Fall Factors?

Y

Yoav_Bar-Ness

Location
Hobart, Tasmania
Hello.
Does anybody know the math for fall factors on lanyard DRT systems? It is quite an important part of rock climbing but I've yet to see any discussion of its role in arborist lanyarding.

Fall factor is distance of fall divided by amount of rope in play.

For example, if a rock climber falls after climbing a bit above a belay ledge and falls past the ledge to an equivalent distance below, thats a fall factor 2, which may snap a climbing rope or other gear.
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What about DRT?
On a split tail, you've got:
harness to
anchor clip to
anchor knot
to anchor side of rope
to branch
to control side of rope
to control friction knot
to control friction knot cordage
to harness

How do you calculate FALL FACTOR? for this system?

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Hi, Yoav;

For our climbing systems, fall factor can be calculated about the same as for rock climbers. All connections are part of the system and do not alter the fall factor calculation. Calculate the fall factor like in rock climbing but double the amount of rope used in the calculation. 2 parts of line, twice as much rope.

Joe
 
Since arbos rarely climb with slack in their ropes or climb above their anchors, fall factor is factored into the system.

Our climbing system is referred to as work positioning not a fall arrest system. That's why we can climb with a front support. The NAA has fought long battles with OSHA about defining the category of body support.
http://www.bstorage.com/speleo/Pubs/rlenergy/default.htm

This article might give you more info on rope loads.

Tom
 
That's good information Tom. Where can I look to find out more about how our equipment is labeled in respect to safety like you are mentioning? I guess I need some comparisons to understand what is actually ment when it's said our climbing system can be attached in front of us as opposed to what is considered a fall arrest system.

Also, thanks for posting the rope physics link. For some reason I haven't been able to access it until you posted it.

Joe
 
<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>Originally posted by joe:
Hi, Yoav;

All connections are part of the system and do not alter the fall factor calculation.
Joe<HR></BLOCKQUOTE>

I have to disagree with you here Joe. I think that there are some extra variables in trees that differ from rock (small though they may be)

The anchor in tree climbing can very often be dynamic. (wood bends)

The friction hitches and other devices on the market have some slip to them.

Both of these factors can significantly reduce impulse.

I also dont think that doubling the rope length is correct either. More rope should reduce the forces by increasing the amount of stretch. In our DDRT system, that extra rope would actually reduce the stretch.

I also have to disagree with Tom. I think that lots of arbos climb above their tie in. Every time we double lanyard up a tree we are probably guilty of standing up above our anchor. Remember that even a short distance above can give a drastic impulse on the climber. especially you guys with steel core lanyards. Dont forget too how we scramble up a few limbs and then tighten our line. this is normally a small amount of slack but the closer you are to the anchor, the more the fall factor increases.

Dave
 
Hi, Dave;

I've missed our debates.
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I think there needs to be some clarification of what is being asked. The gentleman asked how to calculate fall factors with the DdRT like in our climbing system.

To calculate fall factor, divide the distance of fall by the amount of rope out. I explained how I'd calculate fall factor using the DdRT.

I agree with what you have written, but we're discussing 2 different concepts.

These links should help clear away any confusion.
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http://www.fishproducts.com/tech/fallfactor.html

http://www.beal-planet.com/anglais/beal3.html

http://www.petzl.com/statique/sport/ENG/tech/html/shock1.html

Again, thanks for the lead Tom.

Joe

[ September 25, 2002: Message edited by: joe ]
 
Dave,

Thanks for sharing the smiley picture! Looks good
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That rope is looking a little worn. Don't you think that some nice yachting rope or accessory cord would look good?

You're right about climbing above a lanyard. There are times when that happens and has a very real opportunity for a hard fall.

My lanyard is made with 3/8" static that's about 20' long so I'm really careful about placement. The lanyard is double ended and both end has an adjuster. With this setup I can generally get the lanyard into a branch union and not have it sliding along a trunk.

Has anyone ever taken a hard fall onto a rope or lanyard? I've taken swings many times and slid down a limb or two. When I'm in a position that might lead to a slip I keep my rope snug in order to reduce the shock load on my bag of bones.

Tom
 
I feel that in order to have a 'hard fall' when tree climbing, you would need to be directly below (or above)your tie-in point with slack in the line. I've slipped a few times, sometimes from way out on a limb. Rather than falling and getting snapped to a stop, I was redirected by my lifeline and swung back towards the trunk. So I believe the ANGLE of your lifeline between the climber and tie-in point needs to be taken into account on any 'study' of fall factors.
 
Fall Factors are nothing but a CONCEPT to explain the relative SEVERITY of a fall. The actual peak force will change by many variables coming into play.

In the UK we allow 50cm of slack in the system to allow for body thrusting. In tests I've done, the force from such a fall was between 550Kg and 650KG. This is around what our HSE find acceptable. In the field, the forces are likely to be less with more rope above the climber and possible energy absorption of the anchor branch.

A factor 2 fall with a doubled rope system measured around 1200Kg - enough to seriously injure or kill a climber. So - don't climb above your anchor point!

There were lots of other tests and conclusions in the project - I'll be glad when its finally published!
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Mr. Pez;

Since you are stating some material about the subject of fall factors etc. from tests you've been involved with is being published, may I be informed when and where to get these published results?

Joe

[ October 03, 2002: Message edited by: joe ]
 
No worries Joe.

I finished the project in May. It should be ready by the end of the year. It was undertaken for our industry advisory group. When its finished, I'll post it here.

With the developments that are coming through with Huberts lock jack type devices, it is important people realise the benefits of friction hitches. The only drawbacks so far are tying them correctly and friction heat damage. Following my conclusions with the project, a cord is being developed and tested to handle heat.

It would be interesting to test the lockjacks and see at what load they break the rope.
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<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>Originally posted by MrPez:
That is interesting Dave!

At what load does the device slip, and for how far?<HR></BLOCKQUOTE>

The load that the rope slips depends on the type of rope being used, and the wear on the device (being a lockjack or mimono). It is also affected by other friction in the system- ie: friction saver or not, rubbing branches.
The devices will slip at a certain value depending on the above variables and dissipate the energy until the force drops to a level where the device will hold.
One of the first tests I gave a prototype mimono was to drop a 300 lb weight 7 ft. I used highvee blue rope and a pulley up top so all the friction was on the device. The load slid for 3.25 ft before coming to a stop. Try this experiment with a blake and stand back!
More recent tests with production models have slipped to absorb anything over 300lbs for some ropes to 1300lbs with others. It is important to realize that neither device runs. They only absorb the force until it drops below the slip level.

Dave

Dave

[ October 05, 2002: Message edited by: dave spencer ]

[ October 05, 2002: Message edited by: dave spencer ]
 
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