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STRUCTURAL RELIABILITY AND
RISK ASSESSMENT
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WOOD RIVER -KETCHUM 138 kV LINE #433
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TABLE OF CONTENTS
EXECUTIVE SUMMARY
THE PROJECT
THE STUDY
THE RESULTS
RECOMMENDED ACTION
1.INTRODUCTION
1.1 The Project
1 .2 Project Overview
1.3 Review of Project Tasks and Organization of
Final Report
2.FIELD INSPECTION
2.1 Introduction
2.2 Structural Inspection
2.3 Electrical Inspection
3.STRUCTURAL AND ELECTRICAL RELIABILITY ASSESSMENT
3.1 Introduction
3.2 Wind and Wind on Ice Loading
3.3 Structural Analysis and Reliability Methodology
3.4 National Electrical Safety Code (NESC)Considerations
3.4.1 Introduction
3.4.2 Analysis of H-Frame Structures
with NESC Medium Loading
3.5 Structural Reliability of Wood Poles
3.5.1 Implied Reliability for Structures
Designed for Medium Loading
3.5.2 Structural Reliability of Line #433
3.6 Assessment of Structural Component Adequacy
3.7 Assessment of Deadend and Angle Structure Adequacy
3.8 Electrical Reliability
4.LINE CONDITION ASSESSMENT
Structure Condition Assessment
Electrical Condition Assessment
5.RECOMMENDATIONS FOR UPGRADES
5.1 Introduction
5.2 Recommendations for Structural Upgrade
6.SUMMARY AND RECOMMENDATIONS
7.REFERENCES
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APPENDICES
EDM\IDP012.RPT
TABLE OF CONTENTS
EXECUTIVE SUMMARY
THE PROJECT
THE STUDY
THE RESULTS
RECOMMENDED ACTION
1.INTRODUCTION
1.1 The Project
1 .2 Project Overview
1.3 Review of Project Tasks and Organization of
Final Report
2.FIELD INSPECTION
2.1 Introduction
2.2 Structural Inspection
2.3 Electrical Inspection
3.STRUCTURAL AND ELECTRICAL RELIABILITY ASSESSMENT
3.1 Introduction
3.2 Wind and Wind on Ice Loading
3.3 Structural Analysis and Reliability Methodology
3.4 National Electrical Safety Code (NESC)Considerations
3.4.1 Introduction
3.4.2 Analysis of H-Frame Structures
with NESC Medium Loading
3.5 Structural Reliability of Wood Poles
3.5.1 Implied Reliability for Structures
Designed for Medium Loading
3.5.2 Structural Reliability of Line #433
3.6 Assessment of Structural Component Adequacy
3.7 Assessment of Deadend and Angle Structure Adequacy
3.8 Electrical Reliability
4.LINE CONDITION ASSESSMENT
4.1 Structure Condition Assessment
4.2 Electrical Condition Assessment
5.RECOMMENDATIONS FOR UPGRADES
5.1 Introduction
5.2 Recommendations for Structural Upgrade
6.SUMMARY AND RECOMMENDATIONS
7.REFERENCES
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APPENDICES
EDM\IDP012.RPT
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
EXECUTIVE SUMMARY
THE PROJECT
Idaho Power Company's (IPCo)Wood River-Ketchum 138 kV line #433 consists
of 12.4 miles of two pole wood H-Frame tangent structures with three pole wood angle
and deadend structures.The wood structures primarily support 2 -5/16"HS shield wires
and #4/0 (Penguin)ACSR conductors with a few spans supporting 397.5 kcmil (Ibis)
ACSR conductors.The line was originally built in 1962 and was initially operated at 46
kV.This line replaced two long feeders from the Hailey Substation and is currently the
only service of electric power to the Ketchum-Sun Valley community.
Concerns for extending the life of the line within the requirements of the National
Electrical Safety Code (NESC)prompted IPCo to assess its status and to develop
strategies to ensure long-term performance.The team of Engineering Data
Management,Inc.(EDM),POWER Engineers,Inc.(POWER),and INTEC Services,Inc.
combined to provide services for a structural reliability and risk assessment of the line.
The reliability and risk assessments utilize the results of a field evaluation of each
structure performed by Davey Tree (a wood pole inspection company contracted by
IPCo).
THE STUDY
Reliability and risk calculation methods were applied to assess the structural and
electrical performance of the line.The results of a field evaluation of each structure were
utilized in this assessment.Nondestructive evaluation (NDE)and visual inspection of
each wood pole structure,were performed.The PoleTest™wood pole strength analyzer,
developed by EDM with the cooperation of the Electric Power Research Institute,was
utilized by Davey Tree to predict in-place wood pole strength,
measurements of pole strength and observed conditions,detailed calculations were
performed to determine the structural capabilities of the line.An assessment of NESC
structural loading requirements versus current structural capacity was completed.
Based on field
A risk and reliability assessment was performed.Theoretical reliability levels for
local loading conditions were calculated for the H-frame structures.Following the
reliability assessment,recommendations forstructure upgrades/replacements were made
based on maintaining selected minimum reliability levels,
assessments of structural conditions were used to override reliability assessments in
cases where structure or component replacements were mandated by criteria other than
pole strength at groundline.
Results of the field
EDM\IDP012.RPT -I-
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
THE RESULTS
Critical loads for wood pole H-frame tangent structures result from wind and/or
wind on ice conditions.Results of a study of local loading conditions showed that
(considering line location and terrain),of the data that are available,wind records for
Boise are most valid for use in assessing the structural reliability of this line.These
records show an annual fastest mile wind velocity of about 48 mph with a standard
deviation of about five (5)mph.Wind on ice conditions were also considered.A weekly
fastest mile wind velocity of 31.7 mph and a standard deviation of 7 mph were coupled
with a mean radial ice thickness of 0.25 inches and a standard deviation of 0.1 0 inches.
The wind on ice conditions were found to control in most cases.
Adequacy of existing structures was assessed by comparing results of structural
analyses with NESC Grade B construction "new"and "at replacement"requirements.
