Structurally Overloaded Poles

Utility poles in service today, although adequate at the time of installation, may no longer meet structural loading requirements for various reasons.  While strength loss due to deterioration at the groundline may eventually cause a pole to fall below the strength required to remain in service, a separate issue that is a cause for concern involves additional load applied to the pole beyond what it is designed to hold.  When a pole's load exceeds the loading requirements set by a utility's recognized loading codes and standards, that pole is considered overloaded and must be rectified to be in compliance. Overloaded poles left on the system run the risk of premature failure during storm events which effects system reliability, protection of assets, and, most importantly, public safety.

The previous issue of the T&D Times contained Pole Loading 101 which outlined the basics of conducting a pole loading analysis, including requirements based on NESC, GO95, SCS, AS/NZS 7000.  For a basic understanding of structural load, please refer to this article.

Common Causes for Overloaded Poles

There are several common causes for poles becoming overloaded while in service. If proper evaluation was not conducted before the additional load was applied, the pole is at risk of overload. When searching for lines at risk, focus on circuits that may exhibit any of these issues.

  • Addition of new distribution lines or equipment:  If a line received an additional phase, a transmission line added distribution underbuild, or transformers were added to a pole, it's important to verify the current pole size is rated to carry the new, greater bending load.
  • Reconductoring with larger diameter conductors:  If a circuit needs more electrical capacity to feed a growing power demand, it may be necessary to change out the current conductors with larger capacity (larger diameter) cables. The larger diameter will add more wind load to the pole, potentially causing an overload condition.
  • Joint use/third-party attachments:  If a joint user attaches to an existing pole and there is not proper coordination between the pole owner and the joint user concerning load evaluation, the attachment may unknowingly cause an overload condition. Telephone, cable, fiber, and antennas all need to be properly considered in the overall structural loading of a pole.
  • Critical circuits targeted for structural upgrading:  While not technically an overload concern based on code requirements, some utilities have adopted loading criteria above and beyond what is required on certain critical lines.  This practice is commonly referred to as line hardening or structural upgrading. If a pole or line does not meet the higher loading requirement, it would be considered overloaded based on the upgrading initiative.
With increased power demands on the electric grid, telecommunications, and fiber build out growing, and system hardening and reliability becoming a larger focus in the industry, these sources of structural overload are becoming more common.   States and regulatory bodies have taken notice and some are starting to put plans in place for pole loading analysis requirements. 

Identifying Overloaded Poles

As discussed in the Pole Loading 101 article, there are pole loading analysis software products available on the market today that can be used to create detailed loading analysis of a pole in question.  While it would be beneficial to have a full analysis on record for every pole in a utility's system, there may not be funds or man power available to accomplish this is a timely manner.  By focusing on potential problem areas using the common causes listed above, utilities can prioritize pole loading on suspected poles. 

Another method to help prioritize where full loading analysis is needed would be to implement a load screening program in conjunction with a pole inspection program. An inspector, equipped with data collection/load calculation software can quickly enter in pole construction information (length, class, circumference, span lengths, estimated conductor sizes, estimated mounting heights, equipment, etc.) and immediately determine an estimated load percentage.  Poles estimated to carry just below or over 100% of their allowable load can be prioritized as poles requiring a full pole loading analysis.

Once a pole has been shown to be overloaded though detailed pole loading analysis, something must be done to correct this compliance violation.

Correcting Overloaded Poles

The obvious solution to correct an overloaded pole is to replace it with a larger pole of sufficient strength to correct the issue.  This is often the most complex and expensive option because it requires a new pole, certified electrical labor, and coordination with joint-use attachers.  Before selecting a pole replacement option, consider the following alternatives:

  • Rearrangement of facilities on the existing pole: The higher an attachment is placed on the pole, the greater it contributes to the pole's bending load. If there is space to move attachments closer to groundline without violating clearance requirements, it will reduce the bending load and possibly alleviate the overload condition.
  • Correction or addition of guying:  Improper guying may be found on overloaded poles.  Using pole loading analysis software, a designer is able to determine if a modification to the location of an existing guy or the addition of a new guy(s) will correct the overload.
  • Addition of a mid-span pole:  The span length greatly affects a pole's bending load because the greater the span, the more surface area there is for wind pressure to be applied. Overloaded poles with long spans may be corrected by installing a new pole mid-span and attaching power and/or communications. This shortens the span length and transfers some of the original load onto the new pole.
  • Installation of a capacity upgrading truss:  A steel truss used to upgrade the capacity of an in-service wood pole maybe used as a low-cost alternative to pole replacement. The truss works in conjunction with the sound wood pole to create a combined structural bending capacity greater than the original pole. These trusses are manufactured from high-strength steel and engineered to exceed the strength requirements needed to correct the overload condition.  They can be sized appropriately to accommodate most pole sizes and overload conditions, or used to upgrade the pole by 1, 2, 3, or more classes for a line hardening initiative. Installation is faster than replacement and does not require service interruptions, coordination with attachers, or result in double wood.

These alternatives should be evaluated for each overloaded pole before considering replacement while keeping in mind cost, timeliness, complexity, and safety. Regardless of the solution used, finding and addressing overloaded poles is a wise practice to improve system reliability and reduce liability.