20170303IPC to Tidwell Attachment 18.4.pdf

r/7S PROJECT NO.:
COPY NO.:
ISSUED TO:
148029-01
Final Report
IDAHO POWER COMPANY
STRUCTURAL RELIABILITY,
RISK ASSESSMENT &
AVALANCHE LINE LOADINGS
FOR
WOOD RIVER -KETCHUM
138 KV LINE #433
October,1994
FOR INFORMATION REGARDING
THIS DOCUMENT CONTACT:
•BILL EISINGER,PM
PROJECT MANAGER \)
3940 GLENBROOK DRIVE
P.O.BOX 1066
HAILEY,IDAHO 83333
[phone]
TABLE OF CONTENTS
EXECUTIVE SUMMARY
PROJECT OVERVIEW
RELIABILITY AND RISK ASSESSMENT.
RESULTS
RECOMMENDED ACTION
3
3
4
4
1.INTRODUCTION
1.1 Background
1.2 Project Overview
5
5
AVALANCHE STUDY
2.1 Encounter Probability
2.2 Procedures &Definitions
2.3 Avalanche Computation Procedures
2.4 Assumptions Used in Calculations ..
2.5 Avalanche Maps
2.
7
8
8
9
9
STRUCTURAL RELIABILITY ASSESSMENT OF IN-SERVICE STRUCTURES
3.1 Introduction
3.2 Avalanche Loading Criteria for In-Service Structures
3.3 Structural Analysis and Reliability Methodology
3.4 Assessment of In-Service Structures Under Avalanche Loads
3.4.1 Introduction
3.4.2 Structural Analysis of Wood Pole
3.4.3 Structures Under Avalanche Loads
3.5 Structural Reliability of In-Service Wood Poles
3.5.1 Introduction
3.5.2 Implied Reliability for Structures Designed for Medium Loading
3.5.3 Structural Reliability ofLine #433
3.6 Assessment of Structural Component Adequacy
3.
11
11
12
13
13
13
13
15
15
17
19
4.STRUCTURAL ASSESSMENT OF PROPOSED WOOD AND LIGHT
DUTY STEEL STRUCTURES
4.1 Introduction
4.2 Avalanche Loading Criteria for Proposed Structures
4.3 Structural Analysis Methodology
4.4 Assessment ofProposed Structures Under Avalanche Loads
4.4.1 Introduction
4.4.2 Analysis of Proposed Wood Pole Structure
4.4.3 Analysis of Proposed Light Duty Steel Pole Structures
20
20
20
21
21
22
5.RECOMMENDATIONS FOR UPGRADES
5.1 Recommendations for Structural Upgrades 24
6.SUMMARY AND RECOMMENDATIONS 25
7.REFERENCES 26
2PEI-HLY 55-0864 148029-01 (09/94)ab
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POWER Engineers
PROJECT OVERVIEW
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.A portion of the line (approximately 1.25 miles)is to be re-routed to minimize the
impact on a golf course which falls within the line's current route.Seventeen single pole light
duty steel structures and one new wood pole structure are proposed for use in this re-route.
This line was originally built in 1962 and was initially operated at 46 kV.It 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 providing adequate line reliability in an avalanche environment prompted IPCo to
perform an avalanche risk assessment on the line,assess the reliability of structures in
avalanche prone areas,and develop strategies to help ensure long-term performance.The team
of Power Engineers,Inc.(POWER),A.I.Mears,P.E.,Inc.,and Engineering Data
Management,Inc.(EDM)combined to provide services for the avalanche risk assessment and
evaluation of structure performance under avalanche loading.The reliability and risk
assessments utilized line inspection/condition data obtained from a previous reliability and risk
assessment of this line (1).This previous study included a field evaluation of each structure's
condition and an assessment of the strength of each wood pole using nondestructive evaluation
(NDE).
RELIABILITY AND RISK ASSESSMENT
Reliability and risk calculation methods were applied to assess the structural performance of
the line under 50 year return period avalanche loads.The results of a previous field evaluation
of each wood pole structure were utilized in this assessment.The PoleTest™wood pole
strength analyzer,developed by EDM with the cooperation of the Electric Power Research
Institute,was utilized previously to predict in-place wood pole strength.Based on field
measurements of pole strength and observed conditions,calculations were performed to
determine the structural capabilities of the wood pole structures under avalanche loads.A risk
and reliability assessment was performed and theoretical reliability levels for avalanche loading
conditions were calculated for the existing wood pole structures.