Results show that none of the structures were overstressed at ultimate design loads.A
complete reliability analysis conducted using the Boise high wind and wind on ice
loadings and incorporating the predicted existing wood pole strengths (and variations)
from field measurements indicated that in general,the line has a high level of structural
reliability.In fact,most structures have significant excess capacity which could be used
for a future upgrade to a larger conductor,if electrical requirements dictate such action.
Most of the poles were found to have reliability indices greater than 2.5.In addition to
the reliability assessment,a deterministic analysis of the critical buckling loads for all of
the three pole guyed structures and H-frame deadend structures was conducted.This
analysis revealed that all of these guyed structures have excess buckling capacity in their
existing condition.
Through the reliability assessment of all the structures in the line,a curve was
established for the deterioration rate of structural reliability as shown in Fig.1.
Calculations for each pole established the predicted life of each wood pole (structure)
in its existing condition.Finally,results of the reliability assessment,buckling capacity
assessment,field condition assessment and pole inspection by Davey Tree were
combined to provide the basis for the recommended upgrade/replacement schedule
provided in Table 1.
EDM\IDP012.RPT -II-
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
TABLE 1
Recommended Upgrades/Replacements
Modification/Replacement Units
Structure Replacement 1
Wood Pole Replacements 2
Crossarms Replacements 10
Insulator Repairs/Replacements 18
RECOMMENDED ACTION
The structural inspection revealed that the structures are generally in good
condition.Relatively minor problems were noted that should be corrected in a
maintenance program.
Implementation of the proposed upgrades will help ensure that Line #433 will
operate at acceptable reliability levels.
EDMMDP012.RPT -III-
r
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Wood River-Ketchum 138 kV Line #433
Idaho Power Company
December 1993
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Figure 1 Average Reliability Degradation Rate Curve for Western Redcedar Poles
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Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
STRUCTURAL RELIABILITY
AND
RISK ASSESSMENT
OF THE
WOOD RIVER-KETCHUM 138 kV LINE
FINAL REPORT
1.INTRODUCTION
1.1 The Project
Idaho Power Company's (IPCo)Wood River-Ketchum 138 kV (Fig.1 .1)line #433
consists of 12.4 miles of two pole -frame tangent structures with three pole wood angle
and deadend structures.The wood structures support 2-5/16"HS shield wires and
ACSR conductors.Number 4/0 (Penguin)ACSR conductor is supported by most of the
structures,however,a few structures support 397.5 kcmil (Ibis)ACSR conductors.The
line was built in 1 962 and was initially operated at 46 kV.This line replaced two long
feeders from the Hailey Substation and is currently the only service of electric power to
the Ketchum-Sun Valley community.
Concerns for extending the life of this line in view of the requirements of the
National Electrical Safety Code (NESC)(2)*prompted IPCo to seek engineering services
to assess the status of the existing line and develop a replacement and/or upgrade
strategy to ensure long-term performance.The team of Engineering Data Management,
Inc.(EDM)of Fort Collins,Colorado,Power Engineers,Inc.(POWER)of Hailey,Idaho,
and INTEC Services,Inc.(INTEC)provided the engineering and inspection services.The
reliability and risk assessments utilized the results of a field evaluation of each structure
perfortned by Davey Tree (a wood pole inspection company contracted by IPCo).
1.2 Project Overview
Reliability,life expectancy and risk calculation methods were used to assess the
performance of each H-frame structure in the line.The assessments utilized the results
of a thorough field evaluation of each structure.Results from a conventional inspection
'Numbers in parentheses indicate references used in the preparation of this report.
-1-EDM\IDP012.RPT
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Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
performed by Davey Tree on each pole at and below groundline were combined with
nondestructive evaluation (NDE)strength assessments to better assess the overall
condition of each pole.The PoleTest™field unit developed by EDM was utilized to
predict wood pole strength (10).A thorough visual inspection of each wood pole
structure was performed by INTEC.
Additionally,INTEC performed a climbing inspection on 11 structures at
approximately one mile intervals.During the climbing inspection,the conditions of
structural framing components and electrical components including shield wires,
conductors and insulators were assessed.Conductors were specifically inspected to
assess corrosion and wear.
Detailed calculations were performed to determine the structural capabilities of the
line based on field measurements of pole strength and observed condition.An
assessment of structural loading requirements versus current structural capacity was
completed.As a result of the field inspection and calculations,recommended
replacements/upgrades of poles,crossarms,and braces were developed.A risk and
reliability assessment was performed,and the reliability levels determined for the line
were compared to selected minimum reliability levels.Following the reliability
assessment of the structures in the line,a set of recommendations for upgrade and/or
replacement was developed for IPCo evaluation.
1.3 Review of Project Tasks and Organization of Final
Report
To provide a basis for the organization of the Final Report,the Project Tasks are
reviewed in Table 1.1.The Final Report is organized to follow the logical sequence of
work which was required to accomplish the tasks outlined.Complete records and
results are provided in the Appendices in cases where they are too voluminous to be
included in the body of the report.
-3-EDM\IDP012.RPT
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood R'rver-Ketchum 138 kV Line #433
Table 1.1.
Structural and Electrical Reliability and Risk Assessment Project Task List
BRIEF DESCRIPTIONTASK
Field Inspection and Assessment
A visual inspection of each structure in the line was performed.Poles,
crossarms,braces and insulators were inspected.The conditions of
electrical components were assessed for potential electrical and
clearance problems.Questions arising from the review of the
conventional inspection reports were addressed.A climbing inspection
was performed on a representative sample of structures and the
conductors were visually inspected for corrosion and wear.Visual
examination of crossarms,braces and insulators was performed.
1
Structural Reliability Assessments
Wood pole strength data and field inspection results were utilized to
calculate the structural reliability of each structure in the line.
2
Electrical Reliability Assessment
Theoretical outage rates were calculated utilizing local soil resistivity
and lighting strike data obtained from the BLM's Initial Attack
Management System data base.Theoretical and actual outage
records were compared.
3
Line Condition Assessment
Results of wood pole inspections for condition were evaluated in
combination with reliability assessments to provide the basis for
concepts for upgrade/repair and/or replacement.
