Utilities Make Tradeoffs When Selecting Pole Types

Jun 1, 2003 12:00 PM
By Andy Stewart, EDM International Inc.

For more than 20 years, I've been dealing with poles involving specifications, design, manufacturing, treatments, economics, hardware, testing, repair, inspection, recycling, disposal and failure investigations.

Each pole type has particular advantages, and the usage of different materials shifts with time as utility requirements, manufacturing technologies and the economics of pole materials change.

In assessing system economics, first cost rarely should be the determining factor when designing the transmission and distribution system. As a critical part of the delivery system, pole maintenance and pole life must be considered when selecting the best materials for the job at hand. Instead of thinking in terms of discrete measurable components, it is necessary to look at a project holistically, because poles are only a single component of a transmission or distribution project. In this respect, poles are usually not the most expensive component of such projects. A reasonable breakdown of cost contribution for a typical line project would include: engineering (8%); right-of-way (R/W) preparation (25%); structure costs (15%); hardware (7%); wire and insulators (25%); and construction with foundations (20%).

A caveat regarding these estimates is that there can be pretty wide swings from this rule-of-thumb presentation. For example, R/W could be more heavily weighted for an urban project and less for a rural project. Structure costs account for only 15% of the total cost of the line. Therefore, the pole selection is not of paramount importance, because even a 25% increase in structure cost translates to less than a 4% increase in overall project cost. Instead of concentrating on the pole's first cost, a more suitable approach would be to devise a cost-effective solution for the project by considering the respective characteristics of available pole types as tools to optimize the use of R/W, conductor characteristics and construction methods.

Survey Results: Utilities' Perceived Advantages of the Various Pole Materials
Material	Advantages
Wood	Cost, availability, workability
Light-duty steel	Service life, known strength, engineered product
Fiberglass	Service life, lightweight
Spun concrete	Service life, durable

The available poles are usually wood, steel, concrete, fiberglass-reinforced plastic and hybrid. Variations within each of these categories include:

For wood, natural round and glued laminated.
For steel, direct embedded and base-plated.
For concrete, centrifugally cast and static cast.
Fiberglass reinforced plastic, filament wound, pultruded and centrifugally cast.

Characteristics and Capabilities

The variables in this category have a direct interrelationship with both the conductors and insulators and R/W, which are the most significant cost components of a typical project. By considering these variables first, the list of options may quickly narrow. In general, the important variables are height limitations, transverse load capacity, stiffness, connection capacity and climbability.

Pole height and load-capacity limitations control allowable span length either on the basis of ground clearance or ability to support heavy wind and ice loads. Where R/W is at a premium, single-pole structures may have significant advantages, and choice may be dictated by the height and capacity limitations of the various pole options. In cases where single-pole structures are preferred, foundation capacity can be limiting. In configurations with large overturning moments resulting from wind loads or at sites with weak soils, it may not be feasible to use direct embedded poles. In these cases, base-plated poles that can be attached to concrete foundations may be the best option. Stiffness is important for single-pole structures with restrictive R/W width limitations. Length limitations help determine if it is practicable to transport a particular pole to its installation site. Connection capacity must be considered for manufactured poles, particularly for poles with thin walls. Finally, climbability is important, especially if poles cannot be accessed with bucket trucks.

Handling and Installation

While weight is the single most important differentiator of poles, it is only a factor under certain conditions. For typical construction, the differences in weight among steel, wood and fiberglass poles are almost inconsequential because equipment commonly used for line construction has sufficient capacity to handle them all. Spun concrete, however, could require the use of a different class of lifting equipment, thus, equipment availability and capacity must be considered.

For the most part, weight is only an issue when access is difficult or transport distances are long. Weight can become a significant factor when terrain or access requires the use of helicopters for construction or for back-lot access where it is advantageous to be able to carry the poles to the site. In urban locations that require tall poles, using multi-piece poles helps avoid the need for special permits when transporting long poles down city streets.

Design Issues

Important design issues include project-specific criteria, required analysis capabilities, and pertinent codes, specifications and standards. For example, nonlinear behavior is an important consideration for pole structures, especially those that will support large ice loads, transformers and forces imposed by guys and braces. When nonlinearity is an issue, the structural analysis used during design should account for this behavior and for differences in stiffness among the poles under consideration. The procedure and the criteria governing pole design must ensure sufficient capacity to tolerate the loads a pole will be exposed to during its service life.

