Minimum strength for anchor cordage & for prusik

Questions:

1.  What is the minimum breaking strength suggested for cordage for anchors for single strand, and for usage as prusik.  Unknotted.

2.  Any cordage or rope MBS is reduced by 25-40% after a knot is put on it.  So do two or more anchor points, assuming for perfect equalisation, add up in strength?  And exactly how so?  

So, let’s say for ease of illustration here, two bolted anchor points.  Each, let’s say considering the type of bolt, aging of the bolt, rock in which it is, placement expertise, etc is currently rated at 6 kn and 15 kn respectively.   And for further ease, both for pull out and shear strength.   Would that mean 21 kn of cumulative or 15 kn of force needs to be applied before failure?

I don't know about breaking strength, but a cordalette for wrapping around boulders and horns, 7mm is good. If you're only using it to clip gear for an anchor, 6mm is fine.

A prusik only holds body weight, so it's more a function of what size will bite on the rope (the difference in diameter between your prusik and your rope). I use 6mm.

Maybe you're wanting some scientific analysis, but don't over think it. Sorry if I didn't answer your question.

Your scenario you present is fairly complicated and a little unclear. What is rated 6kn and what is rated 15kn?

Question 1 - I use 7mm for cord, but a hollow block as a rap back up.

Question 2 reads like asking someone how fast their car can go, expressed in miles per gallon.

Avery Angarshli wrote:

Would that mean 21 kn of cumulative or 15 kn of force needs to be applied before failure?

 I think you are asking assuming one bolt will break at 6 kN and the other bolt will break at 15 kN.  However, its dependent on the geometry of the anchor setup.  

Read the section on "Force Balance", the link on "Here's the Math" will show you how to calculate the actual forces depending on the angles.

Link

To answer your loading question, for fun, in a simplified way:

If you were to a achieve perfect equalization, which is very difficult to do, and anticipate the direction of the load perfectly, again which is very difficult to do then,

You could, theoretically put a 12 kN load on it and that load would be shared evenly by both pieces, six each.

Now, in the fraction of a second that you exceed 12 kN, the six kN piece will fail, and the entire load would go to the other piece.  If you achieved perfect no extension,  which is very difficult to do,  then, only 12 kN will go to that piece.

In reality, one piece is going to take a greater  percentage of the load.  So, your rigging should direct the load very slightly towards your strongest piece. The results will vary.

The answer is definitely not 21 as the pieces do not add up.  Your anchor is only as strong as your strongest piece.

 A more likely scenario is if you apply a 14 kN load to your anchor and there is a bias towards your stronger piece, the anchor will hold. But if there is a bias towards the weaker piece, the 14 kN load would blow that one, and,  depending on your rigging, knots, hitches, type of cord, extension, that 14 kN load could spike above 15 and blow your anchor.

Thanks all.  For your responses to point two.

still searching for a specific answer for point one.

let me ask in another way.  What should be minimum ideal strength of the master point on an anchor?  Expressed in terms of 10, 12, 14 kn, etc?

Question 1 is 2 questions.

Avery Angarshli wrote:

let me ask in another way.  What should be minimum ideal strength of the master point on an anchor?  Expressed in terms of 10, 12, 14 kn, etc?

The second I say this someone will jump in and correct me but I don't think there is an ideal minimum strength expressed in kN.  For cordolette/personal-anchor-setup personally I haven't seen many people, if anybody, go less than 6mm nylon which is 7-8 kN.  Obviously if you use stronger material you can go with a skinnier diameter.  But, per the discussion, how that translates to the strength of your master point is highly dependent on the geometry of the setup.

I use 6mm, 21ft and carry 2. 

Also, 5.5 tech cord may be strong, but that shit is so slippery... It doesn't hold knots well If you're using it like regular cords and only throwing 2 overhands to close system, it may slip out and become untied. That's why they recommend a triple fisherman when closing systems with tech cord be it a cordalette or reslinging a cam.

I'd go no lower than 6mm IMO. 7mm is nice too, anything thicker is just not needed in recreational climbing settings unless your program requires it.

Here it is.  As recommended by UIAA.  The magic number at the master point.  13 kn.  now, a function of working backwards to determine, what the anchor cordage strength should be, taking into consideration the age of it, and the reduction in strength via knots.

zibircut wrote:

Here it is.  As recommended by UIAA.  The magic number at the master point.  13 kn.  now, a function of working backwards to determine, what the anchor cordage strength should be, taking into consideration the age of it, and the reduction in strength via knots.

None of that has anything to do with the UIAA or rock climbing.

Jim Titt wrote:

None of that has anything to do with the UIAA or rock climbing.

Please elaborate?  clearly that is a document provided per UIAA requirements to get the belay device pass the certification.  This is the Grivel Shuttle manual.  The device, I understand has the primary purpose of being used for ‘climbing’.  So what is missing here.

The real question here is: Why do you want to know?

Tradiban wrote:

The real question here is: Why do you want to know?

Have you followed the whole thread?  I can’t seem to find what the minimum recommended strength for the anchor cordlette.  All I get is recommendations for diameters.  So if one can understand the minimum requirement at the master point, one can work backwards to feel comfortable about each leg of an anchor.

Avery Angarshli wrote:

Have you followed the whole thread?  I can’t seem to find what the minimum recommended strength for the anchor cordlette.  All I get is recommendations for diameters.  So if one can understand the minimum requirement at the master point, one can work backwards to feel comfortable about each leg of an anchor.

Ya, I get that but what are you looking to do? Make a stronger master point? Find the smallest cord possible to use in an anchor? 

Tradiban wrote:

Ya, I get that but what are you looking to do? Make a stronger master point? Find the smallest cord possible to use in an anchor? 

Just trying to identify the MBS of the cordage I need for building anchors.  Diameter is not a criteria.  Because the MBs seems to differ from brand to brand.

Avery Angarshli wrote:

Just trying to identify the MBS of the cordage I need for building anchors.  Diameter is not a criteria.  Because the MBs seems to differ from brand to brand.

"Minimum breakage strength"? It's the KN rating of the cord. Is this for a climbing application or are you trying to do something different?

For any climbing anchors 6mm cord that is sold for climbing will do.

Ryan Swanson wrote:

Edit to add:  I used to use 6 mm. But the numbers just didn't work for me. I feel the extra bulk from seven millimeters is really insignificant and provides a better margin

And, based on what Jim has to say below, I feel justified.. 

Avery Angarshli wrote:

Just trying to identify the MBS of the cordage I need for building anchors.  Diameter is not a criteria.  Because the MBs seems to differ from brand to brand.

We work with diameters because the standard (EN 564) does so any certified accessory cord has a defined minimum breaking strength:- 4mm-3.2kN, 5mm-5.0kN, 6mm-7.2kN, 7mm-9.8kN amd 8mm-12.8kN.

Commonly used minimum values for each anchor point vary in the 6-8kN region so for single-strand applications 8mm is the minimum. For the actual strength of the masterpoint there is nothing to be gained in being stronger than the karabiner clipped into it and also nothing to be gained making it weaker so work on ca 20kN.