MRA Research

1
Energy Absorption in Multipoint Anchors - a
designed approach
Bob L. Zimering, PhDJoel R. Hayes,
MSEMCSO-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

2
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)

3
Objectives

• Establish peak failure load of low-stretch 7/16
  inch rope for three belay devices without
  screamers Petzl ID, 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.

4
Experiment 1

• 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

ANCHOR
Dyno
Belay device
Results at failure are tabulated below
load
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Experiment set 2

• 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

ANCHORS
2mm cord
Load cell
SS quicklink
Dyno

• 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

Prusiks
load
6
Experiment set 3

• 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.

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Experiment set 3, cont
ANCHORS
2mm cord
Screamer
Load cell
Load cell
SS quicklink
SS quicklink
Dyno
Dyno
Series
Parallel
Belay device
Belay device
load
load
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Experiment set 3 results - practical

• 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
  doesnt 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.

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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

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Experiment set 3 results - belay load interactions

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

• 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.

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Experiment set 3 results - side leg 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

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Experiment set 3 results - 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

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Experiment set 3 results - statistics
Dynamometer (belay device) load

• 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.

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)
Load cell (anchor leg) load

• Screamer orientation effect is only 60
  statistically certain.
• Anchor leg load is driver for catastrophic
  failure - orientation is potential driver

Rope Slip

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

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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 ID, 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

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Literature review future work

• Lion 1996 Load/slip distance with Petzl ID 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 teams
  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

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

• Traverse Rescue
• Petzl
• PMI

• Yates
• Charlet Moser

We would like to thank the following people for
their helpful discussions

• Werner Hueber
• Kirk Mauthner

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.
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Appendix A - Budget
Costs (USD) 540? Rescue Belay.225 x 1 22
5 Petzl I'D 150 x 1 150 400 ft 7/16
low-stretch rope.. 0.66 x 400 264 100 ft 8mm
low-stretch rope. 0.55 x 100 55 Charlet
Moser Nitro 3... 28 x 6 168 Yates Zipper
Screamer.. 21.50 x 6 129 1 Tubular
Webbing. 100 x .40 40 Dynamometer
Rental 95 x 4 380 Miscellaneous Costs
(quick links, paperwork, etc.) 100 Total
.1411
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Appendix B - Photos
Prussiks after experiment 2, (partially welded to
rope)
Screamer deployed after experiment 1
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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.