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Replacement Criteria for Broken Strands of Cable

State WY
Description Text Our maintenance personnel are asking how many strands of cable can be ruptured before cable replacement becomes necessary.  I believe that Trinity has advised that if 3 or fewer strands in a single bundle are ruptured, the cable will still have adequate reserve.  I believe the cable is 3 bundles, with 7 strands each (3x7).  Please advise.
Other Keywords cable barrier, cable, rupture
Date September 27, 2012

Response The decision on when to replace damaged 3x7 wire rope (3 strands of 7 wires each) in a cable system should be kept conservative due to the critical function it serves. In the past we have noted research into wire rope damage for rigging applications that suggested that several wires could be damaged or fractured in a strand as long as the fractured wires were relatively far apart. The argument for this approach was that the friction developed by the weave of the cable wires and strands should help develop the fractured wire over significant cable length. 

However, this type of recommendation does not apply readily to wire rope used in cable barriers because of the difference in the application. For rigging applications, there is generally a significant factor of safety and the wire rope is not stressed near its ultimate capacity. In addition, the wire rope must have the ability to absorb energy in order to develop higher loads when used in a barrier impact. Due to these requirements, we would recommend replacing wire rope on a cable barrier if there is a single fractured wire in a strand or if there is visible plastic deformation and necking of individual wires in a strand. If wire fracture or necking of one wire in a strand is observed, it is safe to assume that the other wires in the strand were loaded at or near their plastic limit as well. Thus, the remaining ductility, internal energy, and capacity of the strand with the damaged wire is likely very low and subsequent loading of the cable will reach the limit of the strand capacity more quickly due to the reduced internal energy dissipation in the strand. 

We would not recommend that you follow the Trinity recommendation of 3 damaged wires in the strand. This would indicate a strand that had very little remaining capacity and we would not consider it fit for service. 
Date September 28, 2012

Response Thank you for your response.  One additional point of information.  The fractures don't appear to be caused by yield strain, but by some feature of the impacting vehicle actually cutting strands.
Date September 28, 2012

Response end of response
Date October 22, 2012

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