The proposed steel structures were evaluated under avalanche loads and the groundline
moments resulting from these loads were compared to groundline moment capacities provided
by the manufacturer.For the proposed wood structure,groundline stresses resulting from
avalanche loads were compared to American National Standards Institute 05.1 (ANSI 05.1)
designated fiber strength values.
PEI-HLY 55-0864 148029-01 (09/94)ab
RESULTS
Adequacy of existing structures was assessed by comparing results of structural analyses using
structure specific avalanche impact loads to the ANSI 05.1 specified fiber strength for western
redcedar poles.Results show that based on ANSI 05.1 fiber strength,all of the structures
have adequate capacity to withstand a 50 year recurrence avalanche.
To better assess the abilities of the in service wood poles to resist avalanche loads,a reliability
assessment was performed which incorporated actual wood pole strength and defect data
combined with local wind loading 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 a weekly fastest mile wind
velocity of31.7 mph with a standard deviation of 7 mph.
A complete reliability analysis was conducted using the Boise loadings and incorporating the
predicted existing wood pole strengths (and variations)from field measurements.The
reliability analysis indicated that the structures in the line have a high level of structural
reliability with regard to avalanche loading.Most of the poles were found to have reliability
indices greater than 2.5 using 50 year recurrence avalanche loads.Idaho Power Company
(EPCo)has selected a minimum target reliability index of 2.5 (based on annual loads)for this
line and other lines of similar importance.Reliability calculations indicate that for the
structures evaluated in this study a reliability index of 0.62 based on 50 year recurrence loads
provides approximately the same structural reliability as a reliability index of 2.5 based on
annual (one year recurrence)loads.A high level of reliability is demonstrated by the structures
evaluated in avalanche susceptible areas as is demonstrated by an average reliability index of
3.6 based on 50 year recurrence loads.
RECOMMENDED ACTION
The reliability assessment indicates that all of the existing wood pole structures have adequate
reliability in their current condition with regard to the avalanche loadings.While structure
#511 demonstrates adequate reliability,it has a significantly lower reliability than the other
structures evaluated and consequently,may be considered a candidate for upgrade.The lower
reliability of structure #511 results from low pole strength and exposure to the highest
potential avalanche impact loading.
Structural analysis of the proposed wood and steel structures indicates that they have adequate
bending capacity to resist avalanche loadings with a 50 year recurrence interval.
PEI-HLY 55-0864 148029-01 (09/94)ab 4
1.INTRODUCTION
1.1 Bacteround
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.A portion ofthe line (approximately 1.25 miles)is to be re-routed to minimize the
impact on a golf course which falls within the line's current route.Seventeen single pole light
duty steel structures and one new wood pole structure are proposed for use in this re-route.
This 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 providing adequate line reliability in an avalanche environment prompted IPCo to
perform an avalanche risk assessment on the line,assess the reliability of structures in
avalanche prone areas,and develop strategies to help ensure long-term performance.The team
of Power Engineers,Inc.(POWER)and Engineering Data Management,Inc.(EDM)
combined to provide services for the avalanche risk assessment and evaluation of structure
performance under avalanche loading.The reliability and risk assessments utilized line
inspection/condition data obtained from a previous reliability and risk assessment of this line
(1)*.This previous study included a field evaluation of each structure's condition and an
assessment ofthe strength ofeach wood pole using nondestructive evaluation (NDE).
1.2 Project Overview
For structures which were found to be located in avalanche prone areas,reliability and risk
calculation methods were used to assess structure performance under avalanche loading
conditions.Calculations were performed to determine structural capabilities based on field
measurements of pole strength and observed condition.A comparison of the effects of
avalanche loadings to current structural capacity was completed.A risk and reliability
assessment was performed,and the reliability levels determined for structures under avalanche
loading were compared to selected minimum reliability levels.
Numbers in parentheses indicate references used in the preparation of this report.
5PEI-HLY 55-0864 148029-01 (09/94)ab
The assessments utilized the results of a previous reliability evaluation performed on this line.
During the previous reliability assessment (1)a thorough field evaluation of each structure was
performed.Results from a conventional pole inspection performed by Davey Tree (An
inspection company contracted by IPCo.)on each pole were combined with nondestructive
evaluation (NDE)strength predictions to better assess the overall condition of each pole.The
PoleTest™field unit developed by EDM was utilized to predict wood pole strength (11).
Following the reliability assessment of the structures in the line,a set of recommendations for
upgrade and/or replacement was developed for IPCo evaluation.
