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Yes that being said, a back-up knot is a good idea... depending upon what type of working load you're doing. I took a newbie out to a popular pit a few days ago and we discussed the rigging.
While I didn't back up the figure 8 knots that I used directly to the (main anchor) I did use additonal back-ups to the bolts that were there. In this case the main anchor was the tree and a prussik knot was tied around the main line and secured to bolts a few feet away from the tree (towards the pit). With another rope there were two bolt/hangers in line from one another. Again a figure 8 knot directly to the farthest one and then a butterfly to the closest to the lip. Neither 8's were back-ed up.
However; if there were to be a rescue load then certianly to prevent possible slippage a back up of a half-sheetbend/fisherman's knot would be a good idea at the base of the 8 knot.
It all depends I think upon what type of vertical work you're planning to do that day. If it's straight no-non-sense type vertical SRT then less is more. If you're doing tandem type work or pick-off/self-rescue type practices then yeah a back-up wouldn't be such a bad idea.
Actual rescue rigging of course is resplendid with redundancy, and thus backups and rebelays and so forth are the norm.
While I didn't back up the figure 8 knots that I used directly to the (main anchor) I did use additonal back-ups to the bolts that were there. In this case the main anchor was the tree and a prussik knot was tied around the main line and secured to bolts a few feet away from the tree (towards the pit). With another rope there were two bolt/hangers in line from one another. Again a figure 8 knot directly to the farthest one and then a butterfly to the closest to the lip. Neither 8's were back-ed up.
However; if there were to be a rescue load then certianly to prevent possible slippage a back up of a half-sheetbend/fisherman's knot would be a good idea at the base of the 8 knot.
It all depends I think upon what type of vertical work you're planning to do that day. If it's straight no-non-sense type vertical SRT then less is more. If you're doing tandem type work or pick-off/self-rescue type practices then yeah a back-up wouldn't be such a bad idea.
Actual rescue rigging of course is resplendid with redundancy, and thus backups and rebelays and so forth are the norm.