One of our ongoing objectives at Technical
Rescue has been to clarify/verify/disprove some of the long held beliefs in
rope rescue which appear to have their roots in a long ago whispered opinion
but which have never really been put to the test, at least not so’s the
wider community has been made aware. There are a number of teams and
organizations carrying out systems and components testing but very little of
the information seems to get out. Probably this is because many are carrying
out less than scientifically prescribed testing which is difficult if not
impossible for others to duplicate and therefore verify. But we believe that
real world tests - outside of perfect lab conditions are what we really need
in rescue - systems checks with all sorts of real world variables built in -
if you can demonstrate a possible problem or solution to a problem then we
all need to hear about it and if your working conditions are similar to
somebody else’s then maybe they’d also need to carry out a similar test.
Most of the problems picked up during such tests are once in a blue moon
occurrences - probably never happen during your rescue lifetime - just like
Piper Alpha (Oil rig) , Space Shuttle Challenger, Clapham and Granville Rail
disasters or the Titanic should never have happened.
Our Rescue Belay test program uses test parameters which should never occur
in rescue -a factor third fall whilst negotiating an edge - but it could and
does happen very occasionally, it may happen to you once in 10 years but on
that one occasion you’d want to know that your equipment which has worked
well on the previous 1000 drops is going to work now.
There is no value in scaremongering for the sake of it but any anomalies
picked up during training or on rescues need to be properly disseminated so
that other teams can make a decision as to whether or not the same problem
could affect them. So don’t be afraid to pass on informal test information -
maybe it will end up being invalidated but it would be useful to at least
have the opportunity to check out possible problems or solutions. You can
put it out on the Internet, send it to magazines like us or to in-house
service magazines but most importantly get it out there. Rescue is probably
the one remaining technical discipline in which research and testing is in
its infancy and where we’re still making up (and breaking) the rules as we
go along.
TEST PARAMETERS
The following test figures are being
presented for interest only - this is not a scientific document - if indeed
such a thing can even exist in rescue. These figures are specific to rope
type, environment, mechanical components etc. and method of use by an
individual so exact figures will vary but the broad picture should be
basically the same. Figures are shown in kilograms for ease of comparative
reference. Force should be shown as
KiloNewtons one of which is more or less
equivalent to 100 kilograms eg 100kg = 1.0 kN
A) INTERMEDIATE EDGES
Every time your main load bearing rope(s)
contact any surface between the anchor and you and/or your casualty on the
end there will be diversion of part of the total load away from the anchor.
This could be quite good couldn't it? We've got a dodgy anchor so we'll pass
the rope over a 90 degree round but high friction edge and halve the anchor
load. Well, probably not, but the following figures should highlight the
fact that anchor failure may be a very secondary concern to rope failure
over an edge because the loads on this very small point of contact are quite
high. This is a concern not just on the edge but at contact points further
down the face - carry some canvas rollup protectors in your pockets to pad
edges during the descent:
| A | 75 kg load | Load at anchor (kg) | Load at edge (nominal) |
|---|---|---|---|
| 1 | Straight Drop (no intermediate edges) | 75 | nil |
| 2 | Rope passes over a 2” hand rail at 90° to the anchor | 40 | 35 |
| 3 | Rope passes over a 90° 6mm edge on a T section girder | 32 | 43 |
| 4 | Rope passes over Petzl Edge Roller on 90° Girder edge (as 3) | 48 | 27 |
| 5 | Rope passes over Troll Edge Rollers on 90° Girder edge (as 3) | 58 | 17 |
A1) With the rope in contact with no other
surface the load on the anchor is more or less equivalent to the load on the
end of the rope.
A2)
A relatively smooth, galvanized metal 2”
hand rail rigged so that the rope enters at 180° and leaves at 90° diverts
just under half of the load away from the anchor. You may consider that if
the 2” bar is clean and smooth then diversion of almost half the load
represents a rather useful load sharing scheme!?
A3) Again entering at 180° to the edge before
passing over the flat top section and passing a 6mm edge at 90°. The edge
was smooth with a very slight curve (2-3mm) rather than being sharp. Being
more severe this edge accounts for over half of the load leaving the anchor
components with an easy 32kg to hold. Much greater risk here of rope failure
at the edge and padding is definitely recommended.
A4) The Petzl edge rollers have small diameter
rollers mounted on the three inside faces of a stainless steel box channel
section. Even though the rollers are of a smaller diameter than the hand
rail and the rope is not moving there is stretch on the rope when loaded
which is not restricted by the rotating rollers and more load is transmitted
to the anchor.
A5) Larger diameter rollers on the Troll model
(1.5”) demonstrates greater efficiency and more load transmitted back to the
anchors.
Very few rescue scenarios permit a clear
drop from the anchor to the load and you can therefore assume that a well
padded 90° edge will be taking around half of the load at the end of the
rope. If this were a belay scenario then quite significant impact forces
will be experienced at this edge which may not have entered your thinking
when establishing a bombproof anchor which ultimately may only be subjected
to half the load.