4
Concepts for Upgrade.Repair and/or Replacement
Utilizing conclusions drawn from reliability calculations and line
condition assessments,concepts for line/structure upgrade,repair
and/or replacement were developed.
5
Final Report
The final report encompasses a complete record of the field inspection,
condition reports and reliability assessments as well as the concepts
for line upgrades.
6
-4-EDM\IDP012.RPT
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Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
2.FIELD INSPECTION
2.1 Introduction
An inspection of the wood structures was conducted by Jim Treat of EDM and Les
Hinzman of INTEC during October 25 through October 29,1993.The data gathered
during this inspection were combined with Davey Tree inspection results to develop a
more complete understanding of the condition of the structures.
The data obtained during the field inspection were used in combination with the
structural reliability assessment to make the final decision concerning any action
required to maintain the reliability of a structure.In some cases,where the reliability of
the structure based on pole strength is sufficient to allow the structure to remain in
service,repair or replacement may be warranted due to other aspects of the structure's
condition.Discussion of the line condition assessment is provided in Section 4.
Complete records from the structural inspections are provided in the Appendices.
2.2 Structural Inspection
Each structure was visually inspected for information including pole species,
length,class,and treatment as well as conditions which could compromise serviceability.
Overhead portions of the structures were viewed with binoculars.Hardware and
connectors were inspected.The location and type of each observed structural problem
were recorded.The results of the inspection are provided in the Appendices.
The wood members (poles,crossarms and bracing)were examined for fractures,
splits,decay,fire damage,mechanical damage,insect attack,woodpecker attack and
other miscellaneous forms of damage.If a problem appeared severe enough to warrant
immediate attention,a comment to that effect was provided.Data on damage at or
below groundline were obtained from previous Davey Tree field inspection results.
Joints and connections were examined to ensure that hardware was intact and
tight.Missing or loose hardware or gaps at joints were encoded on the field data forms.
For comparison to Davey Tree NDE results,nondestructive bending strength evaluations
were conducted at the groundline of several poles using PoleTest™.While it is
recognized that the maximum bending stress in the poles in braced structures will
typically occur at an X-brace attachment location,PoleTest™was used near groundline
because it is generally in this region where the majority of the strength degradation
occurs.In most cases,PoleTest™provided strength values within a generally expected
range.
-6-EDM\IDP012.RPT
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
Idaho Power Company
December 1993
Unadjusted strength values are presented in Appendix A (Inspection Results).To
assess the strength of poles with decay and/or other damage at or near groundline as
reported by Davey Tree,PoleTest™strength values obtained above groundline were
adjusted for the loss of section modulus caused by the damage.The loss of section
modulus was determined using the Davey Tree inspection records combined with more
complete damage records obtained during the INTEC/EDM inspection of the line.While
PoleTest™accounts for strength loss due to decay when a test is conducted through a
decayed section,all groundline strength values were adjusted for the appropriate section
modulus loss.This strength reduction added conservatism to the reliability assessment.
Adjusted strength values are provided in Appendix B (Structural Reliability Assessment
Results).
2.3 Electrical Inspection
Along with the structural inspection,each structure was visually inspected for
hardware wear,grounding and bonding integrity,insulator breakage,evidence of
structure burning and inadequate ground clearance.The location and type of each
observed electrical problem were noted.The results of the inspection are provided in
the Appendices in conjunction with the structural inspection results.
Insulators and hardware were observed from the ground through binoculars.A
climbing inspection was performed on selected structures.The emphasis of the
climbing inspection was to assess the integrity of the conductor and shield wire.During
the climbing inspection,the conductor and shield wire were photographed and were
visually inspected for wear and corrosion.
-7-EDM\IDP01 2.RPT
STRUCTURAL
AND ELECTRICAL
RELIABILITY ASSESSMENT
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
3.STRUCTURAL AND ELECTRICAL RELIABILITY ASSESSMENT
3.1 Introduction
Structural analyses were performed using the results of the field inspections
(Section 2)to assess the existing reliability of the structures.Results of inspections of
the electrical system provided the required data base for the electrical reliability
assessment.The goal of the reliability assessments was to place all structures on an
equal basis such that those structures with substandard reliability could be earmarked
for selective upgrading or replacement.
Structural reliability was evaluated using both local wind and wind on ice data as
well as NESC Medium Loading (4 psf wind on 1/4 radial inch radial ice)conditions.A
reliability calculation performed for the "At Replacement"Overload Capacity Factor (OCF)
of 2.67 enabled a structure-by-structure evaluation of the satisfaction of NESC
requirements.The methodology utilized also provided the means to assess long-term
performance through the use of "reliability versus time in service"curves established for
the line.
In addition to the structural reliability assessment of the wood poles,structural
analyses were performed to evaluate the integrity of crossarms and braces.These
analyses are discussed in Section 3.6.
3.2 Wind and Wind on Ice Loading
The critical loads for wood pole H-frame tangent structures result from wind
and/or wind on ice conditions combined with the dead weight of the structure.Several
load combinations were investigated for the H-frame structures in the line.The
combinations considered were:
Dead Load and NESC Medium Loading
Dead Load and Local Wind (only)Loading
Dead Load and Local Wind on Ice Loading
To evaluate the local loading conditions,results from a previous study were used.
Results of this work were provided to IPCo on July 24,1989 in a report entitled,"High
Wind and Wind on Ice Information,Pocatello and Boise,Idaho."The key points and
conclusions were:
-9-EDM\IDP012.RPT
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
Considering line location and terrain,the Boise wind information is
considered the most representative for the line in this study.
Boise data result in a mean velocity (annual fastest mile)of 47.83 mph with
a standard deviation of 5.01 mph (coefficient of variation (COV)=0.1048),
Type I Extreme Value Distribution.
In evaluating wind on ice,the recommended mean velocity and ice
thickness are 31.7 mph (COV =0.2172)and 0.25 inches (COV =0.40),
Type I Extreme Value Distribution.
