How to Specify a Spun Concrete Pole
A spun concrete pole, like any pole supporting conductors, has various loads applied by line tension, changes of line direction, wind, and ice loadings if applicable, on both the conductors and the pole itself. Because of their durability and reliability, spun concrete poles have a favorable load factor specified in ANSI transmission and distribution line requirements. When specifying poles for distribution and transmission use, it is common to select a pole based on tip load requirements or by design.
Concrete poles are grouped in alphabetically labelled classes which have a bending capacity appropriate to the design requirements. The class is defined in terms of a guaranteed minimum ultimate transverse load applied 2 feet down from the tip of the pole.
The ground line moment capacity depends on the length of the pole since that moment is the product of the class minimum ultimate transverse load and the distance between the point of application (2 feet from the tip) and the ground line.
Note, the wood classification system is based on an average strength where the concrete pole class is based on minimum strength, so the two classification systems cannot be directly compared.
The strength of a concrete pole is defined as a minimum strength which does not vary over time. Poles are designed and manufactured so that all poles will exceed the specified minimum strength which will be present for the life of each pole. In contrast, because wood is a naturally grown product with natural defects such as splits and knots, strength is defined as the average strength of all poles of that species and dimension at the time of installation. Since some poles in a batch will have strengths less than the average and because there is further deterioration with time, a strength factor is utilized.
The NESC has determined the strength factor for concrete as 1.0 compared to 0.65 and 0.85 for wood in Grade B and Grade C construction, respectively. This strength factor is especially important when converting wood poles to spun concrete poles so that the wood pole is replaced with a concrete pole that is designed to the appropriate strength.
For higher voltage lines with large conductor spacings and multiple circuits, there may be major reactions distributed over a substantial length of the pole. There may also be many different load combinations along a line, depending on pole spacing and line angles.
For these types of applications, it is typical to specify poles with a load tree for each pole loading variation along the line. This can be specified by the customer or determined by StressCrete using multiple elevations at which loads can be applied so that the required capacity of the pole can be accurately computed along its entire length.