Wood Pole Structures for Electrical Transmission Lines: Recommended Practice for Design and Use

Used as structural members for centuries, round wood poles account for most overhead utility line construction. Wood poles are primarily used in one or more of the following structural configurations: self-supporting, guyed, and framed. These configurations may include transmission, distribution, and/or communication wires. Self-supporting structures are commonly used in tangent construction where the wire tension creates relatively little or no transverse or longitudinal loading on the pole. Guyed structures are commonly used for installations with line angles and/or unequal tensions to allow smaller pole sizes than would be required without guy wires. Braced H-frame structures typically support larger transverse and vertical loads than single-pole self-supporting structures, resulting in longer allowable span lengths and/or the use of larger wire sizes. These structures can also be designed to carry significant longitudinal loading by guying or other methods.

Design of wood pole structures must address several considerations to result in acceptable, efficient solutions. Each variable must be adequately defined by calculation, testing, or assumption to ensure the design will have an acceptable level of confidence. Before proceeding with design work and ultimately procuring wood poles for construction, the engineer must consider and establish several physical characteristics. The length of wood poles used to support overhead lines is based on the required clearances to the attached facilities and the required embedment depth. The magnitude and direction of all design loads determine the required class of a wood pole. Finally, when designing a wood pole structure and specifying a pole size and preservative treatment, the engineer must consider the ability to access those areas of the pole where hardware is attached.

This chapter describes the mechanical properties, applicable standards, and specifications used to manufacture wood poles. In the North American utility industry, the latest versions of ANSI Standard O5.1 and Canadian Standards Association (CSA) Standard CAN/CSA-015 provide wood poles’ physical properties. These properties include the strength and stiffness properties of the materials comprising the structures, which govern the response of structures to applied loads. For wood poles, the most important measure of stiffness used in analysis and design calculations is the modulus of elasticity. The chapter also briefly discusses the preservative treatments and conditioning of the pole. The American Wood Protection Association and CSA standardize preservative treatments. The two basic categories of treatments are oil-soluble or miscible systems and water-soluble systems.

Wood poles are a cost-effective structural material that is advantageous in the design of overhead lines. This chapter uses the best material property data available from multiple sources and applies current structural engineering principles to recommend appropriate design values and considerations, including the design principles and methodologies of load and resistance factor design. The desired outcome is accurate and reliable wood pole designs that are consistent in performance with other types of structural materials, such as concrete, tubular steel, lattice steel, and so on, when used in the design of overhead lines. Drilled holes will reduce the strength of a wood pole by decreasing the area at a given cross section and thus decreasing the section modulus as well. Resiliency describes the overall robustness and recovery characteristics of overhead line infrastructure.

The physical attachment to wood poles of arms, brackets, insulators, climbing provisions, and so on is an important consideration in the overall structure design because these connections serve the explicit purpose of transferring load to the supporting pole or frame structure. Most pole line hardware connects to wood poles using galvanized through bolts. Round wood poles are well suited for attaching hardware by banding with formed steel bands, steel strapping, or cast steel chain links. In designing connections, the engineer must consider eccentric loads and torsional moments, combined stresses, localized stress concentrations, and other loads resulting from the connection’s configuration. Failure of connections may occur in the wood fibers or the metal components. The chapter explains several of the particular modes. The manufacturer’s instructions regarding bolt tightening should be followed for all attachment connections.

Generally, wood poles are embedded directly, making the foundation a part of the pole and yielding a cost-effective means of supporting an overhead line. During the foundation design process, the engineer identifies the maximum pole groundline forces and applies them to the soil strength properties. Foundations for wood pole structures are typically classified as either direct embedment with native or engineered backfill or as special foundations that are engineered for a specific site. Continuous loads from wire tensions due to line angles or dead-end applications cannot be placed on wood poles without guying the load. Guy wires direct the loads from the pole to an anchor—of which the chapter discusses several types—installed in the ground.

When manufactured with effective quality assurance, natural round wood poles’ inherent qualities provide the engineer with a reliable structural material for design. This chapter explains components of manufacturing and quality assurance that enable poles to perform as intended. The variability of wood poles is quantified by the coefficient of variation. The manufacturing process starts by harvesting select timber and transporting it to a manufacturing facility. After peeling and additional preparation, poles are sterilized before the pressure treatment process to eliminate internal decay fungi. Any good quality control program at a wood utility pole supplier encompasses two basic disciplines: physical material quality and preservative treating results. These disciplines, of which the chapter describes specific aspects, are species and preservative dependent and are considered minimum requirements.

This chapter discusses handling, transportation, assembly, and erection of wood pole structures. Because several manufacturers exist and various species of wood poles are used, handling, assembly, and erection recommendations may vary. When purchasing poles from a specific manufacturer, the engineer is strongly recommended to follow the manufacturer’s guidelines in handling, assembling, and erecting those poles. The manufacturer usually predrills wood poles, but they are also easily drilled in the field using standard equipment and the appropriate wood drill bits. Once again, the manufacturer’s recommendations shall be followed in the field drilling process. As a natural product, wood poles are susceptible to damage due to natural conditions. The chapter describes a few of the most common damaging conditions and provides recommendations to protect the poles against them.

An important focus for owners of overhead wood pole lines is optimizing the service life of existing facilities while ensuring safety and structural reliability and resiliency. Implementation of an effective inspection, maintenance, and repair program is a key element in achieving this. This chapter provides an overview of relevant issues and processes. Variation in pole species, preservative treatment, environmental conditions, structure types, and so on leads pole owners to develop customized, cost-effective strategies for inspecting and maintaining their wood pole assets. The chapter describes the procedures and tools of traditional wood pole inspection. A key element of an inspection program is the criteria used to determine if a weakened pole meets code strength requirements. Unlike most safety code requirements for steel or concrete structures, the United States and Canada allow some deterioration in wood poles before requiring structural rehabilitation or replacement.