Structural Resiliency Affects System Resiliency*

By: Nelson G. Bingel III, NESC Chairman

The destruction and service interruptions caused during Hurricane Sandy in 2012 brought national attention to the resiliency of the electric grid and telecommunication systems. The Department of Energy, the Office of Electric Reliability, and the National Institute of Standards and Technology, among others, are looking at developing resiliency metrics for system performance in major weather events.

System resiliency for overhead electric and telecom lines is explained as the ability of a system to withstand a major storm and minimize service interruptions along with how quickly service is restored. Many factors contribute to system resiliency, including system monitoring and communications, sectionalizing, redundancy, overall utility storm preparedness, and many others. Another factor—the structural resiliency of the wood poles that support overhead lines—is often overlooked.

However, data and experience indicate that in major storm events, wood pole structural resiliency has a direct impact on the number of service outages, the cost of restoration, and the time of restoration. Every time a wood pole fails in a storm, the time and cost of restoration increases dramatically and more customers are likely to be without service. The lower the structural resiliency of an overhead system, the more likely it is to have increased pole failures, time of restoration, and cost of restoration.

Wood Pole Strength
Utility structures are loaded in multiple directions; vertical, longitudinal, and transverse. 


There is a tendency to be concerned about the vertical loading caused by the weight of wires and equipment. However, the governing design criterion, especially for distribution lines, is usually the transverse loading, which is caused by the wind force that is perpendicular to the wires, pole, and equipment. The transverse loads tend to blow the structure over sideways, while the bending strength of the structure resists those loads.

The Accredited Standards Committee O5 is responsible for publishing standards for the manufacture of wood poles and crossarms. The document ANSI O5.1-2017 Wood Poles: Specifications and Dimensions establishes the required bending capacity for all lengths and classes of wood poles.  Although the various species of wood poles might have different wood strength (fiber strength), the circumference requirements are adjusted so all species of poles that are the same length and class have similar bending capacity.

A horizontal load applied two feet from the tip of the pole establishes consistent bending capacities.  This load represents the wind blowing on the wires attached to a crossarm. The values of the horizontal load are shown below.  Keep in mind this is a mean load, so half of a pole population would be expected to fail before reaching the value and half would exceed the value. The division of classes between telco, distribution, and transmission is shown as typical frequency of use, not hard limitations.  This listing also demonstrates that the class of a pole is determined by the number and size of attachments, length of spans, and other factors.  The length of a pole is based on the number of attachments and the vertical clearance required for each attachment.

Wood Pole Loading
The National Electrical Safety Code (NESC) sets the basic requirements for safety of overhead and underground lines. Section 23 addressed the clearance requirements that result in a required pole length. Sections 24 through 26 specify wood pole strength and loading requirements.  Similarly, General Order 95 establishes the strength and loading requirements for overhead lines in California.  The loading and strength variables specified in the NESC include the following:

  • Wind pressure
  • Ice thickness
  • Strength factor
  • Load factor
  • Grade of construction

Even though the NESC and GO 95 are safety standards, these values are in fact applied as design criteria by utility companies for the construction of distribution systems and, in large part, for transmission systems. Therefore, the NESC and GO 95 ice- and wind-loading, plus the strength and load factors for the grade of construction, are primary factors in providing the original "structural resiliency" of the lines. A utility company's choice of construction grade can vary the initial structural resiliency by a factor of two.

Once poles are in service, the subsequent structural resiliency depends on how well utility companies retain the original structural resiliency.

Groundline Decay
Wood pole manufacturing includes full-length treatment with an original preservative that helps to prevent decay deterioration for many years. However, at some point, in-service wood poles may decay in the section from groundline to 18 inches below, out of sight.

There is usually not enough oxygen to support decay deeper in the soil.  

Decay in the groundline zone causes a direct reduction of structural bending capacity and thereby structural resiliency. The NESC and GO 95 allow for up to a one-third reduction in the required bending strength before a pole is deemed a "reject" and must be restored or replaced.

