Utility Pole Failures - Expert Article

The structural failure or collapse of a wood utility pole is often the result of advanced rot, vehicle impact, overloading, or some combination thereof. As part of a forensic investigation, experts will often seek to determine the pre-failure condition of the pole to understand if maintenance operations or loading considerations were contributing factors.

In this article structural engineer, Anthony Volonnino, P.E. discusses the engineering involved in utility pole design and some of the technical concepts relevant to their failures.

Utility Pole Failures - Expert Article

Design Criteria

The structural engineering that goes into a utility pole design is deceptively complex. While they may appear to simply be poles stuck in the ground, design variabilities such as clearance requirements, regional weather conditions, and the spacing between poles all potentially affect the anticipated forces that each pole needs to support; each pole’s design parameters are unique.

As an example, in certain regions utility poles must be designed to support ice loads on the wires. Ice loads will vary depending on the region, but the problem is more complex than simply the weight of the ice. Ice accumulation creates an enlarged surface area, which increases wind resistance as the wind blows on that enlarged area of wire.

Utility pole placement (ex: urban vs. suburban, or near railroad tracks) may affect clearance requirements and pole height. Spacing between poles will affect a pole’s design; poles with differential wire sag on either side will experience unbalanced forces. Guy wires may be used to stabilize poles when they are subjected to unequal forces.

The depth to which a utility pole is embedded in the ground is typically dictated by the forces it must carry. Forces are imposed near the top of the pole, and those forces must be safely transferred to the ground. If the forces are unbalanced, then those forces must be restrained at the ground.

Utility Pole Failure Causation

The consequences of a pole failure can be catastrophic. Significant damage and/or injuries can occur when wood poles, weighing upwards of 1,000 pounds, crash to the ground. Electrical shock hazards are created when energized lines and equipment contact the ground or other structures.

There are many reasons why utility poles can fail, including:

  • Neglect/oversight/abandonment,
  • decay,
  • natural events (such as a tree falling on a wire),
  • vehicle impact,
  • overload,
  • improper use,
  • alteration or removal of guy wires or their anchors,
  • improper pole setting or erosion,
  • a combination of the above.

With decay, deterioration is often hidden from sight. Decay typically occurs at the ground line and below, resulting in an unseen but common and foreseeable hazard.

Standard of Care for Inspection and Maintenance of Utility Poles

Wood utility poles are often treated with a chemical preservative to extend their life expectancy. However, even treated poles will decay over time as a result of moisture exposure from being directly embedded in the ground. That moisture also promotes wood fungal decay, reducing the structural capacity of the pole. Chemical preservatives degrade overtime. Accordingly, utility poles must be regularly inspected and maintained. Maintenance is required by the owner of the pole and is sometimes dictated by the state’s Board of Public Utilities (BPU) or equivalent governing body.

The minimum requirements for utility pole inspection and maintenance are prescribed in the National Electric Safety Code (NESC). The NESC standard includes the requirements for lines and equipment to be inspected “at such intervals as experience has shown to be necessary.”

Proper maintenance of a wood utility pole will include a physical inspection, a record of the observed conditions, and the corrective actions. Noted defects must be tracked within the maintenance program to prevent the development of dangerous conditions. The NESC specifies deterioration thresholds at which wood poles should be replaced or rehabilitated. Any equipment posing imminent danger to life or property must be promptly corrected or isolated.

Alterations or additions of new wires (ex: conductors) and equipment such as transformers may also affect pole requirements. For instance, if a new or heavier conductor is introduced to an existing pole, it must be able to support that conductor and its associated loads. Therefore, the pole would need to be reassessed considering the new forces. The NESC sets requirements for poles and components that are rehabilitated to compensate for additional loads. If the utility pole cannot achieve these requirements through reinforcement or repair, it must be replaced.

Wood utility poles are surprisingly complex structures, and consequences of their collapse or failure are grave. Owners must properly maintain their poles; proper maintenance considerations include inspection frequency, qualifications of the inspectors, and record keeping.

Investigation of Utility Pole Failures

The experts in the Structural Engineering practice group at Robson Forensic are experienced in investigating incidents involving wood utility pole failures and collapse. When utility poles fail, issues of concern may include ownership of the pole, all corporate entities who used it, the series of events which led to the collapse, the associated hazards, and if proper notice was provided. The investigation and analysis vary on a case-by-case basis since no two utility pole failures are alike.

Invasive testing may be necessary to determine causation, so evidence should be preserved following proper evidence protocols. Laser scans and aerial drone photography may be useful for capturing field measurements and other conditions in the environment.

Our Electric Utility experts investigate issues related to line contacts, downed lines, clearance adequacy, and protective device performance. We evaluate equipment installations in accordance with National Electrical Safety Code (NESC), as well as maintenance and repair issues.

For more information, submit an inquiry or call us at 800.813.6736.

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