddy in the back of the line.

Which causes me to ask WIlliam Tucker, is there any chance that this is really what happened in the original story? I know if I encountered a Y-hang with one side slacked, I would be tempted to correct the situation, especially if I had missed the original discussion about why. And you said that the rub spot was no big deal in your estimation. Just wondering if you were the last person down that drop or if there was any chance that someone else may have "fixed" it for you. :shrug:
Andy Armstrong
American Carbide Council
User avatar
Anonymous_Coward
NSS Hall Of Fame Poster
 
Posts: 892
Joined: Feb 3, 2006 1:40 pm
Location: Inside the Beehive
NSS #: 45993RL FE
Primary Grotto Affiliation: Paha Sapa Grotto
  

Re: double figure 8 question

Postby William Tucker » Mar 21, 2014 3:06 pm

Andy,

I know that this is what happened and that noone changed anything about the rigging. We had all discussed it and I watched him rig it. I agreed with the goal of trying to avoid the rub point. This scenario has been repeated more than once and we have always rigged with a double 8 and have tried to offset it on a few of those occasions. It does work for a while. I was second down the drop and last of 4 up in the most recent situation (IIRC). It is a long drop and quite bouncy at the bottom. I did not consider an unequalized double 8 to be any worse than a butterfly with all the load on one anchor and the other anchor used for backup. Experience has now taught me that if you want a purposefully unequalized rig, start with separate knots, not a double 8. The loops of a double 8 are not completely independent. I, like you, usually prefer equalized over backup on a two bolt anchor; but, situations can make the backup option look a bit more attractive. The anchors are bomber. If the same situation gets rigged the same way in the future, I will simply say "it probably won't stay offset" and will rap on it anyway.
User avatar
William Tucker
Frequent Poster
 
Posts: 52
Joined: Mar 7, 2007 7:03 pm
Location: Wolfforth, TX
Name: William Tucker
NSS #: 55566RL
Primary Grotto Affiliation: Lubbock Area Grotto
  

Re: double figure 8 question

Postby Shane S » Mar 21, 2014 3:21 pm

If you cut through a loop in a static caving rope such as PMI then you have some major issues going on somewhere. You should always put a carabineer inside of the bolt so that the rope is on a rounded edge.
User avatar
Shane S
Prolific Poster
 
Posts: 116
Joined: Mar 7, 2013 1:15 pm
Location: Cartersville GA
Name: Shane
NSS #: 64742
Primary Grotto Affiliation: Clock Tower Grotto
  

Re: double figure 8 question

Postby William Tucker » Mar 21, 2014 4:54 pm

Shane S wrote:If you cut through a loop in a static caving rope such as PMI then you have some major issues going on somewhere. You should always put a carabineer inside of the bolt so that the rope is on a rounded edge.


Agreed, Shane. All this talk of cutting one of the loops is more theoretical than practical if proper technique is used.
User avatar
William Tucker
Frequent Poster
 
Posts: 52
Joined: Mar 7, 2007 7:03 pm
Location: Wolfforth, TX
Name: William Tucker
NSS #: 55566RL
Primary Grotto Affiliation: Lubbock Area Grotto
  

Re: double figure 8 question

Postby johncwoods » Mar 21, 2014 4:59 pm

You might want to try a double figure 9.
johncwoods
Infrequent Poster
 
Posts: 10
Joined: Mar 25, 2012 4:56 pm
NSS #: 10503
Primary Grotto Affiliation: So Cal Grotto
  

Re: double figure 8 question

Postby Anonymous_Coward » Mar 24, 2014 9:56 am

William Tucker wrote: I did not consider an unequalized double 8 to be any worse than a butterfly with all the load on one anchor and the other anchor used for backup. Experience has now taught me that if you want a purposefully unequalized rig, start with separate knots, not a double 8.


I'm not sure this is what you meant, but just to be clear for anyone studying this post for actual rigging advice: If you rig with the butterfly and single eight, it still can be tensioned into a load-sharing Y hang. It does not have to be a primary/backup situation just because you choose those knots.

Thanks for the explanation in your example that the knot was definitely not re-rigged. Just thought it was worth asking.
Andy Armstrong
American Carbide Council
User avatar
Anonymous_Coward
NSS Hall Of Fame Poster
 
Posts: 892
Joined: Feb 3, 2006 1:40 pm
Location: Inside the Beehive
NSS #: 45993RL FE
Primary Grotto Affiliation: Paha Sapa Grotto
  

Re: double figure 8 question

Postby Extremeophile » Mar 24, 2014 1:03 pm

Anonymous_Coward wrote:The bolts I use most frequently can hold about 1,200-1,500 lbs. each.

I assume this is the working load limit (WLL), and not the full strength? Most bolts I use (probably the same that you use) have a failure rating of 4,000-5,000 lbs, which is generally 4-5x the WLL. At least this is the rated strength for a steel expansion bolt of roughly 3" length and 3/8" diameter. Of course these ratings all assume the bolt fails before the surrounding rock.
User avatar
Extremeophile
NSS Hall Of Fame Poster
 
Posts: 761
Joined: Dec 7, 2009 7:37 pm
Location: Littleton, CO
Name: Derek Bristol
NSS #: 34941
Primary Grotto Affiliation: Colorado Grotto
  

Re: double figure 8 question

Postby Anonymous_Coward » Mar 25, 2014 9:41 am

WLL, yeah....that's the ticket. I should have said that instead of "can hold".
Andy Armstrong
American Carbide Council
User avatar
Anonymous_Coward
NSS Hall Of Fame Poster
 
Posts: 892
Joined: Feb 3, 2006 1:40 pm
Location: Inside the Beehive
NSS #: 45993RL FE
Primary Grotto Affiliation: Paha Sapa Grotto
  

PreviousNext

Return to On Rope!

Who is online

Users browsing this forum: No registered users