For too long, the majority of the U.S. industry has used safety code requirements as the basis for pole design. Beyond the fact that safety codes should not be used as design codes, this approach is problematic because for years there have been inconsistencies in the criteria embedded in these codes that lead to inconsistencies in the relative structural reliabilities of poles of various materials. There are other standards used in the design process that lead to poor design that need to be corrected.

For example, the most recently released edition of ANSI O5.1, which is the most commonly used standard in the United States for wood pole properties, in my opinion, contains incorrect information that will lead to designs that are inconsistent with what the designer has assumed.

It is encouraging that the industry seems to be moving in the direction of adopting a Load and Resistance Factored Design (LRFD) approach built on the foundation of reliability-based design principles. EPRI was visionary in this regard, when more than 20 years ago it began an initiative to develop reliability-based design procedures for overhead lines. Similar procedures have already been adopted for almost all engineered structures outside the power-line arena. In fact, several other countries have embraced the concept of LRFD for their power-line design procedures. Reliability-based design embodied in an LRFD format holds the promise of enabling the engineer to design different types of lines, supported on different types of structures, and subject to different loading conditions to more consistent levels of structural reliability.

Durability and Maintenance

The most common forms of pole degradation are a function of exposure to temperature, moisture, biological agents and ultraviolet light. The impact of these variables can be minimized with coatings and preservative treatments. Too often decisions are made to save on first cost, and the protection potential offered by preservative treatments is not fully realized. In this instance, the relatively low cost of adding protection represents a trivial extra cost in the overall cost of the project and represents a prudent investment.

Beyond these variables, there are site-specific anomalies that should be considered when selecting a pole. For example, if a line is to be built in a corrosive environment, it is important to consider the impact of long-term exposure to alkaline soils, salt spray and salt fog along the coast. If a line is to be built through an area where nearby lines have been exposed to extensive woodpecker damage, this fact should be considered in making the selection. And, if forest or range fires are prevalent, selection should be based on this possibility.

Finally, it is important to consider how maintenance will be performed in the future. For example, if access to a line is difficult, then a proper choice would be a pole that requires less maintenance in the given environment.

Environmental Characteristics

It is unfortunate that some pole vendors, from time to time, criticize competitors for being environmentally irresponsible. The industry needs to consider all environmental implications of the various facets of the products to develop a balanced and rational program regarding these issues, which would include:

Environmental impacts of the raw material resources.
Energy requirements for manufacturing the product.
Pollution generated during the manufacturing process.
Environmental effects of the treatments and coatings used.
Disposal and recycling.

This industry, like all others, must be environmentally prudent and responsible. Wood pole producers should seek to improve the environmental friendliness of the preservatives, while advancing the treatments for their poles. At the same time, steel pole producers must promote energy-efficient production of the poles and control fugitive hazardous-coating materials. Concrete pole manufacturers must ensure that cement kilns from which they purchase their materials are not introducing hazardous materials into the atmosphere. And, fiberglass-reinforced plastic pole producers must control the release of volatile organics during manufacture.

Some site-specific issues have environmental implications where prudent avoidance should be practiced such as when a line might cross a sensitive habitat, a wetland area, for example. In such a circumstance, a pole made from an inert material would be a better choice than a pole that might leach. Where the habitat supports birds of prey and phase-to-ground clearances are minimal, a nonconductive pole material may be a better choice.

Initial and Life-Cycle Costs

The time value of money diminishes the significance of cost differences for maintenance among the different pole types, because the costs for maintenance are usually small relative to the cost of the pole and because of the long time that elapses between maintenance activities. For example, the time between installation and the first maintenance cycle under most conditions is usually 10 to 20 years, and the time between each subsequent cycle is in the range of 5 to 20 years. The net present value of these costs is small, so there may not be a need to perform life-cycle cost analysis unless the difference between the top two contenders for selection is small and determining the results may affect the ranking of the options. Life-cycle cost analysis should include: impact of environmental conditions on maintenance requirements, projected service life, inspection costs, maintenance costs, and disposal and recycling costs.

As in every engineering activity, there is no substitute for personal experience. Thus, it is important to temper the results of any rigorous analysis by relying on experience and good engineering judgment to ensure that the final decision is based on facts rather than perception.

Andy Stewart joined EDM International Inc. (Fort Collins, Colorado, U.S.) in 1983 and is now president of the company. He holds BS and MS degrees in civil/structural engineering and has devoted much of his career to the design, thermal rating, inspection, maintenance and operations of T&D lines and related R&D. Stewart is director of Intec Services Inc., a provider of inspection and maintenance services, and Barlow Projects, a developer of renewable energy projects. He is currently chairman of the IEEE Working Group on Management of Existing Overhead Lines.
astewart@edmlink.com