MRA Research - the Role of Energy Absorption Energy Absorption in Multipoint Anchors - a designed...

MRA Research - the Role of Energy Absorption

Energy Absorption in Multipoint Anchors - a designed approach

Bob L. Zimering, PhDJoel R. Hayes, MSE

MCSO-MR/CAMRA

Agenda

• Objectives• Experiment 1 - Baseline loads• Experiment 2 - Side vs middle leg failure• Experiment 3 - Screamer type, orientation,

belay Design of Experiments• Belay load, leg load, slip results• Noise and significance of results• Comparison with past work• Lessons learned & next steps• Appendix A : project budget• Appendix B : photos


Hypothesis

• Using a “screamer” will prevent failure when one point of a 3 point belay anchor fails because of impulse force when catching a 600 lb load.

• Experimental Factors:•Different belay devices•Load sharing vs. load distributing•Middle vs. side leg of anchor fails•Different types of screamers•Screamer orientation (parallel or series)


Objectives• Establish peak failure load of low-stretch 7/16 inch rope for three belay devices

without screamers : Petzl I’D, Traverse Rescue 540, and tandem Prussiks. • Determine the peak load on the belay device and remaining legs of a multi-point

anchor when one leg fails• Evaluate utility of screamers, used either in series with or parallel to the legs• Evaluate relative utility of two brands of screamer : Charlet Moser and Yates.• Create recommendations for optimal use of screamers in rescue scenarios.

Failure Criteria• The load is dropped.• The sheath is damaged enough to show the core.• The core is damaged enough to create an easily palpable soft or hard spot.• More than 24 inches (60 cm) of rope slips through the belay device.


Experiment 1

ANCHOR

Dyno

Belay device

load

• Experiment set 1 baselined the failure loads of the belay devices used.

• The device is set in series with the load and a digital dynamometer

Results : •The tandem prusiks withstood the highest amount of energy before failure. •The ID generated higher load than the 540 rescue device but stopped the fall with less slip

Results at failure are tabulated below

device fall distance peak load slip distance failure mode

prussiks 42 in 1960 lbf 10 in damage

ID 24 in 1455 lbf 29 in rope slip

540 24 in 1320 lbf 34 in rope slip

1850


ANCHORS

2mmcord

Prusiks

load

• Experiment set 2 established the relative forces in legs of a 3 point load equalizing anchor system after failure of one leg

• The experiment compared middle leg to side leg failure

• Due to budget, only one load cell was available. The belay device was attached to the dynamometer

Experiment set 2

1850

1850Loadcell

Dyno

SS quicklink

Results : • Side leg failure created 2200 lbf force,

compared to 1345 with center leg failure• Maximum force generated in the

(remaining) side leg 750 lbf


• Experiment set 3 culminated represented a combination of 1 & 2 with the addition of “screamer” energy absorption devices.

• Only side leg failure of a 3 point load distributing anchor was studied

• Screamers were put alternately in series or parallel• Two different screamers were used : Charlet Moser and Yates• Each combination series/parallel and CM/Yates were permutated

with the 3 belay devices (Prusiks, ID, 540)• One load cell was used to measure load on the side leg, and the

digigal dynomometer was attached to the belay device.

Experiment set 3


Experiment set 3, con’tANCHORS

2mmcord

Belaydevice

load

1850

1850

Loadcell

Dyno

SS quicklink

load

1850

1850

Loadcell

Dyno

SS quicklink

Screamer

Belaydevice


• Range of load measured by dyno were 680 - 1935 lbs - all configurations with screamer kept peak load below failure.

• Load measured in leg by load cell typically about 30% of dyno load - about 40 % of energy is absorbed in friction of the load distributing system.

• Prussiks have high propensity to cause minor damage to rope, and generate highest loads - load distribution by prussik body melts rope but doesn’t break it.

• Slip with 540 device and screamer is much greater than slip with either of other devices - 540 load limiting feature makes screamers irrelevant.

Experiment set 3 results - practical


Experiment 3 results - belay load main effects

• Belay device greatest driver for dyno force• Force with Prussiks ~ 25% higher than with other devices• Negligible effect of screamer type, orientation on max load at belay device


Experiment set 3 results - belay load interactions

• Nonlinear interactions between belay device and screamer type. Yates seems to limit load better in most cases, but physics cannot be explained without more experiments.

• Interaction between orientation and screamer effectiveness - Yates does slightly better job limiting load in series orientation than CM screamer


Experiment set 3 results - side leg load

1000

600

2001000

600

200

scream

belay

series

trav

id

pr

yts

cm

Interaction Plot - Data Means for load

• Load in leg strongly dependant on screamer orientation

• Force with Prussiks almost twice that with other devices

• No discernable influence of screamer type on load in leg

• Nonlinear response not significant

• Results corroborate Dynamometer results• Load in leg strongly dependant on

screamer orientation• Force with Prussiks almost twice that with

other devices• No discernable influence of screamer type

on load in leg


Experiment set 3 results - slip

40

20

0

40

20

0

scream

belay

series

trav

id

pr

yts

cm

Interaction Plot - Data Means for slip

• Slip is noisiest response in experiment

• Slip with 540 device in conjunction with screamer is order of magnitude greater than others

• Slip not dependent on screamer type, or orientation

• Slip not strongly dependent on type, slightly on series. When using 540 device, prefer parallel to minimize slip

• Series/parallel vs screamer type reverses slip magnitude. Use Yts in parallel or CM in series


Experiment set 3 results - statistics• We infer that statistical significance

of belay & belay interactions is ~80%• Practical significance - the greater

load generated by prussiks is more likely to damage the rope than are the two mechanical belay devices.

