By Gouri S. Bhuyan, Ph.D., P.Eng. and Dave S. Chetwynd, P.Eng.
A study by B.C. Hydro's, Dave Chetwynd and Powertech Labs' Gouri Bhuynan could "refresh old philosophies'' about the lifespan and use of wood poles, says Chetwynd.
They have been studying poles that have been in service as well a new poles, and their results may come as a surprise.
"We found that poles that have been in service up to 35 years are, strengthwise, equal to new poles being put into service," Bhuyan said. "Every pole is different depending on its conditions."
The testing of old poles shows the fibre strength of these poles can be sustained through the years..
"If wood is looked after, there's no reason why it should lose its strength," Chetwynd said.
And that's in an area that sees mostly mild temperatures and a great amount of moisture throughout the year.
"We've got the worst decay conditions in Canada,'' Chetwynd said.
Both men know that these findings may be opposite the belief that a pole's strength decreases at a constant rate from the day of its installation.
The wood poles used in the B.C. Hydro overhead system are mainly western red cedar (WC) or lodgepole pine (LP). There are approximately 800,000 distribution size and 100,000 transmission size poles in service, with a total asset value of approximately $1.6 billion. To assist in the improvement of the pole management system, several grading, residual strength and reliability issues (see figure 1) are being studied.
Non-destructive Evaluation for Grading Strength of Poles
Wood pole grading at B.C. Hydro is done visually based on physical dimensions (CSA standard O15),
and guarantees pole size but not pole strength. Strength is indirectly determined through the
designated fibre stress, although it can vary widely among poles of the same size. The CSA O15
system assigns a single fibre stress value for all poles of a particular size within a specific species,
leading to the under-utilization of some poles and the over-utilization of others.
Non-destructive evaluation (NDE) of wood pole strength can improve the reliability of the overhead system by rejecting weaker poles and by utilizing stronger poles in high load areas. An NDE method based on the longitudinal stress wave technique has been used to evaluate the strength grading of approximately two hundred transmission and distribution size poles, both new and old, at B.C. Hydro's pole yard. Use of the new method resulted in upgrades to a higher strength class for about 75% of the tested poles, while about 8% of the poles were downgraded. If the upgraded transmission size poles were put into service by matching the strength class with the appropriate load level, there would be a 25% increase in value to the utility from economic considerations. Figure 2 illustrates the effectiveness of this NDE technique over visual grading for the strength classification of new distribution size WC poles.
Residual Strength of Poles
The long-term reliability of a wood pole is determined mainly by the structural load it bears (and
variability of load) and by its rate of biodegradation. Initial wood preservation treatment can improve
a pole's service life by delaying the biodegradation process. Where decay has started, retreatment
with preservative chemicals during regular maintenance generally inhibits further biodegradation of
the sound wood, and extends the pole's service life.
In order to decide whether to replace or refurbish old WC poles, many utilities and pole inspection companies use strength degradation rates that range from 82 psi to 117 psi per year. These rates assume that pole strength begins to decrease from the year of installation - yet this is a disputed point. The effect of time of exposure on the strength of wood fibre is unclear, other than the loss in cross-section from decay.
To try to shed some light on this issue, the residual modulus of rupture of old transmission size WC poles was measured (see figure 3). The butt-treated poles had been removed from the ground after 35 to 38 years of service. Breaking tests were done. On some of the poles, clear specimens were then machined from a section near to the groundline and from a section well above the groundline. The specimens of WC fibre that had been in service for 36 years gave a mean residual modulus of rupture value of 4,650 psi.
Residual strength of specimens taken well above the groundline was found to be approximately 10% to 15% higher than that of the fibre near or below the groundline. When compared with the mean strength of new transmission size poles, the residual strength of the WC poles that had been in service for almost four decades was found to be similar, with no strength degradation of fibre.
Maintenance Strategies for Poles
B.C. Hydro commences pole maintenance inspections after poles have been in service for 20 years
and then inspects at 8 year intervals. Each year about 50,000 poles are inspected. During the regular
maintenance cycles the physical condition of these poles is inspected using conventional sounding and
boring techniques. Based on existing guidelines for distribution poles, the test-and-treat contractor
decides whether the pole can remain in service or is to be recommended for stubbing or for
replacement. Distribution poles that are recommended for refurbishment or for replacement must be
re-evaluated using an instrumented drill called a Resistograph. This drill has been found to be
effective in detecting and quantifying hollow heart, shell rot, internal pockets and insect damage. A
final decision is then made on the serviceability of the pole. A serviceable pole goes back into the
maintenance cycle and will be re-inspected in eight years.
A different maintenance strategy has been developed by Powertech for transmission size poles (see Figure 4), and is being implemented in the B.C. Hydro system. It consists of two levels of assessment. The first level is intended to be carried out by the test-and-treat contractor. If no decay is found in a transmission size pole, or the decay is within allowable limits, the contractor may declare the pole to be serviceable. If the detected decay exceeds the allowable limits, then the serviceability of the pole must be assessed in detail using the Resistograph. This second level of assessment is expected to reduce the number of poles that are replaced unnecessarily.
Assessing Reliability of Poles
The procedure using the instrumented drill serves to map the extent of decay. The localised decayed
portion of the pole, the soil conditions, and any loading from attachments are modelled. The pole is
analyzed using a finite element computer program to establish whether it is serviceable, or will be
after stubbing, or if it needs to be replaced. A typical mapping of groundline decay after drill
measurements and finite element modelling is shown in Figure 5.
The reliability of any in-service pole can be assessed using this finite element approach based on the probabilistic method. Probability of failure or the reliability index is calculated, taking into account the extent of groundline decay, the existing attachments, the variability in fibre strength, and the randomness of wind and ice loading. Figure 6 shows the effect of internal rot at the groundline on the reliability of a 45' long pole supporting a 3 phase line spanning 50 m. This information can be used to decide whether to extend the life of a standing pole or to allow additional loads during upgrading of the line. Changes in reliability due to changes in loadings or the effect of decay can be readily established.
Non-destructive evaluation using the longitudinal stress wave technique could become an effective method for strength classification of removed older poles, leading to environmental and economic benefits. Using the NDE technique, strength grading of new transmission sized poles and new distribution size poles for supporting transformers is also preferable; it helps to maximize the use of each pole.
The present strength degradation schemes used for estimating the remaining life of western red cedar poles are not proven.
The Resistograph instrumented drill is being used as an integral part of B.C. Hydro's maintenance strategy for distribution and transmission poles. When the results of Resistograph measurements on transmission size poles show levels of decay that exceed allowable limits, a more detailed assessment of serviceability is undertaken using a finite element analysis computer program.
This computer program can be extended to detailed reliability analysis of any in-service pole from life extension or upgrade considerations.
Gouri Bhuyan is with Powertech Labs Inc. in Surrey, B.C.. Dave Chetwynd is with B.C. Hydro T & D Maintenance. Powertech Labs will be pleased to provide wood pole assessment services to utilities using these developed tools and the results obtained from various wood pole related work during last five years at BC Hydro. For further information please contact Gouri at (604) 590 7407.