3.3 Structural Analysis and Reliability Methodology
Structural analyses and reliability calculations for the loading conditions cited in
Section 3.2 were performed using the computer program FRAMER*.Program FRAMER
is a computer program which performs either the analysis,design,or reliability
assessment of planar wood transmission structures,such as H-frames.Input data
include descriptions of structure geometry,nominal material properties,conductor
(including overhead ground wire)sizes and locations on the structure,and transverse
and vertical loadings.
The load and resistance probability distributions were described and the program
computed the reliability index,/?,of each of the support poles in the structure
configuration.
To account for wood pole degradation with "time-in-service"and to provide a basis
for predicting the "time-to-replacement"for each pole,the following procedures were
utilized (results obtained from calculations for each structure using FRAMER provide the
necessary reliability indices):
Using the results of reliability assessment calculations for each structure,
a reliability versus time-in-service curve was established as shown in Fig.
3.1 (a best fit curve defined by the equation (3 =yS0e'bT is used to represent
the average rate of degradation for all the results.)
Program FRAMER was developed under the sponsorship ofthe Electric Power Research Institute
(EPRI),Palo Alto,California as part of research project RP1352 conducted by Colorado State
University.
-10-EDM\IDP01 2.RPT
Reliability Degradation Curve Applied to each Pole B =B^"1*
4
Predicted Replacement Time (TR)
Reliability of Existing Poles (BT)
3
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Minimum Reliability
JQ 2
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<D
tr
Time in Service (T )
1
o T
0 10 20 30
80
40 705060
Years in Service (T )
Figure 3.1.Reliability Degradation Curve for Assessing Time to Replacement
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
where:
Reliability Index at time T (in years)P
Projected Reliability Index for new structures (T=0)Po
Base of natural logarithme
Constantb
Structure-by-structure calculations were performed to obtain the existing
reliability index,/?T,for each pole and the predicted replacement time,TR,
based on the average rate of degradation for all the structures in Line
#433.
Given the results of these calculations,final choices for reliability level,frequency
of pole inspection,replacement strategy,etc.can all be made by rational economic
analysis.
Assessment of the reliability of structural components other than the wood poles,
such as braces,crossarms and connections,requires separate analysis.Since no
method is available for predicting the strength of these components,their adequacy was
assessed by comparing stresses and forces occurring in these components to design
capacities.Details of these calculations are provided in Section 3.6.
3.4 National Electrical Safety Code (NESC)Considerations
3.4.1 Introduction
This section provides results of calculations to assess the adequacy of
wood poles in the line with regard to NESC (2)requirements.The line is considered to
be governed by NESC Medium Loading.Structural analysis of the structures in the line
provided a basis for assessing general compliance with NESC provisions and a means
to develop reliability measures for current and long-term performance of the structures.
It should be noted that NESC loading does not directly recognize local
conditions including wind magnitude,direction and profile,nor does it provide a direct
means to utilize data on the strength of individual wood poles.Thus,it is necessary to
interpret the "spirit"of NESC requirements when using NESC loads in concert with ANSI
05.1 (1)wood pole strength values in any reliability assessment procedure.
-12-EDM\IDP012.RPT
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
Analysis of H-Frame Structures with NESC Medium
Loading
3.4.2
The H-frame structures in the line were analyzed with NESC Medium
Loading to determine their existing structural capacities.The following procedures were
used:
NESC Medium wind loading was applied to the structures with the
specified 0.25 in.radial ice on the #4/0 (Penguin)or 397.5 kcmil
(Ibis)ACSR conductors and 5/16"HS shield wires
ANSI 05.1 specification minimum dimensions were used for the
poles to ensure "worst case"maximum stresses.
The weight span was assumed to be 20%greater than the wind
span.
The results of the application of NESC Medium loading to each of the
structures are provided in Appendix B.In all cases the maximum stresses under the
NESC medium loading (OCF=4.0)fall below the ANSI 05.1 designated fiber stress
values 6000 psi for western redcedar and 8000 psi for Douglas-fir indicating that the line
design is adequate to meet NESC structural requirements.The stresses tabulated in
Appendix B are the maximum combined axial and bending stresses occurring in the
poles.
Examples of structural analysis results for structures that were deemed to
be representative averages for the line are presented in Table 3.1.For unbraced H-
frames the maximum stress in the poles was found to occur near the groundline.As is
typical for braced H-frames,the maximum stress was found to occur at the upper X-
brace attachment.Figure 3.2 illustrates the basic H-frame structural configurations and
Table 3.2 provides descriptions of the various H-frame structure types.
-13-EDMMDP012.RPT
Table 3.1
Examples of Structural Analysis Results
For NESC Medium,Grade B Loads
MAXIMUM
STRESS*(psi)
OCFs 2.67
STR SPANSTR POLE
HEIGHT
MAXIMUM
STRESS*(psi)
OCFs 4.0
POLE
CLASS#TYPE («)
(ft)
2336A3499575495602
24883728A527600652
36495466A253966875
19282889AX265465490
122818391AX51265503
11841774AKX349755544i
23583533AKX269075521i
310046441AKX85995491
22353348AKX165190492
'Maximum stresses are based on ANSI 05.1 minimum dimensions for western redcedar poles.
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Configuration Configuration Configuration
Figure 3.2 H-Frame Structural Configurations
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
Table 3.2
Descriptions of H-Frame Structures
DescriptionStructure
Type
H-frame Tangent,Unbraced CrossarmA
H-frame Tangent with X-Brace,Unbraced CrossarmAX
H-frame Tangent with X-Brace,Outside Vee-BracesAKX
H-frame Deadend with X-Brace,Unbraced CrossarmGX
H-frame Deadend with X-Brace,Inside and Outside Vee
Braces
GKX
3.5 Structural Reliability of Wood Poles
Implied Reliability for Structures Designed for
Medium Loading
3.5.1
To assess the reliability implied under NESC Medium loading conditions,
wood pole strength values must be known.Recent testing programs at Colorado State
University (3,4,5)have provided data on new and in-service wood poles.These data
can be used to assess the implied reliability by considering the NESC loading condition
to be a constant combined with the variability of the wood pole strength.Table 3.3
provides the ANSI 05.1 values for new wood poles based on the minimum pole
dimensions (ANSI dimensions)typically used by designers.The results of reliability
calculations considering NESC loads and grades for new pole structures designed to the
maximum stress limit (at the critical stress location)are provided in Table 3.4.
