When is a Pole “Rejected”?
Either by a formal or informal inspection, every year a Utility has a number of poles that are “rejected” to be changed out. Depending on the circumstances, an individual may opt to have a pole removed for any number of reasons. For this discussion, I will limit it to when a pole should be rejected for reduction of strength value.
Each pole within a specific height and class has a predicted breaking load, where a specific load applied two feet from the pole tip, will break 50 percent of the poles in that height/class. By that reasoning, approximately 50 percent of poles will break with a lesser load applied and approximately 50 percent of poles will need a greater load applied to break. The range for a natural wood product that is grown and is subject to knots, density variables and other natural anomalies is greater than would be required for a manufactured product, such as steel or fiberglass.
Taking these variables into account, when a line is built, the NESC recognizes different grades of construction based upon the criticality of the structure. Grade B, being the highest grade of construction, applies to specific conditions such as highway, railroad and other conductor crossings, to name a few. Grade C applies to the vast majority of all other distribution pole installations.
Transverse loading typically is the controlling design criteria in new pole installation. Measured in terms of a Safety Factor (SF) where a SF of 1 indicates the breaking stress of a pole to the estimated maximum stress in ordinary use, traditionally Grade B construction has been built to a SF of 4 while Grade C construction has been built to a SF of 2.67.
Taking the above traditional construction intents into account, in general, the NESC recommends replacement of a pole when it has assumed 2/3 of its required safety factor for the construction type.
Over time, equipment and conductors may be added to a structure, creating an overload situation, thereby reducing the amount of degradation required to meet that required strength requirement. In short, pole loading may be increasing while pole strength is unchanged, technically bringing a pole to reject status.
Due to the large number of variables required to be taken into account in a current pole status assessment, a number of “rules of thumb” have been applied. Additionally, pole loading software has been developed to take some of the guesswork out of strength loading.
External degradations of a pole, especially out of the line of lead, affect pole strength to a much greater degree than internal degradations (See Figure A). The problem with internal degradations is that they are not uniform and unseen. Short of cutting the pole in two to examine it, there is an additional factor which has to be built in for the unseen variables of decay. For all internal decay, there is a certain amount of wood that is still intact, which is measurable, but contributes very little in terms of strength to the pole.
The other factor with internal degradation versus external degradation is that it is more difficult to control. External degradation can be removed completely, measured and treated thoroughly with a wood preservative, while internal degradation cannot easily be measured, removed and treated. In short, this will likely cause an inspector to reject a pole more quickly with internal decay than it will with a pole with external decay, if presumed strength values are the same.
Most poles are subject to degradation due to decay and mechanical damage is in a zone from 10” below groundline to approximately a foot above groundline. Therefore our reductions for strength are centered largely towards inspections in this specific zone.
Summary:
· Most distribution poles are rejected when they reach a SF of approximately 1.8 (Grade C). This is determined with a mix of art and science.
· There are parameters to describe when a pole should be replaced, but rejecting a pole can also be subjective based on the discretion of
the pole owner
Best Regards.
Joel
· Most distribution poles are rejected when they reach a SF of approximately 1.8 (Grade C). This is determined with a mix of art and science.
· There are parameters to describe when a pole should be replaced, but rejecting a pole can also be subjective based on the discretion of
the pole owner
Best Regards.
Joel