Structural Rehabilitation: Cost-effective Alternatives to Pole Replacement
As the transmission and distribution infrastructure continues to age and revenues decrease or at best remain steady, utility companies must seek creative and cost-effective solutions to problems that naturally accompany aging assets.
By Matt Gardner, P.E.
As the transmission and distribution infrastructure continues to age and revenues decrease or at best remain steady, utility companies must seek creative and cost-effective solutions to problems that naturally accompany aging assets. The use of repair and restoration products provides utilities with significant economic advantages, and structural and environmental benefits.
Several proven products exist to rehabilitate utility poles that no longer meet the code requirements as a result of decay or mechanical damage. Strengthening solutions that add bending capacity to overloaded poles are also available.
Groundline Decay and Damage
Utility companies 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 maintenance program is to ensure the structural reliability and safety of power line systems. Identifying problems such as groundline decay 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.
|Steel reinforcement of a wood utility pole|
A Proven Track Record
There are a number of vendors and contractors that offer various pole 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. Pole 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.
|Installation of a composite repair|
The most common and proven rehabilitation system that helps restore groundline strength to 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. Steel trusses are usually galvanized to reduce corrosion potential. They are driven to pre-defined depths and secured to the pole with high-strength steel banding. By driving a truss and securing it alongside a wood pole, bending loads applied to the pole because of wind pressure, equipment, ice and wire tension are transferred to the truss, which carries the load to the ground-effectively bypassing the decayed or damaged areas. These trusses, when properly sized and oriented, are engineered to have the equivalent strength of the original wood pole.
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 conventional 80,000 psi, or less, material. Because of higher steel strengths, the thickness can be reduced for a given truss capacity that reduces the amount of steel used and enables easier installation.
A critical component of this, or any repair, is the application of remedial treatments to arrest the decay. Failing to remedially treat the decay will significantly shorten the life of the repair.
|Completed composite repair|
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 repairs give pole owners another option for restoring poles that might not be reinforcable using a truss. By wrapping a pole in specially-designed, field impregnated composite fabric, poles can achieve full bending strength in all directions-360 degrees around the pole. This repair methodology is suitable for poles with groundline decay and decay much higher on the pole.
Composite repairs 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 ultra-violet deterioration to ensure the repair 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, and a resin material is applied to each layer to bond the fibers together and seal the repair. Remedial treatment is a key component of the long-term success of this repair system.
|Higher grade steel reinforcement of a wood utility pole|
Class Upgrades for Overloaded Poles
The critical element for many utility poles is the bending load generated by wind blowing perpendicular to a pole's wires and equipment. The pole's bending load is usually the greatest near the groundline. The rapidly increasing number of attachments added to poles after installation is causing a rampant number of overloaded poles across the country. These new wire and cable attachments might not have been considered when the pole was originally designed and installed. In such situations, the amount of bending load at the groundline exceeds the amount allowed by National Electric Safety Code (NESC) or GO95 standards. There is currently an increase in system hardening programs designed to increase the capacity of structures along key circuits. This proactive approach will help prevent pole failures and provide additional reliability.
Extended steel trusses are designed to help increase a pole's load carrying capacity and correct overload conditions. These extended steel trusses can increase a pole's class designation by up to 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 problem of overloaded poles and provide a system hardening solution.
The 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 many tens of thousands of trees each year. Reducing the need for new poles 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.
Numerous pole replacement alternatives are available. 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 pole life. Technical standards personnel, project planners and specifiers with utilities are urged to consider these cost-saving products.
About the author: Matt Gardner, P.E., is product engineer for Osmose Utilities Services Inc. in Tyrone, Ga. He is a registered structural engineer with more than 12 years of experience in the utility and construction industries. For additional information about utility pole rehabilitation products and methodologies, please contact Gardner, at firstname.lastname@example.org.
Pole Loading Software
Pole loading software has become a commonplace tool for determining load conditions of in-service poles. Many pole owners, in fact, now mandate that attachers submit pole loading calculations as part of their attachment applications.
Photogrammetry technology and other advanced field data collection methods have been integrated with pole loading applications to allow for quick and efficient pole model creation. Having accurate pole calculations and detailed information about loads allows utility designers and planners to make informed decisions on how to address overloaded poles.