PEI-HLY 55-0864 148029-01 (09/94)ab
Table 3.2
Descriptions of Wood Pole StructuresI
Structure
Type
Description
I
H-frame Tangent,Unbraced CrossarmA
AX H-frame Tangent with X-Brace,Unbraced Crossarm
AKX-HD H-frame Tangent Hold Down Structure with X-Brace,Outside Vee-Braces
H-frame Deadend with X-Brace,Unbraced CrossarmGX
H Three Pole Structure
3.5 Structural Reliability of In-Service Wood Poles
3.5.1 Introduction
No standard minimum reliability criteria exist for extreme local loading conditions such as
avalanches.Therefore,for the purposes of this reliability assessment,reliability indices
resulting from avalanche loads were compared to reliability indices implied by the NESC.As
the Wood River-Ketchum line is located in the NESC medium district,comparisons were made
with NESC Medium implied reliability indices.
It should be noted that the NESC does not directly recognize local loading conditions
including wind magnitude and direction,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 (2)wood pole strength
values in any reliability assessment procedure.
3.5.2 Implied Reliability for Structures Designed for Medium Loading
To assess the reliability implied under NESC Medium loading conditions,wood pole strength
values must be known.Testing programs at Colorado State University (4,5,6)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.
Table 3.3
ANSI 05.1 Wood Pole Strength Data
StandardPoleStrength Coefficient of
Variation(psi)Deviation
(Psi)
Species
5200 946WesternRedcedar 0.192
PEI-HLY 55-0864 148029-01 (09/94)ab 15
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Table 3.4
Implied Reliability Indices
for NESC Design of New Western Redcedar Poles
Overload Implied
Reliability
Index
(Annual Loads)
Capacity Design
Stress*
(psi)
Factor
Grade (OCF)
B 4 1500 3.91
*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 results of reliability
calculations for the "at replacement"condition.The calculations 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 (4,5,6,7).
Table 3.5
Implied Reliability of Western Redcedar Poles for NESC
At Replacement Condition
Original Reduced
Mean
Implied
Reliability
Index
(Annual Loads)
Design
Stress
Standard
Strength*Deviation
(psi)(psi)Grade (psi)
1500 3484B 1070 1.84
*Reduced (deteriorated)mean strength 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.
In particular,the reliability index which will satisfy the NESC "at replacement"criteria provides
a guide for setting the minimum acceptable reliability level.
PEI-HLY 55-0864 148029-01 (09/94)ab 16
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 avalanche 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.
3.5.3 Structural Reliability of Line #433
As noted in Section 3.2,structural reliability calculations considered local avalanche conditions
and utilized wind data from the Boise airport.Pole strengths (and variations)were assessed
using PoleTest™during a previous project.Modifications in pole capacity were made if
warranted by decay or pole damage noted during the previous inspections.
In accordance with the intent of the NESC,IPCo has selected a conservative minimum target
reliability index of 2.5 (based on annual loads)during a previous reliability assessment of this
line.This target value is also used by IPCo and other utilities for other lines of similar
importance.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 as discussed in
Section 3.5.2.
As adequate avalanche load data are not available to determine mean annual avalanche impact
force values,it was necessary to base the reliability assessment on avalanche loads which have
a 50 year recurrence.Reliability indices based on differing load recurrence intervals should not
be compared directly.Consequently,the reliability values provided in this reliability assessment
do not compare directly with the 2.5 target reliability index which is based on annual loads.To
determine a reliability index based upon 50 year recurrence loads which is approximately
equivalent to the 2.5 (annual recurrence)target reliability index,calculations were performed
using the average strength and coefficient of variation values of the wood poles located in
avalanche hazard areas.These strength values were obtained through non-destructive
evaluation (PoleTest™)performed during a previous study and were adjusted to account for
pole defects.Due to the unavailability of annual avalanche load data,these calculations were
based on the variation of material strength alone and represent only a guideline to provide a
better understanding of the reported reliability indices which are based upon 50 year loads.
The calculations indicate that a reliability index of 0.62 (50 year recurrence)will provide
approximately the same structural reliability as a 2.5 (one year recurrence)reliability index.
Results ofreliability calculations to determine reliability indices ((3's)based on 50 year loads for
each wood pole structure in avalanche hazard locations are provided in Table 3.6.The
structures evaluated exhibit a high average reliability index of 3.6.
17PEI-HLY 55-0864 148029-01 (09/94)ab
!
