While wood poles are important, the real work in carrying the conductor wires, transformers and other equipment is done by the wood crossarm. To many, crossarms appear as a simple, rectangular piece of wood bolted at the top of poles. But in reality, they are actually durable, engineered products designed to carry the unique loads in overhead electrical systems.
Click on the video to review today's wood crossarms
Today's wood crossarms are the culmination of more than a century of use. They are designed to take on weather extremes as well as unique structural challenges to provide decades of service in place.
History of crossarms
When wood poles were first used in 1844, they typically carried one or two light telegraph wires. The development of electricity generation at the turn of the century brought new load demands, leading to the use of wood crossarms to carry heavier lines, transformers and other equipment.
Wood crossarms first appeared in the 1920s to carry multiple wires and insulators. These crossarms were primarily made of untreated wood and were produced with a “round roof” or heavily beveled edges at the top to allow for moisture run-off.
In the 1930s, with crossarm use expanding, automated drilling was adopted to consistently place holes needed to affix crossarms to the pole. Some of that original drilling technology is used today.
A variety of species have been used for crossarms over the years. However, the most popular species used is Douglas fir, noted for its strength, stability and availability. In addition to Douglas fir, crossarms are also made from Southern Yellow Pine.
Preservative treating was started in the 1940s, with creosote and pentachlorophenol used to extend the life of crossarms from years to decades in place. At first, preservative treating was done through a thermal dip process, with crossarms immersed into a tank of heated preservatives, followed by dipping into a “cool” tank.
Treating for durability
Crossarms today are pressure treated to intergrate protective preservatives into the wood. Manufacturing starts at the sawmill, where crossarms are cut and graded at the mill based on the grading rules for the respective species.
The freshly cut crossarms are shipped to the treating company, where they are inspected, stacked and prepared for drying. Dry kilns provide controlled conditions to sterilize the wood and reduce the moisture content to the required 19 percent or less.
After drying, Douglas fir crossarms are incised, with small slits cut into the wood to get preservative deep into the wood. The crossarms then move to a boring center where holes are drilled in consistent locations for equipment that will be attached later. Drilling holes prior to pressure treating allows the preservative to move into those cut areas to protect the wood.
Next, the crossarms are placed on a cart and moved into a cylinder called a retort for pressure treating. The retort is sealed, a vacuum is applied and liquid preservatives are pumped in to fill the cylinder. Pressure is applied over time to force the preservatives into the wood fiber.
The wood is then removed from the retort and allowed to dry. Crossarms then undergo final assembly, such as adding end plates or customer-requested equipment, and then are bundled or packaged based on customer requests.
The most profound changes for crossarms have come in the past 40 years, as national standards were created. The first standards came through the ANSI Accredited Standards Committee O5, or ASC O5, which was organized in 1924.
The ASC O5 Committee includes representatives from wood pole and crossarm producers as well as utilities and the general public and it maintains the chief standard for crossarms, ANSI O5.3 Solid-Sawn Wood Crossarms and Braces: Specifications and Dimensions. For utility co-operatives, the Rural Utilities Service, or RUS, offers a similar standard in Bulletin 1728H-701.
Both standards define the minimum quality levels for crossarms, including structural grades, sizes and moisture content. They also define strict dimensional tolerances, requiring crossarms to be no less than 1/8th of an inch oversized and no smaller than the specified size.
Once a finished crossarm is attached to a utility pole, it becomes a critical structural component for electricity distribution. As such, the design properties for crossarms must be incorporated into the engineering of the loads and design stresses in overhead systems.
Crossarms work in concert with the wood pole to carry dynamic loads which can change depending on conditions such as winds, snow, ice and damage to other poles.
Given these stresses, wood crossarms must be exceedingly strong. One indicator of strength is the Modulus of Elasticity (MOE), which measures the stiffness. Douglas fir crossarms are assigned an MOE of 1.7 million pounds per square inch, or psi, making them very stiff and able handle such stresses without bending excessively.
Alternative materials are often promoted as stronger and more durable than wood. Many of these claims, however, are not substantiated. From an engineering standpoint, comparing materials can be an apples and oranges question, as wood offers some distinctive qualities not found in other materials.
Wood poles and crossarms offer a unique resiliency beyond their stated capacities. Standards for wood poles and crossarms define a 20 percent coefficient of variation, or COV compared to a 5 percent COV for materials such as steel or composite fiberglass. So when exposed to extreme loads, a higher percentage of wood poles and crossarms can survive vs. materials with a lower COV.
The choice in materials for crossarms is increasingly driven by environmental and sustainability considerations. Too often, these choices are based on incomplete information or erroneous assumptions not supported by science. By many independant, objective standards, wood offer numerous environmental benefits.
Wood crossarms come from a sustainabile raw material, with timber growth far exceeding the volume harvested each year. With treating, crossarms last for decades and contribute to sequestering carbon in place. The estimated 100 million wood crossarms in use in North America represents 90 million metric tonnes of carbon dioxide, or CO2, removed from the environment. That volume is equivalent to the CO2 emissions from burning 10.2 billion gallons of gasoline.
Independent life cycle assessments, or LCAs, offer the best way to compare the environmental impacts of materials. Recent LCAs for utility poles offer insights that can be applied to wood crossarms as well.
Preserved wood utility poles use less energy and resources, and have a significantly lower environmental impact when compared to concrete, steel and fiber-reinforced composite utility poles. When normalized, the LCA data shows the production and use of fiber-reinforced composites generate 8 times more greenhouse gases, require twice as much fossil fuel use and 27 times more water use compared to preserved wood.
For more than a century, wood crossarms have been the top choice of utilities throughout North America. They have an unmatched record of long-term performance in service. They are made from a sustainable, renewable resource and with pressure treating sequester carbon for decades in service.
Most importantly for utilities, wood crossarms are much more cost effective than alternatives, with prices often three to four times lower than alternative materials. And they are readily available in good or bad weather to keep the lights on. When you add it all up, there is only once choice in crossarms to bring you power: Wood!