Utility poles must be designed and built with certain safety aspects in mind. The National Electrical Safety Code (NESC) published by the Institute of Electrical and Electronics Engineers, Inc. (IEEE) covers the basic provisions for the safe installation and operation of aerial and buried power and telecommunication facilities within the Untied States. NESC specifies strength and loading rules based on the construction grade of the line and environmental loading district (wind and/or ice) to meet basic safety requirements. Because of these NESC-specified safety requirements, it is necessary to perform a Pole Loading Analysis (PLA) for the design of new pole installations as well as existing poles when the facilities on the pole will be changed. In particular, NESC C2-2012, Part 2: "Safety Rules for Overhead Lines", Section 25 "Loading for Grades B and C" provides rules that utility poles must withstand in storm loading conditions. This article provides a brief description of these loading requirements below.
Note: Safety requirements specific to California are based on the General Order No. 95 specifications, Canadian requirements are based on the CSA standards, and the AS/NZS 7000 standard are the requirements used within Australian and New Zealand.
Many factors go into the PLA including the strength of the
pole, how the wires and equipment are attached to the pole, and the environmental
factors surrounding the pole. Some
factors are more critical than others when it comes to the overall loading on
the pole. Due to the complexity of these
various interacting factors, it is common that most PLAs are performed
utilizing specialized software designed explicitly to determine the mechanical
loading on the pole.
The NESC specifies strength and loading rules based on three different "grades of construction." The grade of construction generally determines different margins of safety. Higher grades of construction translate to higher levels of structural reliability and safety to withstand the environmental conditions of ice and/or wind loading. The three NESC-defined construction grades are:
Grade B - This grade of construction provides the highest margin of safety and is required when the pole supports spans that cross limited access highways, railroads, and waterways.
Grade C - This grade of construction is most common and provides a basic margin of safety. It is often utilized for the typical power and joint-use distribution pole.
Grade N - This is the lowest grade of construction and is most often used for emergency and temporary construction.
Note: GO 95 grades of
construction are A, B, and C, and are defined based on circuit classes attached
to the pole. CSA defines pole grades as
either 1, 2, or 3, and are a combination of upper most circuit class and
situation at crossing.
NESC also specifies two environmental loading conditions (wind and ice-winter storm) which the poles must withstand. These environmental conditions are divided into three loading zones on the mainland United States plus warm Island loading zones used in locations such as Hawaii, Puerto Rico, and the Virgin Islands.
Heavy Loading Zone - This loading zone incorporates 1/2 inch of radial ice buildup and 4 pounds per square foot of wind pressure.
Medium Loading Zone - This loading zone incorporates 1/4 inch of radial ice buildup and 4 pounds per square foot of wind pressure.
Light Loading Zone - This loading zone corresponds to 9 pounds per square foot of wind pressure without adding ice to wire diameters.
The diagram below indicates where these loading zones are utilized within the US. The NESC also specifies rules for extreme wind conditions that represent a summer storm. And extreme wind and ice conditions in very isolated regions with known high winds and/or for very tall (above 60ft) poles.
In California, as defined by the GO 95 standards, there are two major loading districts:
Heavy Loading District - This loading district incorporates 1/2 inch of radial ice buildup and six pounds per square foot of wind pressure for poles located at altitudes of greater than 3000 ft.
Light Loading District - This loading district corresponds to nine pounds per square foot of wind pressure without adding ice to wire diameters for poles located at altitudes of less than or equal to 3000 ft.
The diagram below indicates where these GO 95 loading districts are located within California.
Ultimately it is the strength of the pole that determines if the pole in use with the various electric and telecommunications equipment and spans attached can withstand the environment loading for the necessary level of reliability (grade of construction). Utility poles, wood poles in particular, are classified by their size and strength into pole classes (Class 4, Class 3, Class 2, Class 1, Class H1, Class H2). As you move down in class number, the diameter and strength of the pole increases. The bending strength of the pole, as defined by the class of the pole, determines its load carrying capacity. The design process typically corresponds to picking the class of pole based on the required grade of construction, loading district, and the equipment/spans that the pole must carry. Keep in mind that since the pole's bending strength is a function of the pole's circumference to the third power, a slight increase of 30% in circumference increases the bending capacity by 123%.
The loading district, which defines the lateral load on the pole based on the amount of ice buildup and wind pressure causes lateral bending moments on the pole. The amount of bending moments is also a function of various parameters of the equipment, spans, and guys attached to the pole. The most significant parameters of the spans that effect loading include attachment height, diameter, span length, and span angles. For the equipment attached to the pole, one needs to understand the shape, size, weight, and attachment height to properly take into account the wind/ice loading. When specifying guys attached to the pole it is necessary to know guy size/type, attachment height, orientation, and distance the anchor is away from the pole.
Finally, one should keep in mind that as the pole ages, the
strength of the pole may decrease due to damage or decay. Existing poles in the field that are having
new equipment or spans attached, should be evaluated for the potential risks of
existing decay and pole strength loss.
This strength loss of the pole is often modeled within a pole loading
analysis by reducing the effective ground line circumference of the pole.
One point of confusion between NESC and GO 95 is how each defines the various factors that must be applied to the pole loading analysis to determine sufficient strength for the applied load. NESC uses two different types of factors: Load Factors and Strength Factors. The NESC load factors increases the applied storm load on the pole based on the required construction grade. While the NESC strength factors decreases the efficient strength of the pole.
GO 95 using a single factor called the Safety Factor. The Safety Factor augments the storm load on the pole. In either case, the standards specifies that the 'applied load' must be less than the 'efficient strength' of pole in order for the load to meet the standards safety requirements. The table below outlines how these NESC Strength/Load Factors and GO 95 Safety Factors are applied, with the last column giving a direct comparison between two standards safety requirements based on construction grade.
In a nutshell, we perform pole loading analyses so that (1) the pole in the field is safe and reliable and (2) so there is a level of confidence that the pole won't catastrophically fail (break and fall to the ground) during severe weather conditions. Making sure that the strength of the structure has been designed to meet the NESC standards also ensures a level of reliability for the electric and telecommunications services on the pole. As an engineer designing a new pole structure or changes to an existing structure, you want to safeguard the facilities on the pole, the utility workers who work at the pole, and the general public who live or pass by the pole.
Since many factors must be be taken into account when performing a pole loading analysis, there are comprehensive pole loading analysis tools, such as Osmoses' O-Calc® Pro software, that help take the guesswork out of load calculations for line design, pole replacement, and joint-use loading. For more information about the O-Calc® Pro structural analysis software for utility poles, please call 716-319-3423 or email [email protected].
Would you like to know more about pole loading? If so, we invite you to learn more about these upcoming educational events:
Webinar: Wood Pole Strength & Loading Basics
April 25, 2016 at 11:00 AM Eastern | Online (WebEx)
meeting NESC requirements with ANSI 05.1 wood poles
May 16 - 19 | Learn more or register here.