Structurally Overloaded Poles
Utility poles in service today,
although adequate at the time of installation, may no longer meet structural
loading requirements for various reasons.
While strength loss due to deterioration at the groundline may
eventually cause a pole to fall below the strength required to remain in
service, a separate issue that is a cause for concern involves additional load
applied to the pole beyond what it is designed to hold. When a pole's load exceeds the loading
requirements set by a utility's recognized loading codes and standards, that
pole is considered overloaded and must be rectified to be in compliance. Overloaded
poles left on the system run the risk of premature failure during storm events
which effects system reliability, protection of assets, and, most importantly,
The previous issue of the T&D Times contained Pole Loading 101 which outlined the basics of conducting a pole loading analysis, including requirements based on NESC, GO95, SCS, AS/NZS 7000. For a basic understanding of structural load, please refer to this article.
Common Causes for Overloaded Poles
There are several common causes
for poles becoming overloaded while in service. If proper evaluation was not
conducted before the additional load was applied, the pole is at risk of
overload. When searching for lines at risk, focus on circuits that may exhibit any
of these issues.
With increased power demands on
the electric grid, telecommunications, and fiber build out growing, and system
hardening and reliability becoming a larger focus in the industry, these
sources of structural overload are becoming more common. States and regulatory bodies have taken
notice and some are starting to put plans in place for pole loading analysis
- Addition of new distribution lines or equipment: If a line received an additional
phase, a transmission line added distribution underbuild, or transformers were
added to a pole, it's important to verify the current pole size is rated to
carry the new, greater bending load.
- Reconductoring with larger diameter conductors: If a circuit needs more
electrical capacity to feed a growing power demand, it may be necessary to
change out the current conductors with larger capacity (larger diameter)
cables. The larger diameter will add more wind load to the pole, potentially
causing an overload condition.
- Joint use/third-party attachments: If a joint user attaches to an
existing pole and there is not proper coordination between the pole owner and
the joint user concerning load evaluation, the attachment may unknowingly cause
an overload condition. Telephone, cable, fiber, and antennas all need to be
properly considered in the overall structural loading of a pole.
- Critical circuits targeted for structural upgrading: While not technically an overload
concern based on code requirements, some utilities have adopted loading
criteria above and beyond what is required on certain critical lines. This practice is commonly referred to as line
hardening or structural upgrading. If a pole or line does not meet the higher
loading requirement, it would be considered overloaded based on the upgrading
Identifying Overloaded Poles
As discussed in the Pole Loading
101 article, there are pole loading analysis software products available on the
market today that can be used to create detailed loading analysis of a pole in
question. While it would be beneficial
to have a full analysis on record for every pole in a utility's system, there
may not be funds or man power available to accomplish this is a timely
manner. By focusing on potential problem
areas using the common causes listed above, utilities can prioritize pole
loading on suspected poles.
Another method to help prioritize
where full loading analysis is needed would be to implement a load screening program
in conjunction with a pole inspection program. An inspector, equipped with data
collection/load calculation software can quickly enter in pole construction
information (length, class, circumference, span lengths, estimated conductor
sizes, estimated mounting heights, equipment, etc.) and immediately determine
an estimated load percentage. Poles
estimated to carry just below or over 100% of their allowable load can be
prioritized as poles requiring a full pole loading analysis.
Once a pole has been shown to be
overloaded though detailed pole loading analysis, something must be done to
correct this compliance violation.
Correcting Overloaded Poles
The obvious solution to correct an overloaded pole is to
replace it with a larger pole of sufficient strength to correct the issue. This is often the most complex and expensive
option because it requires a new pole, certified electrical labor, and
coordination with joint-use attachers.
Before selecting a pole replacement option, consider the following
- Rearrangement of facilities on the existing pole: The higher an attachment is placed on the pole, the greater
it contributes to the pole's bending load. If there is space to move
attachments closer to groundline without violating clearance requirements, it
will reduce the bending load and possibly alleviate the overload condition.
- Correction or addition of guying: Improper guying may be found on overloaded poles. Using pole loading analysis software, a
designer is able to determine if a modification to the location of an existing
guy or the addition of a new guy(s) will correct the overload.
- Addition of a mid-span pole: The span length greatly affects a pole's bending load
because the greater the span, the more surface area there is for wind pressure
to be applied. Overloaded poles with long spans may be corrected by installing
a new pole mid-span and attaching power and/or communications. This shortens
the span length and transfers some of the original load onto the new pole.
- Installation of a capacity upgrading truss: A steel truss used to upgrade the capacity of an in-service
wood pole maybe used as a low-cost alternative to pole replacement. The truss
works in conjunction with the sound wood pole to create a combined structural bending
capacity greater than the original pole. These trusses are manufactured from
high-strength steel and engineered to exceed the strength requirements needed
to correct the overload condition. They
can be sized appropriately to accommodate most pole sizes and overload
conditions, or used to upgrade the pole by 1, 2, 3, or more classes for a line
hardening initiative. Installation is faster than replacement and does not
require service interruptions, coordination with attachers, or result in double
These alternatives should be evaluated for each overloaded
pole before considering replacement while keeping in mind cost, timeliness,
complexity, and safety. Regardless of the solution used, finding and addressing
overloaded poles is a wise practice to improve system reliability and reduce