Shock load

[ QUOTE ]
During the filming of "The Art and Science of Practical Rigging", we dropped a 660 pound log into the dyno and there was a very good 'rope man' running the rigging line, and the dyno read over 4,500 pounds. That's 7X.

I agree with the formula that mass increases exponentially for every 1 foot drop.

[/ QUOTE ]

What was the usual distance of fall to reach those numbers Norm?
 
[ QUOTE ]
[ QUOTE ]
During the filming of "The Art and Science of Practical Rigging", we dropped a 660 pound log into the dyno and there was a very good 'rope man' running the rigging line, and the dyno read over 4,500 pounds. That's 7X.

I agree with the formula that mass increases exponentially for every 1 foot drop.

[/ QUOTE ]

What was the usual distance of fall to reach those numbers Norm?

[/ QUOTE ]

The throat was maybe 2 inches on the cow hitch, plus the distance of an arborist block and dyno. So, from the tie off to the dyno was approximately 2 feet, a 4 foot drop total.
 
During the Rigging Research for the British HSE and Forestry Commission, we tested the rule of thumb discussed in this thread against the forces measured when logs were snatched in a snubbed-off scenario. It did not work all that well, I have to say.

The whole forces in rigging issue does not seem to be all that simple, maybe that's why there are no easy answers. But there is some information in the Rigging Report that may be helpful. The report has become a rather big ... but maybe it is worth for people in this thread to look at. Don't get scared off, it is a lot of pages. Chapter 8 deals with forces in rigging.

12 MB download from http://www.tree-consult.org/html/eng/articles/rigging.htm click at the HSE link button.

The rule of thumb from Don Blair's book is discussed on page 187 and 242 (watch out, the pdf-reader may display different page numbers). In a snubbed-off scenario, the rule of thumb underestimates the shock-load for short logs and overestimtes for long logs (longer than 1.4 m).

Loads are much greater of course when logs are being snubbed off instead of let run. We also let two logs run, and tested tree tops against logs.

The results showed that for the trees that we dismantled and the lowering rope we used, the force at the rigging point was rather constant between 9 and 10.5 times the logs' weights. The tree tops with branches on generated roughly 6 to 7-times their weights as a peak force. When the leaves were still on, it was only 4.5 to 5.5-times. Though the tree tops in leaf were actually snubbed off, the forces were not much greater than for logs let run: these resulted in forces roughly 4.5 times the logs' weights - but we did only two of those.

But of course, the stem's reaction to snubbing off these tree tops was much more violent compared to letting log's run. In these cases, I believe shock load is not the only concern. Strong stem deflection and violent shaking may be more of an issue for the climber aloft ...

Check out the report if you like. There will be some articles in the future, one was in Tree Buzz already and is still on our webpage.
 
Sorry to be a smart
tongue.gif
,

but distance of fall in mechanics is calculated from the center of gravity before the drop to its position after the drop. Peter Donzelli has described that very well in The Art and Science of Practical Rigging.

But please, forget about mass increases with every foot a body drops. That is not the case. The force generated when rope stops a falling body is just a product of its mass and the deceleration. Mass remains constant, and deceleration is how quickly the body is changing its speed from free fall to rest.

If you let a log run, you slow it down over a long period of time, i.e. you gradually decelerate it. If you snubb it off, you stop it in a fraction of a second, and you generate great peak forces (like hitting the brakes of your car pushes your body into the safety belt if you wear one)
wink.gif
 
Thanks for taking the time to post your information. It's nice to see scientific study. When I have used my dyno on job sites as well with Norm and others during demo's snubbed off rigging was generally around 10 times the load quite consistantly. Nice to hear from you Andreas.
 
Todd K, Norm, so what do you use in the field, as a rule of thumb to help estimate the forces that will be applied. Or is it just years of experience, and using a dyno.. I just want something to go off of, so i'm not just cutting and crossing my fingers. I have a rough idea, and usually stay within a safe work limit. But sometimes on the bigger stuff, it's nice to know
 
Thanks to everyone for contributing to this thread!

Adding science to the discussion is so helpful.