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Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
Table 3.3
ANSI 05.1 Wood Pole Strength Data
Standard
Deviation
Coefficient of
Variation
Pole Strength
(psi)Species
(psi)
9465200 0.192WesternRedcedar
7860 1132 0.144Douglas-fir
Table 3.4
Implied Reliability Indices
for NESC Design of New Poles
Overload
Capacity
Factor
Design
Stress*
Implied
Reliability
Index(OCF)Species (psi)Grade
Western
RedcedarB 1500 3.91
4
Douglas-fir 2000 5.18
Western
RedcedarC 3000 2.322
Douglas-fir 4000 3.41
*Design Stress =ANSI Designated Fiber Stress/OCF
These values provide a guideline for comparison to new pole reliability,assuming
that the as-built structures are fully loaded.
Additional information for comparison to actual reliability levels can be obtained
by considering the NESC requirements for "at replacement"conditions.As usually
interpreted,when pole strength deteriorates to a point where the strength is exceeded
by the load effects occurring when an overload capacity factor of 2.67 is applied to the
basic NESC loadings (Grade B),the pole should be replaced.Table 3.5 provides the
-17-EDMMDP012.RPT
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
The calculationsresultsofreliabilitycalculationsforthe"at replacement"condition,
incorporated the new pole strength data provided in ANSI 05.1 (Table 3.3)reduced by
the ratio of the NESC "at replacement"OCF to the OCF for new construction,i.e.2.67/4.0
=.67 and published standard deviations of in-service poles (3,4,5,6).
Table 3.5
Implied Reliability for NESC Design of Wood Poles
for At Replacement Condition
Reduced
Mean
Strength*
(psi)
Original
Design
Stress
Standard
Deviation
Implied
Reliability
Index(psi)(psi)SpeciesGrade
Western
Redcedar 34841500 1070 1.84B
2000 5266Douglas-fir 2106 1.55
Western
RedcedarC 3000 3484 1070 0.45
Douglas-fir 4000 5266 2106 0.60
*Reduced (deteriorated)mean atrength obtained by reducing the original strength by the ratio
of the "at replacement"OCF to the OCF for new construction.
Implied reliabilities for new and "at replacement"conditions are useful guidelines for
setting limits on reliability levels for use in making pole replacement and upgrading
recommendations.
replacement"criteria provides a guide for setting the minimum acceptable reliability level.
These values were used as guidelines as described in Section 3.5.
In particular,the reliability index which will satisfy the NESC "at
Reliability analyses based on NESC requirements cannot be applied using pole-by-pole
loads and actual pole strengths as predicted by PoleTest™.Thus,more complete pole-by-pole
reliability analyses were conducted which use local wind and wind on ice conditions coupled
with predicted pole strengths,and their possible variations.This method results in more
realistic reliabilities and provides a sound basis on which to make final upgrading and
maintenance decisions.It should also be noted that condition assessments must be
considered in this approach and may override calculated reliabilities in cases where severe pole
damage or decay exists.
-18-EDM\IDP012.RPT
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
Structural Reliability of Line #4333.5.2
As noted in Section 3.2,structural reliability calculations considered local wind and
wind on ice conditions and utilized data from the Boise airport.Pole strengths (and variations)
were assessed using PoleTest™predictions (See Appendices for complete listing).
Modifications in pole capacity were made if warranted by decay or pole damage noted during
the inspection or reported by Davey Tree.
Results of reliability calculations to determine reliability indices (fi's)for each pole
the H-frame structures in the line are provided in the Appendices.Average indices resulting
from the reliability calculations are compiled in Table 3.6.Average current reliabilities,along
with benchmark limits based on NESC considerations (Section 3.5.1),provide guidelines for
establishing a reliability versus time-in-service diagram.
Using the pole strength and reliability values tabulated in Appendix B,a graph of
average reliability versus time in service was constructed using a best fit curve as shown in Fig.
3.3 (8,9,14).Based on information from previous studies,IPCo has selected p =2.50 as a
conservative minimum value for yearly reliability.Western redcedar poles with calculated
reliability indices below 2.50 are recommended for replacement or upgrade.The choice of a
reliability index of 2.50 ensures that the NESC criteria for replacement are conservatively met
for each pole.The conservatism of the minimum reliability index is confirmed by comparing
the selected value to the Grade B NESC "at replacement"reliability index of 1 .84 discussed in
Section 3.4.3.The degradation rate established by the average degradation curve shown in
Fig.3.3 provides the basis for predicting future pole replacement rates.Based on a threshold
reliability index of 2.5 average predicted degradation times are provided in Table 3.7.
Nondestructive strength data were not available for structure #572 due to an "unable to
test pole"reading on the left pole.A mean pole strength was,therefore,assumed for this pole
and adjustments were made for loss of section modulus due to decay.Reliability results based
on these assumptions indicate that the structure has a reliability that falls below the threshold
reliability index of 2.5.
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-0.003 T
AVERAGE DEGRADATION RATE CURVE,(3 =4.0 e
4
3
cs
X<D V-a
£
Minimum Reliability
®min =2.50
2?
xi 2
.2
oNJcco
1
0 T
i
80706050400102030
Years in Service (T )
Figure 3.3 Average Reliability Degradation Curve
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
Table 3.6
Reliability Measures For
In-Service Western Redcedar Poles
Average
Reliability
Index
Benchmark
Reliability
Index
Number of
Poles
Time in
Service
(years)
(NESC)
3.91New
4.32<31
3.713831
1.842"At Replacement"
Grade B NESC Reliability for New Pole Design (see Section 3.4.3)
2Grade B NESC "At Replacement"Reliability (see Section 3.4.3)
Table 3.7
Predicted Time-To-Minimum Reliability for
Western Redcedar Poles
Predicted Time-To-Minimum Reliability (years)
1-5 6-10 11-15 16-20 21-25 25-300yrs.
