Wire Ropes Under Tensile Load - Wire Ropes: Tension, Endurance, Reliability - page 167

Stationary wire ropes have to meet all these requirements independent of each
other.

2.7.1 Extreme Forces

To prevent a wire rope breaking due to an extreme force which occurs only rarely,
technical regulations normally require that the minimum breaking force F min is
several times higher than the nominal rope tensile force S

F min m S : ð 2 : 112 Þ

The so-called safety factor m takes the increase of the tensile force due to
possible overloading into consideration as well as the weakening of the wire rope
breaking force due to fatigue occurring over time in the case of fluctuating forces
or by corrosion. Paton et al. ( 2001 ) found a reasonable weakening of the wire rope
breaking force occurs long before the rope breaks under the fluctuating tensile
force. One of their results shows that the breaking force for the spiral ropes tested
is reduced by 15 % at between about 20 and 70 % of their endurance. The 15 %
loss of breaking force occurs late if the endurance is low (N & 50,000) and earlier
if it is high (N & 5 9 10 6 ).
In technical regulations, experts have defined the reference values for the safety
factors based on their own experience combined with theoretical considerations.
Of course, for each individual technical field, the safety factor varies according to
whatever extreme forces may occur there. For example the safety factor for stay
ropes for cranes is about v = 3.2. For steel constructions and bridges, the safety
factor is smaller being about v = 2.2 as the greater part of the forces comes from
their own constant weight.
The wire rope breaking force is valid for wire ropes terminated with either resin
or metal sockets. For wire ropes with other terminations, the wire rope breaking
force (more or less reduced) can be calculated with the breaking force factor f F
from Table 2.7 .
The required minimum breaking force of the wire rope with the terminations
T is then

F minT ¼ f F F min m S :

ð 2 : 113 Þ