As transmission and distribution infrastructure worldwide continues to age, utilities must seek creative and cost-effective solutions to problems that naturally accompany aging assets. Several proven products exist to rehabilitate utility structures that no longer meet the code requirements as a result of decay, corrosion, or mechanical damage. Strengthening solutions that add capacity to poles and towers to enhance structural resilience are also available in the marketplace.
The use of restoration products provides utilities with significant economic advantages, as well as structural and environmental benefits.
Deterioration and Damage
Utilities have become increasingly aware of the need to inspect their transmission and distribution poles on a cyclical basis. The goal of a routine and methodical inspection and life-extension program is to monitor and remediate utility structures to maintain or improve the structural health and safety of overhead lines. Identifying problems such as groundline decay, corrosion, and damage is a key component of avoiding dangerous and costly pole failures. Depending on the severity, pole owners may be able to structurally rehabilitate these weakened assets at a fraction of the cost to replace them.
A Proven Track Record
There are a number of vendors and contractors that offer various structure restoration systems. When specifying and implementing restoration systems, utilities are urged to select engineered products and companies that have a proven installation and safety track record. Every component of a structural rehabilitation system should be tested and documented by the vendor supplying the material. In addition, long-term performance and consistency should be a factor in selecting a restoration system. Not all restoration systems are equal. Structure owners should ensure sound engineering practices have been followed in the restoration system design and that the system has been thoroughly tested and proven over time.
The most common and proven rehabilitation system that helps restore groundline strength to wood utility poles involves the installation of a single, or in some cases a double, steel member known as a truss. Truss systems are widely used by utilities and are the lowest available cost restoration option for poles with groundline decay.
It is highly recommended that steel trusses be galvanized to reduce long-term corrosion potential. Trusses are driven to predefined depths and secured to the pole with high-strength steel banding. When bending loads are applied to the structure, the resulting bending movement is transferred to the truss, which bypasses the decayed or damaged area of the pole and transfers it to sound foundation below ground.
The highest bending loads a utility structure withstands are a result of weather events such as high wind or high wind and ice accumulation acting on the pole and all of its attachments. These trusses, when properly sized and oriented, are engineered to restore the code-mandated strength to the weakened structure, without accounting for any wood strength in the weakened area that may remain.
Most trusses use high-strength steel. Some manufacturers offer specially shaped trusses that use steel with yield strengths as high as 100,000 psi. Use of higher-grade steel means steel reinforcement members can be lighter and more cost-effective than more common, lower yield material. Because of higher steel strengths, the thickness can be reduced for a given truss capacity, which reduces the amount of steel used and enables easier installation.
Additionally, pre-engineered restoration options for corroded and damaged steel structures are available today in the marketplace. A critical component of this, or any restoration system, is the application of remedial treatments to arrest wood decay or coatings to prevent corrosion. Failing to remedially treat decay or protect steel from corrosion may significantly shorten the life of the restoration.
Composite materials, such as fiberglass blends, have been widely used in industrial applications for many years. More recently, they have gained popularity in the utility and construction industries. Composite restorations give pole owners another option for restoring poles that aren’t reinforceable using a steel truss. By wrapping a pole in specially designed, field-impregnated composite fabric, poles can achieve full bending strength in all directions around the pole. This restoration methodology is suitable for poles with groundline decay and decay much higher on the pole.
Composite restorations are useful for poles with limited access and situations where overhead obstructions might prevent driving steel trusses. In locations where the pole’s appearance is important, composite restorations are an aesthetically attractive solution because the final installation can be closely matched to the pole’s appearance, color, and profile at the groundline. Properly designed systems include a protective barrier against weathering and ultraviolet deterioration to ensure the restoration does not degrade over time.
Composite installations involve excavating the pole, cleaning the pole, filling any external voids that might exist, and remedially treating the pole to arrest the decay. Composite fabric sheets are wrapped around the pole in layers with a resin material applied to each layer to bond the fibers together and seal the restoration. Remedial treatment is a key component of the long-term success of this restoration system.
Pole Capacity Upgrades
For many utility poles, the bending load generated by high winds blowing perpendicular to a pole’s wires and equipment results in high stresses starting in the groundline zone and decreasing as you move up the pole. As winds increase, so does the stress on the pole — and the likelihood of failure.
In the U.S., there are minimum structural loading criteria set forth by the National Electrical Safety Code (NESC) that almost all states follow (except California, which follows General Order 95). Other countries have similar standards that dictate minimum allowable loading criteria.
When a pole exceeds the required loading criteria of the applicable code, it is considered overloaded. Rather than be replaced with a stronger structure, the current structure can be strengthened with an engineered steel truss solution for a fraction of the cost of replacement.
The rapidly increasing number of attachments added to poles after installation accounts for a significant number of overloaded poles across the country. Additionally, extreme weather can produce wind speeds that exceed the minimum wind speeds that structures are required to withstand by the code. Evaluating poles to higher wind speeds above the minimum required may be prudent, especially for critical circuits. There is currently an increase in system hardening programs designed to increase the capacity of structures along such key circuits. This proactive approach will help prevent pole failures and provide additional system reliability.
Extended steel trusses are designed to help increase a pole’s load carrying capacity and correct overload conditions. These extended steel trusses create hybrid wood and steel structures that can increase a pole’s bending capacity as many as three classes. By properly designing the truss to match wood stiffness, the pole and truss work together to safely resist applied loads in excess of the original pole’s capacity. This type of reinforcement can resolve the overloaded poles and provide a system hardening solution.
Environmental Benefits of Rehabilitation
It is estimated that as many as four million poles are installed in the U.S. each year. Proper maintenance, along with restoration of in-service poles, can increase average service life by decades and reduce the demand for new poles by hundreds of thousands of trees each year. This not only saves trees, it also decreases the consumption of chemical and petroleum carriers used in their manufacture. The typical penta-treated distribution pole contains approximately 6.4 pounds of pentachlorophenol and 28 pounds of diesel oil. Forestalling the replacement of one million penta-treated poles each year could save as much as 17 million gallons of fuel oil.
Today, more structure replacement alternatives are available in the marketplace than ever before. These rehabilitation systems have proven track records, both in testing and from many years of in-service performance. They help structurally restore and add load-carrying capacity, providing years of additional, reliable structure life. Technical standards personnel, project planners, and specifiers with utilities are urged to consider these cost-saving products. UP
The Authors: Matt Gardner, P.E., is senior director of product management for Osmose in Peachtree City, Ga. He is a registered structural engineer with more than 20 years of experience in the utility and construction industries.
Chad Newton is a director of product management for wood infrastructure for Osmose in Peachtree City, Ga. He holds a B.S. in mechanical engineering and an MBA, both from the Georgia Institute of Technology. In his current role, he oversees the design, development, and implementation of Osmose’s industry-leading wood pole inspection and restoration products and services. He can be contacted at email@example.com.