SRT Basal Anchor Forces - Should I Basal Anchor Across the Yard??

R

rypie37

Location
Minneapolis
Okay, so I just became aware of two competing "truths" that I've had bouncing around in my toolbox for climbing and rigging:

1. While rigging / routing SRT basal systems, it is good to load the spine / spar of a tree or limb, since you deflect a lot of force 'vertically' with the strength of the tree. Basically, the principle behind fishing pole rigging. This is why we put the portawrap at the base of our tree, directly below our anchor more or less. In this way we reduce the side loading of the anchor point and the torque in the system. Right??

2. Vector forces - more open angles reduce the force on the anchor point. I just watched a video of Dr. John Ball mentioning that a basal anchor further away horizontally doesn't 'make a huge difference, but is better than straight down because you open up the vector angle. (He also cautions it is riskier / less ideal to do since you inevitably add more things, obstructions, or chances for error this way)


So....I had always thought that you shouldn't be basal anchoring far away from your TIP due to the torque on your tie-in point. What am I missing? #hopeit'snotobvious :LOL:
 
Not really understanding you question. Are you talking about the difference between spreading your basal anchored load over several limbs versus just one at the top?
 
Okay, so i've been looking, and this short video does a good job of covering what I'm asking about:

Now the question is...is it always better to basal anchor as far away horizontally from the base of your tree as practical? (assuming it's practical and there are no additional safety concerns in between, like moving traffic, equipment, spinning swords, etc..)
 
I think he's talking (asking?) about the difference between a typical basal anchor arrangement where the rope is anchored at the base of the tree the climber's ascending, and an arrangement where the anchor is moved laterally to a different tree, opening the angle between the basal side of the climb line and the climbing side of the climb line.

I believe the only differences between an open angle (base anchored to a different tree) and a closed angle (180deg turn in the climb line at the primary support union, anchored at the bottom of the climbing tree), are that 1) the open angle reduces the total force vector magnitude at the primary supporting union, and 2) the open angle creates non-zero lateral loading on the tree being climbed (side-loading, in other words), resulting in slightly less compression on the tree being climbed for the cost of increased side-loading.

This, to my rationale, doesn't seem to be a good trade-off - decreasing the force on a tree in its strongest dimension (compression along the spar) for increased force on a tree along it's weakest dimension (lateral side-loading).

That clip is just talking about loading limbs in compression,I believe (without having scoured it and watched every second) - the exact thing that's NOT being done with a base-anchored straight-up-and-down-spar were you to move the base anchor away from the tree (to another tree) laterally.
 
Yes! Like, at what point is the open vector worth it vs. the increased side loaded torque / bending moment added to the system. I don't think I've seen the pros/cons contrasted anywhere before.
 
There are many many pros and cons to both. If your looking solely at loads at your tip, opening the angle is the best way. But if you are appreciatively concerned about the very real micro differences in the outside the lab/theoretical force differences pick a better tip.

Sometimes a base tie on the trunk your climbing gets in the way, and is vulnerable to saws. Sometimes a base tie to a neighboring anchor can accidentally at like a speed line (sometimes on purpose). It’s also further removed, out of sight/out of mind.
 
rypie37 said:
Yes! Like, at what point is the open vector worth it vs. the increased side loaded torque / bending moment added to the system. I don't think I've seen the pros/cons contrasted anywhere before.
Click to expand...

In that video, it's a little bit of a different trade-off he's looking at, with the limb walk and all; in the arrangement he's describing, he's actually directing the forces more into compressing the redirect limb (that is, more in-line with the limb) as he opens up the angle, and because he's opening up the angle, there is also a corresponding reduction in the force vector magnitude at that redirect limb as well. So, it's a win-win.

What your original post was referring to with what I imagine to be a classic basal anchor situation - straight vertical pole - is quite different; moving the anchor point away from the tree results in decreasing compression along the spar while increasing side-loading. That's not a trade-off I'd make because the spar will be stronger in compression along it's length, than being torqued by lateral force. At least, I can't think of a situation in which that isn't the case.