00 1 0 1No.of
Poles
2 1
2*0 0 1 0 1No.of 1
Structures
'Note:A total of 3 poles in 3 structures are recommended for replacement based on the combined results of the
reliability and condition assessments.See Section 5 for an explanation.
To assess the reliability of poles in angle and deadend structures,predicted pole
strengths were compared to PoleTest™threshold strengths for Grade B construction as
provided in the PoleTest™Technical Supplement (10).(Threshold strength of Western
Redcedar =2180 psi.)This approach was used because no methods currently exist to directly
assess the reliability of poles loaded in axial compression.
-21-EDM\IDP012.RPT
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
3.6 Assessment of Structural Component Adequacy
It was not practical to perform structural reliability calculations for crossarms and
braces due to the lack of adequate resistance data.Therefore,deterministic analyses
were used to assess the worst case loads and stresses occurring in the structural
components of concern.
To evaluate the structural integrity of the crossarms and braces,three structural
configurations were analyzed using NESC Medium Loads with "at installation"overload
capacity factors of 2.2 for vertical loads and 4.0 for transverse loads as provided by the
1 993 NESC.The three configurations analyzed were "A","AX"and "AKX"type structures.
These configurations enabled the evaluation of the unbraced and braced crossarm as
well as the evaluation of the X-braces.Structures #568,#490,and #491 were analyzed
as these represented the worst case conditions,i.e.the longest spans for the "A","AX",
and "AKX"structure types,respectively.For conservatism,weight spans were
considered to be 20%longer than wind spans.
The structural analyses revealed that the maximum bending stress in the
unbraced crossarm in Structure #568 is 1 1 15 psi under NESC loading.Similar analysis
of Structure #490 indicates a maximum bending stress of 1070 psi in the unbraced
crossarm.These results compare to an assumed design stress of 7600 psi for Douglas-
fir crossarms.Because the bending stress is less than the design stress value,it is
assumed that the crossarms provide sufficient structural reliability in the unbraced
configuration.
For the braced crossarm configuration,the resultant horizontal and vertical loads
at the conductor attachment points were approximately 1,215 lbs and 1,200 lbs,
respectively.These loads cause a bending stress of 1400 psi,which is significantly less
than the ultimate value of 7600 psi specified by the manufacturer.Further,buckling of
the braced crossarm was not deemed to be a problem.It is,therefore,assumed that
the braced crossarms which passed the structural inspection provide adequate reliability.
!The maximum axial load in the vee-bracing for this loading condition was found
to be approximately 3,250 lbs.This compares to a load rating of 20,000 lbs for Hughes
#2025 (3 3/8 x 4 3/8)bracing and a load rating of 15,000 lbs for Hughes #1 135 (2 2/4
X 3 1/2)bracing.
I
Based on the results of the analyses of the structures described above,the
maximum X-brace load was determined to be 1 2,170 lbs for structure #491 .This value
compares to a load rating of 20,000 lbs as provided by the manufacturer.Therefore,it
is assumed that the X-brace reliability is also adequate.
-22-EDMUDP012.RPT
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
Based on the above analyses,the framing in line should ensure adequate
reliability provided that maintenance needed to repair loose or split components is
performed.
3.7 Assessment of Deadend and Angle Structure Adequacy
Because no methods exist to adequately assess the reliability of guyed pole
structures,which carry primarily axial compression loads,the deadend H-frame and
three pole guyed structures in the Wood River-Ketchum 138 kV Transmission Line were
analyzed using deterministic methods.The buckling capacity of the wood poles was
calculated using the procedures presented in an ASCE paper by Gere and Carter (11),
(see Appendix C).This procedure is based upon classical buckling analysis and does
not consider the effects of bending.The 1993 Edition of the National Electrical Safety
Code (ANSI C2)was used as a reference for conductor and structure loading and
overload capacity factors.
The following assumptions were made for the analysis of these structures:
•NESC Medium Loading Zone (1/4"Radial Ice,4 PSF wind)
•Grade B Construction
•Conductor:#4/0 ACSR (Penquin)
•Conductor diameter=0.563 inch Wt/ft=0.291 lb/ft
Conductor Wt/ft with 1/4"radial ice=0.544 lb/ft
Conductor transverse wind load with 1/4"radial ice and
4 psf wind =0.3543 lb/ft
Conductor design tension 3,340 lbs.
Str.#540 -Conductor:397.5 kcmil ACSR (Ibis)
Str.#540 -Conductor diam =0.783 inch Wt/ft=0.5466 lb/ft
Str.#540 -Conductor Wt/ft with 1/4"radial ice=0.868 lb/ft
Str.#540 transverse wind load with 1/4"radial ice and
4 psf wind =0.4278 lb/ft
Str.#540 -Conductor design tension:6,520 lbs.
Shield wire -5/16"HS
Shield wire diameter =0.313 in.,Weight=0.206 lb/ft.
Shield wire weight with 1/4"radial ice =0.381 lb/ft.
Shield wire transverse wind load with 1/4"radial ice and
4 psf wind =0.271 0 lb/ft
Shield wire design tension =2,000 lbs.
1 to 1 guy slope
1/2"EHS guy wire (RBS=26,700 lbs)
Pole species -Western Redcedar
Weight span =1 .2 x wind span
-23-EDM\IDP012.RPT
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
The methods of analysis and calculations are presented below:
GUY ADEQUACY:
(GX Structure,500 ft Wind Span)In-line guvs
1.65 ((1.5)*3340 +(1)(2,000))+2.5(500)(1.5(0.3543)+0.2710)<0 g (26 700)
cos 45
17,780 <24,030
Therefore,one guy is adequate at this conductor position.
Bisector guvs
32.57°Line Angle,528 ft Wind Span
[1.65 (2)(3,340)sin (32.57/2)+2.5(0,3543)(528)cos (32.57/2)]<09(26 700)
cos 45 ''}
[3091 +449]<24(030
cos 45
5010 <24,030
Therefore,one guy is adequate at this conductor position.
