SRT Basal vs. MRT load differences

[MarkBugs]

Hi all,

 

I'm looking to understand the differences on the tie in point between a basal tie off and an MRT system (assuming the same anchor). I know MRT puts 100% of your load on the tie in point, but why does a basal anchor double (~200%) the load? If they are both two ropes coming down? Might be a stupid question but trying to wrap my head around it - thank you.

[carlos]

its because the basal tie acts like a 2;1 pulley system .

this will explain it better than me.

 

Mechanical Advantage Explained - Educated Climber.com

WWW.EDUCATEDCLIMBER.COM

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Crane at Prague Castle, photo courtesy of handshouse.org “Any sufficiently advanced technology is indistinguishable from magic.” – Arthur C. Clarke   “Mechanical advantage is a...

 

[peds]

So you've got a rope going over a good branch up in the tree, you are still on the ground. Grab both strands of the rope with both hands, and lift your feet off the ground: 100% of your weight is now on the branch the rope is going over, with each strand carrying 50% of your weight.

 

Let go of one strand, hold on to the other, and lift your feet again: 100% of your weight is on that one strand, but because there is no counterweight on the other end, none of that load is transferred onto the branch up above, and you land in a heap on the ground. 

 

Now find a friend who weighs exactly the same as you, and grab a strand each: you've both got 100% of your weight on the strand you are holding, which is carried up to the branch, leaving 200% of YOUR individual weight, or both yours and his. Neither of you collapse onto the ground.

 

Thank your friend for his time, tell him politely to go away, and tie his strand off on the base of the tree. The tree will now match your 100% weight, leaving 200% up on the branch.

[MarkBugs]

Legends- thank you guys. Both answers cleared it up for me 👍

[Joe Newton]

SRT climbers also have much bigger willys, increasing the loading on the anchor point

[peds]

Ah come on, that doesn't bring it up to 200% though, does it. 135, tops.

[Joe Newton]

Maybe not for you

[peds]

Absolutely fair point, trees aren't big enough here on the west coast anyway. I only ever use a ladder and a lanyard.

[john87]

IGNORING FRICTION, if you pull on a rope, no matter how it is directed, the tension is the same anywhere in the rope, yes??

 

Now, you tie a rope to the bottom of a tree and throw it over a branch. You now dangle on the rope. Say you weigh 150 pounds..

The tension in the rope is the same everywhere, yes??

 

So, you now have a rope with a tension of 150 pounds in each "leg" over the branch. The downwards force on the branch can easily be seen to be 300 pounds.

 

Follow me??

 

john..

[Mark Bolam]

19 hours ago, Joe Newton said:

SRT climbers also have much bigger willys, increasing the loading on the anchor point

SRT is also twice as safe because there is only one rope to cut instead of two, so 50% of the risk.