Page 208 - J. C. Turner - History and Science of Knots

Studies on the Behaviour of Knots 197

the anchorage and the attachment to the load will be affected by high forces.
If the load is deformable, as is a person, the longer the time a high force is
applied, the greater the likelihood of severe damage. Other consequences of
longer falls at the same fall factor are a greater absolute elongation of the rope,
with a greater chance of hitting the bottom, and a higher speed, with greater
damage if any intermediate object is hit.
UIAA Fall Test
The UIAA has devised a standard fall test (Fig. 4). To qualify, a full-sized
climbing rope has to withstand at least 5 falls of 80 kg with a fall factor of
1.78, with the maximum shock force of the first fall less than 12 kN. Only the
shock force of the first fall must be less than 12
kN; subsequent falls generate higher forces as
the energy-absorbing capacity of the rope di- ♦ I
minishes, reaching substantially higher values
up to the point of failure [18, p. 18][23, p. 77). E
A force of 12 kN on a person is only probably
survivable, depending on how it is distributed
through a harness or the like, but damage could fi
be expected; it is also greater than the breaking o o Y
strength of some of the smaller anchoring de- In X
0°
vices. Consequently, climbers take precautions
to minimise fall factors, and hence shock force,
by placing intermediate running belays at suit-
able intervals; they are also interested in various
kinds of dynamic belay techniques and flowing 402
hitches (see Chapter 9). These are usually in-
tended to limit the shock force to between 2 and
6 kN, considered the best compromise between Fig. 4. The UIAA Fall Test. A
the force imposed on the falling object and the test weight (1) of 80 kg (for a
full-size climbing rope) falls 5.0
amount of rope run past the belay [10]. Static m freely to (2), held by a rope
kernmantle ropes used in caving, rescue and the 2.8 m long, passing over a bypass
like are unlikely ever to need to be called on to edge of 5 mm radius at (Y), 0.3
meet a high fall factor; they are made to have m from the rigid anchorage (X)
much less stretch, specially at low loads, than and forming an angle of 30°
the dynamic climbing ropes. They are therefore tested at lower fall factors,
often 1.0 or less, often in a simpler apparatus without the bypass edge.
Knots
Only a few tests of the strength of knots under shock forces have been
made; kernmantle ropes, either static or dynamic have been used. Some fall
tests have been made on the holding power of friction hitches and flowing
hitches and belay devices (see Chapter 9) and will be discussed below in the