BUCKLING LOAD:(32.57°Line Angle,Wind Span =528,Weight Span =634)
Maximum guy load +weight load =vertical load
[2.2 (2)(3.340)sin (32.57/2)+(4)(0.3543)(528)cos (32.57/2)
+2.2 (0.544)(634)]=Vertical Load
6510 =Vertical Load
For all poles evaluated,the critical buckling loads were found to be significantly greater
than the applied vertical loads.The results of these analyses are provided in Appendix
C.
-24-EDM\IDP01 2.RPT
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
Based on the above analyses coupled with information on shell rot,heart rot,and
pole strength,all of the structures were found to be adequate with respect to pole
strength.The buckling capacity was calculated without considering the effects of
bending.The requirements presented in the NESC code suggest that bending effects
are not considered in the design of guyed wood poles.In general,the bending effects
for a guyed wood pole are compensated for by the overload capacity factors applied to
the loads.The guys in general are adequate.
The anchors were not evaluated as there was no information provided by the field
inspections.
3.8 Electrical Reliability
(Section 3.8 is provided as an addendum to the report.)
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Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
4.LINE CONDITION ASSESSMENT
4.1 Structure Condition Assessment
The purpose of the structure condition assessment is to provide data to assist in
making the final recommendations for structure modifications to maintain line reliability.
The final recommendations are based on consideration of inspection results coupled
with the structural reliability assessment.
To conduct the condition assessment and make final recommendations
concerning structure repairs and replacements,a combination of several sources of
information was used.The EDM/INTEC structure inspection results (Section 2)were
combined with the results of the Davey Tree inspection,the reliability assessment
(Section 3)and the structural analyses of the adequacy of structural components
including crossarms and braces to develop a complete understanding of the condition
of each structure.While the reliability assessment was a key to quantifying the feasibility
of upgrading the line,the final repair or replacement decisions for an individual structure
may be governed by the results of the structural inspection.Therefore,while the
reliability level of an individual structure may be adequate based on the groundline
bending strength of the poles,the results of the structural inspection may dictate repair
or replacement.
The structures in the Wood River -Ketchum 138 kV line are generally in good
condition.Several poles have relatively minor shell rot as is common for 30 year old
western redcedar poles.Many poles exhibit minor woodpecker damage.In most cases
where woodpecker guards have been installed,they have prevented further woodpecker
damage.Two poles,however,in structures #538_and #540 have excessive woodpecker
damage and require repair or replacement.Additionally,it is recommended that a
climbing inspection be performed to evaluate the right pole in structure #558.A
woodpecker hole is located at the top of the pole immediately below the crossarm.
Inspection is required to ensure the integrity of the connection and evaluate the capacity
of the pole to support the loads placed on it.The borings made at and below
groundline by Davey Tree indicate that minimal internal decay is present in the poles in
this line.Loose guys,damaged insulators and split crossarms were noted on some
structures and a maintenance program to repair these minor problems is recommended.
Additionally,it was observed that many framing connections were slightly loose.These
slightly loose connections are not noted in the inspection report as they are common in
this line.A maintenance program to tighten framing connections in all the structures is
recommended.A complete list of the damage noted on each structure is provided in
the Appendices.Table 4.1 lists the observations and the number of recommended
repairs and replacements.
i
-27-EDM\IDP01 2.RPT
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
4.2 Electrical Condition Assessment
Based on field observations and their implications with regard to electrical
reliability of the line,there are no significant occurrences of observed conditions
overriding the overall electrical reliability of individual structures.
However,as described in Table 4.1 ,occurrences of loose or misaligned hardware,
broken ground wires,and broken insulators were observed.While these defects are not
significant contributors to the overall reliability of the line,it is recommended that
corrective measures be performed during the time that other mitigative measures are put
into place.
TABLE 4.1
Summary of Recommended Structure Repairs and Replacements
Based on Condition Assessment1
Repair Reinforce or
Replace
Structural
Component
Wood Poles3 2
Guys 24
Crossarms:2 10
Loose Hardware 10
Ground wires 1
Insulators 18
'See Appendix A for specific condition assessment results.
2An additional 8 crossarms were found to be in moderate condition.
3One of the poles recommended for replacement based on the condition assessment was also
recommended for replacement based on the reliability assessment.
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Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
5.RECOMMENDATIONS FOR STRUCTURAL UPGRADES
5.1 Introduction
Recommendations for replacement and/or upgrade of the structural components
of the line are presented in this section based on the structural analyses,combined with
the results of the field assessment.Implementing these recommendations will upgrade
the line to a level of reliability which will help provide for on-going performance with
minimal maintenance.
5.2 Recommendations for Structural Upgrades
Based on the results of the reliability assessment,line condition assessment
and evaluation of the adequacy of the components in the tangent structures,relatively
minor maintenance is needed to ensure the reliability of the line.A complete list of
structures with loose guys,split crossarms,damaged insulators,etc.is provided in
Appendix A.Recommendations to ensure structural performance are provided in
Appendix B.
A summary of recommended modifications to the wood pole structures is
provided in Table 5.1.Woodpecker damage warrants replacement of the right pole in
structure #538 and the center pole in structure #540.Replacement poles for these
structures may be of the same height and class as the original.It is recommended that
structure #572 be replaced.Due to the development of a surrounding residential
district,pole top extensions have been added to this structure.Additionally,the left pole
has both heart rot and shell rot and the crossarm has a minor split.Nondestructive
strength data were not available for the left pole in structure #572 due to an "unable to
test pole"reading.A mean pole strength was,therefore,assumed for this pole and
adjustments were made for loss of section modulus due to decay.Reliability results
based on these assumptions indicate that the left pole has a reliability that falls below
the threshold reliability index of 2.5.Due to the low reliability of the left pole,condition
of the framing,and the presence of the pole top extensions,it is recommended that the
entire structure be replaced even though the right pole provides adequate reliability.
This structure may be replaced with a type "AX"structure using 75'class 2 poles.
Crossarm replacement is recommended for those crossarms which have
un-repaired splits.Insulator replacement is recommended for those structures with
chipped or broken insulators.