Table 3.6 Avalanche Reliability Assessment
for In-Service Wood Structures
Wind PoleStructure
Number (Type)Span Height Pole Class Pole Diameter Pole Strength Prediction*
(ft)(ft)
Left Right Center Left Right Center Left Right Center
(in.)(in.)(in.)(psi)(psi)(psi)
496 (A)590 2 265 N/A 17.3 15.7 N/A 3830 4370 N/A
497 (A)558 60 2 2 N/A 14.6 14.5 N/A 4160 3820 N/A
N/A 16.0498(A)558 60 2 2 17.5 3940N/A 4430 N/A
2 2 15.8499(A)570 60 N/A 15.4 N/A 4130 4140 N/A
621 65 2 2 N/A 15.7500(A)18.4 N/A 4780 3930 N/A
508 (GX)835 70 2 2 N/A 18.6 19.8 N/A 3560 3600 N/A
511 (GX)962 2 2 N/A 17.470 16.4 N/A 3690 3600 N/A
688 2 2 N/A 18.5542(HDA-AKX)75 18.0 N/A 4470 3940 N/A
600 3 2 N/A 15.8548(A)60 16.8 N/A 4160 4460 N/A
551 (A)601 65/75**2 2 17.0N/A 16.4 N/A 3860 4240 N/A
552 (GX)920 60/70**2 3 N/A 18.4 16.0 N/A 3760 3950 N/A
555 (A)505 3 3 N/A 15.6 15.970 N/A 3960 4030 N/A
556 (H)1030 2 2 16.3602 17.4 17.2 4020 4270 4430
Determined based on the results of a previous project.*
Left Pole /Right Pole
PEI-HLY 55-0864 148029-01 (09/94)ab 18
\
3.6 Assessment of Structural Component Adequacy
It was not practical to perform structural reliability calculations for crossarms and X-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.
None of the H-ffame structures identified as being in avalanche hazard areas had braced
crossarms.
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To evaluate the structural integrity of the crossarms and X-braces,the worst case structure
(#511)was analyzed using its specified avalanche load with an overload capacity factor of 1 .0
for both vertical and transverse loads.No wind loads were considered in this stress analysis.
The configuration analyzed was a "GX"type structure.This configuration enabled evaluation
of the unbraced crossarm as well as the X-braces.For conservatism,the weight span was
considered to be 20%longer than the wind span.
The structural analyses revealed that the maximum bending stress in the unbraced crossarm in
Structure #511 is 400 psi under avalanche loading.These results compare to an assumed
design stress of 7600 psi for Douglas-fir crossarms.Further,buckling of the crossarm was not
deemed to be a problem.It is therefore assumed that the crossarms provide sufficient
structural reliability in the unbraced configuration.
Based on the results of the analyses of the structure described above,the maximum X-brace
load was determined to be 310 lbs.This value is due to the low location of the avalanche load
on the structure and 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.
Based on the above analyses,the framing in the line should ensure adequate reliability with
regard to the avalanche loadings.
Table 3.6 Avalanche Reliability Assessment
for In-Service Wood Structures
Structure
Number
(Type)
Avalanche
Center of
Preuure (ft)
Avlanche
Impact Force
PerPole (lbs)
Reliability Index
(50 Yr.Avalanche Load +Weekly
Wind)
Combined Bending and Axial Stress
(50Yr.Avalanche Load +Nominal
Dead Load)
Left Right Center Left Right Center
!e?!I lE2iI
496 (A)4.26 9427 3.46 3.63 N/A 1190 N/A1190
497 (A)4.26 9252 2.89 N/A2.65 1250 N/A1250
498 (A)4.26 8070 3.76 4.52 N/A 1110 N/A1110
499 (A)4.26 7353 3.73 3.66 N/A 1010 1010 N/A
500 (A)4.43 419 5.35 4.59 N/A 70 70 N/A
508 (GX)N/A4.43 14724 3.47 3.82 1740 1740 N/A
511 (GX)21937 N/A4.59 1.75 1.04 2680 2680 N/A
\542(HDA-AKX)5.35 4.87 N/A3.94 4328 440 N/A440
5.09 N/A548(A)4.26 346 4.98 70 N/A70
5.25 3.75 N/A551(A)5266 3.52 820 720 N/A
552 (GX)9159 3.74 3.36 N/A5.41 1550 1620 N/A
N/A555(A)5.41 12027 2.02 2.23 2200 2200 N/A
556(H)5.25 9297 2.99 3.71 3.76 1570 1570 1570
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PEI-HLY 55-0864 148029-01 (09/94)ab 19
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4.STRUCTURAL ASSESSMENT OF PROPOSED WOOD AND
LIGHT DUTY STEEL STRUCTURES
4.1 Introduction
Structural analyses were performed to assess the performance ofthe proposed wood and light
duty steel structures to be utilized in the golf course re-route.As non-destructive evaluation
(NDE)data are not available for the new wood poles,their performance was assessed based on
the maximum stress developed at groundline by the avalanche.A check of groundline capacity
versus the moment induced by the avalanche loading was also performed for the light duty
single pole steel structures.