This is just relating to loading forces - there are other factors which might lead you to move a base anchor away from the tree being climbed. As a general guideline though: compression is good, torque/lateral-loading is bad.

Edit: I'm also just talking about a situation where you're climbing a straight spar on the rope directly below the primary support point, from which the rope departs and travels to it's terminal anchor point. Open or close angles to direct forces into compression.
 
Last edited:
rypie37 said:
Yes! Like, at what point is the open vector worth it vs. the increased side loaded torque / bending moment added to the system. I don't think I've seen the pros/cons contrasted anywhere before.
Click to expand...

In the video, he's opening the angle AND decreasing side-loading, or for the purpose of decreasing side-loading.
 
rypie37 said:
Yes! Like, at what point is the open vector worth it vs. the increased side loaded torque / bending moment added to the system...
It does not need to be one or the other. An open angle can also be used to reduce the bending moment. It is something that is incredibly useful and I do all the time because it is so simple with SRS.
Click to expand...
Click to expand...
 
The issue of loads on the TIP has been a sticking point for me for a long time.

Proper life support equipment is expected to have 5k# breaking strength, right? That includes anchors and TIPs. Doing some rough math will show how far a climber with a geared-up weight of 200# can fall and generate that load...not far.

Its always a good idea to rig for climbing or removal to reduce loads. Proper principle.

What I have an issue with is looking at a TIP that we want to use because its in a great location then futtering around to find ways to reduce the load. This has always seemed to be the wrong way to decide what to use as a primary TIP.

This thought process is the same as thinking that its OK to drive too fast because you have a seatbelt, airbags and good brakes. No...drive slower. If things go pear-shaped and the car crashes or the TIP fails it will be a catastrophe.

If a TIP is chosen and the climber decides that in order to use it the rigging needs change because it might fail I say the TIP is wrong. The TIP should stand on its own merits without the belt and suspender sort of thing.
 
LordFarkwad said:
In the video, he's opening the angle AND decreasing side-loading, or for the purpose of decreasing side-loading.
Click to expand...
Right! And the OP was confusing me with talk of moving the basal anchor laterally. In the video, he's not moving the anchor location, he's just redirecting the initial path from the anchor outward.
 
Tom Dunlap said:
The issue of loads on the TIP has been a sticking point for me for a long time.

Proper life support equipment is expected to have 5k# breaking strength, right? That includes anchors and TIPs. Doing some rough math will show how far a climber with a geared-up weight of 200# can fall and generate that load...not far.

Its always a good idea to rig for climbing or removal to reduce loads. Proper principle.

What I have an issue with is looking at a TIP that we want to use because its in a great location then futtering around to find ways to reduce the load. This has always seemed to be the wrong way to decide what to use as a primary TIP.

This thought process is the same as thinking that its OK to drive too fast because you have a seatbelt, airbags and good brakes. No...drive slower. If things go pear-shaped and the car crashes or the TIP fails it will be a catastrophe.

If a TIP is chosen and the climber decides that in order to use it the rigging needs change because it might fail I say the TIP is wrong. The TIP should stand on its own merits without the belt and suspender sort of thing.
Click to expand...
Does this philosophy still apply in situations where the climber would be stringing multiple smaller TIP's/PSP's together for strength/redundancy reasons? Like is often done with canobasing.
 
Tom Dunlap said:
canobasing?

What is that?
Click to expand...
A base anchor, but set high in the canopy. Or, in other words, a canopy anchor that redirects higher over multiple smaller unions, with the idea being to spread the climber's load over the several smaller unions together. The friction combined with potentially more favorable load vectors at each union (compression), on paper at least, enables* the safety factor obtained from climbing supported by any one of the unions alone.

Someone correct me if I've severely mucked the explanation/definition.

*Edit: increases
 
Last edited:
You must log in or register to reply here.

New threads New posts

Kask Stihl NORTHEASTERN Arborists Wesspur TreeStuff.com Teufelberger Westminster X-Rigging Teufelberger
Back
Top Bottom