B) ASCENDING
The load cell was placed at the anchor
between a heavy duty 3” Spanset sling and a fig 8 knot in the rope. There
was no intermediate edge between the anchor and the load.
Texas rig (or Prusic rig) using two handled
ascenders (SRT Explorers
This set of tests is horrendously subjective
nevertheless the relative figures give us some idea of the increased forces
at work.
| B | Ascending (kg) | Down-climbing (kg) | |
|---|---|---|---|
| 1 | Attempted good technique (75kg rescuer - fresh) | 100 | 88 |
| 2 | Attempted good technique (75kg rescuer - fatigued) | 104 | 90 |
| 3 | Attempted good technique (84kg rescuer - fresh) | 104 | 94 |
| 4 | Carabiner realignment during downclimb (84kg) | - | 110 |
| 5 | Attempted good technique (84kg rescuer - fatigued) | 107 | 101 |
| 6 | Contrived poor technique (75kg rescuer) | 116 | 136 |
| 7 | Contrived poor technique (84kg rescuer) | 120 | 138 |
B1) Our rescuer ascends the rope in a controlled
manner, making constant steady progress with no snatching at the top
ascender and restrained reloading of the harness during the sit phase.
During the ascent the anchor load is increased by a third of the original
75kg. Downclimbing, which requires much more control on the sit phase to
minimise shock is very good, in fact this probably highlights a slight
deficiency in ascending technique.
B2) Having ascended and downclimbed a few times
(less than 50ft in total) this test highlights the obvious - your technique
deteriorates with fatigue and therefore the anchor loads are higher.
B3) Our more experienced and heavier rescuer
demonstrates slightly better ascending technique and again controls the
downclimb well adding less than an extra quarter to the anchor load.
B4) A common phenomenon when using carabiners
instead of maillons is the slight rotation of the crab on the D-ring
bringing about not only the undesirable loading of the crab away from its
strong spine but a heart-stopping jolt as it clicks back into place. This
rather minor realignment added a momentary 16kg to the anchor load.
B5 & B6) The rescuers purposely rush the ascent in as
realistic a manner as possible rather than trying to purposely shock the
system - you tend to pull harder on the ascender and sit down harder in the
harness. Around a half as much load again was added during the ascent phase
but in the downclimb hard sit-downs have bumped the load up to approaching
double the original load. This is a common problem with folk who are either
out of practice or novices or very fatigued.
C) HAULING REDIRECTIONS
The forces applied to an anchor are dictated
not only by the weight of the load and any additional forces due to hauling
but by the direction in which the hauling is carried out relative to the
load.
For this set of tests a load cell is placed
at the anchor and on the end of the rope to be hauled to try to gauge how
much force (or kg weight equivalent) is used by the hauler as well as the
load to which the anchor is subjected.
Theoretical Mechanical Advantage
With a load of only 20kg a simple 360°
redirection has generated over double the load at the anchor whilst the load
is simply supported by the rescuer. Add a degree of pulling force and this
rockets up to somewhere around 4 times the actual load depending on who is
doing the hauling and how smoothly. The input forces on a 1:1 are, as
expected roughly comparable with the load plus some friction from the
pulley. If we change the direction of hauling we see an escalation of force
at the anchor: The large CMI bearing pulley gives some clear advantages over
the smaller sheaves and especially when compared with a carabiner.
CONCLUSIONS
Everybody focuses heavily on anchors in rope
rescue - we fixate on the biggest, heaviest, most substantial thing we can
wrap a sling around or clip a carabiner into. This is good. But there are
danger points far more tangible in a rope system than anchor failure.
Assuming you haven’t just tied off around a termite infested stump or a well
disguised (but dead) stonefish the anchor could be the least of your
worries. We have heard of many system failure reports in the past few years
but haven’t yet heard of an anchor failure (cue loads of post….). In the
closest thing to a failure we’ve ever had ourselves the sheath on one of two
ropes was being slowly cut by a small burr on an otherwise smooth
intermediate metal guard rail. Any intermediate rope contact point between
the load and the anchor is a serious risk - in some cases this contact point
could be diverting more than 50% of the load away from the bombproof anchor
and onto a possible unprotected surface. The next greatest risk is a hauling
system - the loads being applied can be several times that of the actual
load for which all system components have been calculated. Primary amongst
these risks is the toothed cam capable of severing the sheath of any number
of rope brands but frequently its overloading is caused by too many haulers
or too enthusiastic a hauling action. If you are meeting with a degree of
friction/resistance such that the hauling proves very difficult think about
changing or improving your haul system - possibly by incorporating a
piggyback which makes the act of hauling slightly easier and therefore more
fluid. In both ascending and descending the actions should be as smooth and
constant as possible - Gung-Ho bounding down a face or ‘running’ up a rope
is unprofessional and bad for all system components from rope to carabiners
to slings to anchors (and especially intermediate unprotected edges!)
Thanks to: Pete Rowe, Phil Crook, Kerry
Charlton. Kieth Jones
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