4.2 Avalanche loading Criteria for Proposed Structures
For the purpose of assessing the adequacy of the proposed structures on a stress basis,only the
stresses produced by avalanche loadings are considered.Wind loads are excluded in this
assessment as avalanches typically occur during calm weather conditions.Further,considering
the effects of avalanche loads alone,provides results which can be combined with other
loading conditions if desired.For the structural assessment of the proposed wood and light
duty steel structures the following load combination was considered.
•Dead Load and Avalanche Loading (50 yr.Return Period)
4.3 Structural Analysis Methodology
Structural analyses calculations for the loading conditions cited in Section 3.2 were performed
for the proposed wood pole structures using the computer program POLDAR^(see Section
2.3 for more information).
Groundline stresses and capacities of the light duty steel poles were evaluated using criteria in
the American Society of Civil Engineers (ASCE)manual "Design of Steel Transmission Pole
Structures"(16).Groundline moments developed by the avalanche loads were compared to
groundline moment capacities provided by the pole manufacturer.
For purposes of assessing groundline stresses all proposed structures were evaluated as
tangent structures regardless of guy configurations.This produces conservative groundline
moments for comparison to groundline capacities.
3 Program POLDAR was developed under the sponsorship of the Electric Power Research Institute (EPRI),
Palo Alto,California as part of research project RP1352 conducted by Colorado State University.
PEI-HLY 55-0864 148029-01 (09/94)ab 20
4.4 Assessment of Proposed Structures Under Avalanche Loads
4.4.1 Introduction
This section provides results of calculations to assess the adequacy of the new wood and light
duty steel poles in the line with regard to avalanche loads.Structural analysis provides a basis
for assessing the affect of avalanche loading as it compares with ANSI 05.1 fiber strength for
the proposed wood pole structure.For the light duty steel structures,groundline moments are
evaluated for comparison with the groundline moment capacities provided by the
manufacturer.
4.4.2 Analysis of Proposed Wood Pole Structure
The proposed wood pole structure in the re-route was analyzed with structure specific
avalanche loads combined with dead loads to assess its structural adequacy.The following
procedures were used in the evaluation ofthis structure:
•Avalanche loads were considered without the addition of wind loads.The dead load
(OCF=1.0)of the structure and conductors (397.5 kcmil (Ibis)ACSR conductors and
5/16"HS shield wires)was considered.
•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 these loading conditions to structure #498 (NEW)are
provided in Table 4 .1.For all of the poles in this three pole structure the maximum stresses
under the avalanche loading (OCF=1.0)fall below the ANSI 05.1 designated fiber stress value
of 6000 psi for western redcedar.This indicates that the proposed poles have adequate
capacity on a stress basis.The stresses tabulated in Table 4.1 represent the maximum
combined axial and bending stresses which occur at groundline in the poles.
Table 4.1 Stress Analysis of Proposed Wood Pole
Structure #498 Under Avalanche Loads
Structure
Number
Avalanche
Center of
Pressure
Avalanche
Impact Combined Bending and Axial Stress
(50 yr.Avalanche Load +Nominal Dead
Load)
(Type)Force
(ft)(lbs.)
Left Right Center
(E5!>(psi)(psi)
498 (AHXS)8070 9904.26 900 900
PEI-HLY 55-0864 148029-01 (09/94)ab 21
Table 4.1 Stress Analysis of Proposed Wood Pole
Structure #498 Under Avalanche Loads
Structure
Number
Pole
Pole ClassHeight ANSI 05.1 Groundline Diameter
(Type)(ft)
Left Right Center Left Right Center
i!3l IulI lilLi
498 (AHXS)60/60/55 11 1 16.8 16.8 16.2
*Left Pole /Right Pole /Center Pole
4.4.3 Analysis of Proposed Light Duty Steel Pole Structures
The proposed light duty steel pole structures in the re-route were analyzed with structure
specific avalanche loads to assess structural adequacy.The following procedures were used in
the evaluation of these structures:
•Avalanche loads were considered without the addition of wind loads.The dead load
(OCF=1.0)of the structure and conductors (397.5 kcmil (Ibis)ACSR conductors and
5/16"HS shield wires)was considered.