Pole-Management Practices: Superficial or Comprehensive?
Most utility companies inspect some portion of their poles during an annual program, but their approaches vary and the results related to structural resiliency are widely different. Some utility companies simply require sounding a pole with a hammer, above groundline. This method identifies less than half of current "reject" poles (finding only the worst of the worst) and very few of the poles with earlier stages of decay that require maintenance to halt decay and/or restoration.

In contrast, a comprehensive pole inspection program uses multiple traditional inspection methods and includes excavation below groundline for greater accuracy in assessing a pole's condition. Such a program also includes removing early decay and application of supplemental preservatives to control the process of decay through the next cycle.

Several cycles of a comprehensive pole-management program will maintain the structural resiliency and extend the service life of a pole significantly. Data shows that the national average pole life without pole management is approximately 45 years. The average life occurs when half of the poles are likely to have a remaining bending strength that is below code requirements.

In contrast, the national average life of poles within an effective pole-management program is 73 years. This life extension is accomplished through multiple inspection and treatment cycles that help prevent the reduction in structural resiliency. The total cost over the extended life of the pole is likely to be $250 or less. This total cost represents 5 percent to 8 percent of pole replacement costs when they range from $3,000 to $5,000. This cost yields results of a significant 60 percent increase in average life for an additional 5 percent to 8 percent in cost over the life of the pole.

One aspect of wood pole management that made funding a difficult issue for many utility companies was that most of the program cost was considered operations and maintenance.  Handheld computers used during pole inspection in the 1990s began developing a large database of information on wood poles. This data was used to validate the life extension shown above that results from the application of supplemental preservative treatments during a pole management program.

As a result, many pole owners now capitalize a significant portion of pole-management costs.

Dramatic A/B Comparison of Structural Resiliency Affecting System Resiliency
There have been many instances where one major storm event impacted neighboring utility companies revealing structural and system resilience that was very different. The table below illustrates data taken from public reports on how two neighboring utilities fared following a hurricane.  The numbers for Utility B were adjusted to account for having 60 percent more poles.

The comprehensive groundline management program of Utility A included excavation and supplemental treatment and achieved a 98 percent efficacy for retaining structural resiliency.  Utility B had a less rigorous program that omitted effective supplemental preservative treatments to control decay and retain structural resilience. The efficacy of this program is rated significantly lower as many more weakened poles remain in-service and continue to decay.

Utility A replaced 152 poles following the event, while Utility B replaced 2,790, a factor of 18. The costs of restoration for Utility B were 16 times the restoration costs of Utility A, $310 million versus $20 million, resulting in a factor of 16. Utility A had fewer weakened poles, resulting in fewer outages, faster restoration time, and a far lower cost of restoration.

It's important to point out that while hurricanes provide a dramatic example, weather phenomena such as ice storms—nearly ubiquitous, if unpredictable, across the nation—can be just as devastating. Such storms have wreaked havoc in many states, which led state commissions to require improved pole-management programs.

Though I raise the issue as chairman of the NESC, action on this issue will fall to regulators and utility companies, as the NESC is neither a complete design guide nor a prescriptive how-to manual.

In the wake of a variety of major weather events, regulators in Florida, California, Missouri, Kentucky, and other states have established more-prescriptive pole-management programs. Utility commissions in additional states may find it beneficial to review aspects of pole-owner wood pole-management programs. They should seek to find answers to questions like what kind of supplemental preservatives are incorporated, if the costs of preservative application are capitalized, and whether the program results in lower overall costs to consumers and retention of structural resiliency.

Just to repeat, the new mantra "structural resiliency affects system resiliency" and analysis of data on effective pole-management programs point to a very positive cost-benefit ratio. A more comprehensive, proactive approach goes to the very heart of the traditional mission of utility companies: to provide electric power or communications that is safe, affordable, reliable, and resilient.

*This article was originally published by Nelson Bingel as "Wood Pole Strength and Loading - Key to Resiliency, Require Programs" in Natural Gas & Electricity 34/10 (October, 2017) ©2017 Wiley Periodicals, Inc., a Wiley company. 

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