After reading the replies here and reading a few parts of the Rigging Report I deleted the old rule of thumb for drop loads. That will be stored in the steamer trunk along with the hank of three strand Green Streak manila climbing line.

The 'Rigging Report' is going to be printed and bound. I'm 'thumbing' through it now on line. What a great resource!
 
I use a rule I was taught when I was fresh in my climbing career."bigger piece, bigger rope". There are too many variables to have a rule of thumb, what rope, how much rope, what speices, skill of the rope handler. Any change in the system will have a big effect on the force.
 
[ QUOTE ]
I use a rule I was taught when I was in my early in my climbing career."bigger piece, bigger rope". There are too many variables to have a rule of thumb, what rope, how much rope, what speices, skill of the rope handler. Any change in the system will have a big effect on the force.

[/ QUOTE ]

That is great! Some true sound advice and i appreciate it. I usually switch my rope out on really big wood to a stable braid, however i find if i use a 1/2" line it helps to keep me honest.... If that makes sense.
 
Yeah let it run is half the lesson. The other half is to know how to put the brakes on easy .that’s always the last thought that goes through my mind when the piece leaves is hoping my ground guy doesn’t screw up. But if it really is critical a seasoned man gets on the rope trainees on the not so hard ones.
 
I like the rule, also just keep in mind when I was taught that we were not using any slings or blocks just saws and ropes. So nowadays look out for the sling.(twice the force of the rope.)Also never forget the tree, the true puzle.
 
[ QUOTE ]
...so what do you use in the field, as a rule of thumb to help estimate the forces that will be applied.

[/ QUOTE ]

Based on what Andreas said, and what Peter D. found before him, and what Tod and others have found, I have used the guideline that the load at the block is about 10x's the weight of the piece. For critical situations and/or snubbing, that gets increased. Also need to account for the condition of the equipment etc.
 
Theoretically if you had a long enough distance you could drop a 100 lb log 10 ft and then decelerate it very slowly and the force on the anchor could (theoretically) be 201 lbs (rope tension could be 100.5 lbs).

Conversely, You could drop a 100 lb log 1 ft onto a steel cable with no stretch and have it held taught. The force here could be in the thousands of pounds.

The loads are measured in force over time or "impulse". A good example of this is in cars they have crumple zones that reduce the impulse by decelerating cars over time instead of coming to a jarring stop.

There is no formula to predict the force on the anchor unless you know how far the rope will run, how hard it will be decelerated, how much the rope will stretch and how much the tree will bend. Obviously this can not be predicted accurately so it is best to stick with rules of thumb.

I will see if I can put together a spreadsheet to play with.
 
I would like to add that the knots used are often overlooked as weakest link in the system especially the bowline. What is it like 60% off the strength of your line? Big fan of clove hitch and half hitches myself.
 
Not to bore everyone, but check out the Rigging Report
smirk.gif


We have evaluated a test series that Ken Palmer and Michael Tain did at Samson on the strength of knots in rigging lines and slings when attached to logs. Unfortunately the results are not available in an article yet, so you would have to work your way to chapter 7 of the rigging report.

And yes, it seemed that in our tests the sling was the weakest link, which is somewhat frightening. But on the other hand, using a stronger sling is not really a problem, just a matter of cost and handling maybe. But it does not change the dynamics of the rigging system very much. And from my point of view, the rope should be the weakest link. Definitely not the tree, and rather not the sling ...

That is not to say that using shoestrings would be the optimum
tongue.gif
 
Interestingly enough, I have overloaded a rigging system once and it was the sling that failed. Fortunately in this at the time the system was more of a backup the actually necessary.
 
[ QUOTE ]
Todd K, Norm, so what do you use in the field, as a rule of thumb to help estimate the forces that will be applied. Or is it just years of experience, and using a dyno.. I just want something to go off of, so i'm not just cutting and crossing my fingers. I have a rough idea, and usually stay within a safe work limit. But sometimes on the bigger stuff, it's nice to know

[/ QUOTE ]

I have a green weight log chart with me at all times. I calculate the log weight and how far it gonna fall before the "catch".
 
You must log in or register to reply here.

New threads New posts

Back
Top Bottom