Source DF Seq SS Adj SS Adj MS F Pbelay*series 2 649254 649254 324627 4.2 0.192belay 2 510679 510679 255340 3.3 0.232scream*belay 2 367004 367004 183502 2.37 0.297

Dynamometer (belay device) load

• Screamer orientation effect is only ~60% statistically certain.

• Anchor leg load is driver for catastrophic failure - orientation is potential driver

Load cell (anchor leg) loadSource DF Seq SS Adj SS Adj MS F Pseries 1 199950 199950 199950 1.15 0.397belay*series 2 382317 382317 191159 1.09 0.477

• Rope slip is a potential danger with wrong combination of gear

Rope SlipSource DF Seq SS Adj SS Adj MS F Pbelay 2 2860.2 2860.2 1430.1 1.82 0.354scream*series 1 602.1 602.1 602.1 0.77 0.474

“P” value is a measure of probibility that effect is “real” - like a measure of “signal to noise” ratio. (We would accept a 80% risk in assuming that these terms are not significant in effecting the dynomometer load)


Conclusions• Using a screamer either in parallel or in series keeps peak load on the belay below

failure when one leg of a 3 point anchor fails in catching a 600lb working load using either tandem prussiks or a Petzl I’D, under conditions which would have generated just enough energy to fail the system without the screamer.

• Both Charlet Moser Nitro 3 and Yates Zipper screamers work equally well for this rescue application.

• We recommend to NOT use screamers in conjunction with the Traverse Rescue 540 belay device for this application. The combined effects of load limitation and increased rope slip that both the 540 device and the screamer impart on the system is not advantageous from a load limiting perspective, and can significantly contribute to unacceptable total rope slip.

• We recommend that if screamers are to be used as a load limiting device in a multipoint anchor system, that they be used in the parallel orientation. Parallel orientation eliminates system extension due to screamer extension, and using a parallel screamer orientation is about 70% certain to lower peak load relative to series orientation with prussiks (estimate difference 30 percent), and about 60% certain to lower load in the legs (estimate difference 70 percent).

• Friction in the webbing reduces the loads at the anchor by approximately 50%


Literature review & future work• [Lion 1996] Load/slip distance with Petzl I’D 6-8 kN 40 cm slip. Consistent with

our findings• Screamer devices are known to produce consistent results when new, but are

fabricated with materials that degrade quickly with age and abrasion. Concern is waranted if these devices are used systematically as part of a team’s anchor gear

• Load distributing (equalizing) arrangement of anchor is known to create greater loads than a load sharing arrangement. This was confirmed by experiment 1. Loads in multipoint load sharing anchors are far less dependent on actual anchor geometry since the induced slack is minimal compared with a load distributing anchor. This is consistent with NFPA recommendations.

• Experimentation has shown that prussiks are less likely to fail under a shock load than are mechanical devices such as ascenders [2001]. Our experiments showed the reverse trend, with good correlation (80%). This merits further research.

• Additional statistical analysis to conduct : Regression, Power calculation, and Multivariate Analysis may shed light on (statistical) significance of the results


Acknowledgements

• Traverse Rescue• Petz’l• PMI

We would like to thank the following companies for donating or discounting their products for this study:

We would like to thank the following people for their helpful discussions:• Werner Hueber• Kirk Mauthner

• Yates• Charlet Moser

We would like to thank Neal Jeffers (CAMRA) for the use of his tower, and David Bremson (CAMRA) for his assistance in testing.

Finally, we would like to thank the MRA for providing the grant under which this research was conducted.


Appendix A - BudgetCosts (USD)

540 Rescue Belay………………….225 x 1 = 225Petzl I'D…………………………… 150 x 1 = 150400 ft 7/16 low-stretch rope……….. 0.66 x 400 = 264100 ft 8mm low-stretch rope………. 0.55 x 100 = 55Charlet Moser Nitro 3……………... 28 x 6 = 168Yates Zipper Screamer…………….. 21.50 x 6 = 1291” Tubular Webbing………………. 100 x .40 = 40Dynamometer Rental……………… 95 x 4 = 380Miscellaneous Costs (quick links, paperwork, etc.) 100

Total:……………………………………………………………….1411


Appendix B - Photos

Prussiks after experiment 2, (partially welded to rope)

Screamer deployed after experiment 1


Background• During technical rescue operations, standard practice is to use a belay line as a

back up for safety. When necessary, the belay might be anchored to 3 points which share (or distribute) the load, none being absolutely “bomb-proof”.

• If a 600 lb load (victim + 2 rescuers) shockloads the belay, there is a potential for one of the 3 anchor points to fail - creating an instantaneous redistribution of much greater energy to the remaining legs due to slack in the anchor webbing.

• Energy absorption devices (“screamers”) designed for recreational climbing, in conjunction with choice of belay device may mitigate the risk of a catastrophic failure of the entire anchor due to failure of one point.

• Mitigating risk of anchor failure must be balanced both against risk of rope failure and unacceptable rope slip.

• This study seeks to quantify the relationship between these requirements against the parameters and choices available to the technical rescue team.

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