For those structures which require pole replacement,reliability calculations
have been performed for both the current and upgraded structural configurations.These
-30-EDM\IDP012.RPT
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
calculations assume that IPCo will replace both poles in an H-frame structure.The
results of these calculations are provided in Table 5.2 along with the as-built values.
TABLE 5.1
Recommended Modifications
To Wood Pole Structures
Condition Number of
Structures
To Be
Modified
Modification
Badly decayed or damaged poles;
or structures with insufficient
capacity.
Structure Replacement 1
Badly damaged polesPoleReplacement 2
Split or damaged crossarmsCrossarmReplacement 10
Broken or chipped insulatorsInsulatorReplacement 18
Table 5.2
Structural Reliability Results for Upgraded
H-Frame Structures
Current
Minimum
Reliability
Index (J3)
Upgraded
Minimum
Reliability
Index (J3)
Controlling
Load
CaseStr.Pole
Left Right Left RightNo.Height
Pole Pole Pole Pole
WIND/ICE3.9 2.2 3.51 3.5153875
5722 N/A N/A WIND/ICE3.23 3.2375
1For purposes of evaluating the upgraded reliability of this structure,it is assumed that both poles in this
structure are replaced.
2Actual reliability data are not available for this structure in its current condition.Therefore,its reliability
was estimated.
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Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
6.SUMMARY AND RECOMMENDATIONS
The structural inspection revealed that the wood pole structures are generally in
good condition.Relatively minor problems,including loose guys,split crossarms,
damaged insulators,etc.were noted and should be corrected.Shell rot,woodpecker,
and fire damage were noted on some structures,but in most cases,this damage was
not severe enough to compromise structural reliability.A complete list of structural
defects identified in the EDM/I NTEC inspection is provided in Appendix A.The
information from the inspection was coupled with additional damage information from
the Davey Tree records to develop a complete understanding of the condition of each
pole.This information was then used as the basis for the reliability assessment.
The reliability assessment indicates that most of the tangent H-frame structures
have reliability indices that exceed the minimum reliability index of 2.5 selected by IPCo
and therefore should ensure adequate performance.One pole in H-frame structure
(#538)warrants replacement due to woodpecker damage.A second H-frame structure
(#572)is recommended for replacement due to a combination of low reliability and
framing condition.Nondestructive evaluation data were not available for the left pole in
this structure due to an "unable to test pole"reading from the PoleTest™device.To
estimate the reliability of this structure a typical pole strength was assumed for the left
pole and adjustments were made for heart rot and shell rot damage.The resulting
estimated reliability for this pole falls below the 2.5 threshold value chosen by IPCo.The
current and upgraded reliabilities for these structures have been provided in Table 5.2.
Additionally,the center pole in structure #540 (3 pole structure)warrants replacement
due to woodpecker damage.
A deterministic analysis of critical buckling loads was conducted for all three pole guyed
structures.The results indicate that the structures are adequate in their existing
condition provided that maintenance including repair or replacement of damaged
framing,tightening of loose guys and hardware,and insulator replacement is performed.
-33-EDMMDP012.RPT
REFERENCES
Idaho Power Company
December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
7.REFERENCES
American National Standards Institute,Inc."For Wood Poles-Specifications and
Dimensions."ANSI 05.1 -1987 New York,New York,1992.
1.
American National Standards Institute,Inc."National Electrical Safety Code."
Institute of Electric and Electronics Engineers,Inc.ANSI C2,New York,New
York,1993.
2.
Bodig,J.,J.R.Goodman,G.E.Phillips and G.B.Fagan."Wood Pole Properties."
Palo Alto,California.EI-41 09.
3.
Vol.1:Background and Southern Pine Data.July 1985.
Vol.2:Douglas-fir Data.January 1986.
Vol.3:Western Red Cedar Data and Size Effect.September 1986.
Bodig,J.,R.W.Anthony,J.R.Goodman."Nondestructive Evaluation of Wood
Utility Poles -A Status Report."Palo Alto,California.July 1986.EI-6-5063.
4.
Bodig,J.,and R.W.Anthony."Nondestructive Evaluation of Wood Utility Poles.
Vol.2:Second Generation Nondestructive Evaluation."Palo Alto,California.
November,1987.EL-5063.
5.
Criswell,M.E.and M.D.Vanderbilt."Reliability-Based Design of Transmission Line
Structures."Palo Alto,California.March,1987,EL-4793.
6.
Vol.1 :Final Report
Vol.2:Appendices
Electric Power Research Institute."Pole Analyzer Convinces Utility that Line can
Last 10 or More Years."EPRI First Use,RP 1352,March 1988.
7.
Engineering Data Management,Inc."Comparative Design and Economic Analysis
of Transmission Line Structures for Lifetime Performance."Fort Collins,Colorado.
Report No.WWPI 002.February 1 987.
8.
Engineering Data Management,Inc."Costs of Failure vs.Lifetime Management
for Wood Pole Utility Lines,"Distributed at IEEE/PES 1989 T &D Exhibition,New
Orleans,Louisiana,April 1989.
9.
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December 1993
Structural Reliability and Risk Assessment
Wood River-Ketchum 138 kV Line #433
Engineering Data Management,Inc."PoleTest™Technical Supplement (Model
PT101,Version 1.20)"TechWare Division,March 1988.
10.
Gere,J.and W.O.Carter."Critical Buckling Loads for Tapered Columns."ASCE
Structural Journal.New York,New York,Vol.88,No.ST1,February 1962
11.
Goodman,J.R.and A.H.Stewart,"Wood Pole Management -Utility Case Studies,"
IEEE/PES 1989 Transmission and Distribution Conference,New Orleans,
Louisiana,April 1989.
12.
Hasenoehrl,P.,"Nondestructive Evaluation in Wood-Pole Management."
Transmission and Distribution.December,1987,pp.38-43.
13.
Stewart,A.H.and J.R.Goodman,"Life Cycle Economics of Wood Pole Utility
Structures,"IEEE/PES 1989 Transmission and Distribution Conference,New
Orleans,Louisiana,April 1989.
14.
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