•The steel structures were evaluated in accordance with the American Society of Civil
Engineers (ASCE)manual "Design of Steel Transmission Pole Structures"Second Edition.
The application of avalanche loads to the steel pole structures resulted in groundline stresses
and moments which can be compared to the 65 ksi yield strength of the steel and the
groundline moment capacities provided by the pole manufacturer.The results of the steel pole
structure analysis and the groundline moment capacities provided by the manufacturer are
listed in Table 4.2.All ofthe steel pole structures demonstrate adequate bending capacity with
regard to avalanche loads.
The shear capacity ofthe structures was also checked and found to be adequate.Performance
of the structures under combined vertical (OCF =1),bending and shear forces was evaluated.
Interaction equations indicate that the structures should perform adequately with regard to
avalanche loading under these combined conditions.
PEI-HLY 55-0864 148029-01 (09/94)ab 22
Table 4.2 Structural Analysis Of Light Duty Steel
Poles Under Avalanche Loads
Pole Base
Diameter
Structure
Number
Pole Groundline
Diameter
Groundline
Moment
Capacity
Avalanche Avalanche Groundline
Moment
(ft-ldps)
Bending
StressClassHeightCenterofImpact
Pressure Force (tai)
151 (In-)151 2^1
LD-3 19.88 18.50499R75 250 4.26 12457 53.1 12.4
LD-3 19.88 18.50 4.43500R75250 10565 46.8 11.0
LD-3 19.88 18.50 4.43501R75250 15190 67.3 15.8
LD-3 20.60 19.15 268 4.26502R80 5010 21.3 4.7
232LD-3 19.15 17.85 4.26503R70 5879 25.0 6.3
LD-3 19.88 18.50 250 4.26 6522504R75 27.8 6.5
LD-1 16.88 15.88 183 4.43514R75 16507 73.1 23.4
PEI-HLY 55-0864 1480294)1 (09/94)ab 23
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5.RECOMMENDATIONS FOR STRUCTURAL UPGRADES
5.1 Recommendations for Structural Upgrade
The reliability assessment indicates that all of the existing wood pole structures have adequate
reliability with regard to avalanche loading.All structures have reliability indices based on 50
year recurrence loads of 1.04 or greater.These indices are greater than the 0.62 fifty year
recurrence value which approximates an annual reliability index of 2.5.
As an option structure #511 may be considered a replacement candidate due to the fact that its
reliability is significantly lower than the reliabilities of the other structures in the line.The left
and right poles in this structure have reliability indices of 1.75 and 1.04 respectively which are
significantly lower than the average reliability index of 3.6 for the wood poles exposed to
avalanche hazard.The results of calculations for optional upgrade of structure #511 are
provided in Table 4.1 along with the as-built values.The upgrade reliability calculations
assume that both poles in the H-ffame structure are replaced with either class 1 or class 2
ANSI 05.1 wood poles.
Table 5.1
Structural Reliability Results for Optional Upgraded
H-Frame Structure #511*
Current Upgraded
Reliability
Index (P)
(50 yr.Loads)
Reliability
Index (P)
(50 yr.Loads)
Pole Pole
Height Class
Left Pole Right Pole Left Pole t Pole
2 1.04701.75 2.22 2.22
70 1 2.74 2.74
For purposes of evaluating the upgraded reliability of this structure,it is assumed that both poles in this
structure are replaced.
PEI-HLY 55-0864 148029-01 (09/94)ab 24
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6.SUMMARY AND RECOMMENDATIONS
An extensive analysis has been performed to assess the reliability of the structures in the Wood
River-Ketchum 138 kV Line #433 with regards to avalanche loading.For in-service
structures this analysis considered individual pole condition and strength data collected during
a previous inspection and reliability evaluation ofthe line.
Fifty year recurrence avalanche loads were developed for each structure located in an
avalanche hazard area.These structure specific avalanche loads were combined with local
weekly wind loads obtained from Boise airport data and individual pole strength data to
evaluate the reliability of each in-service pole which is exposed to an avalanche hazard.
The reliability assessment indicates that all of the in-service wood poles exhibit reliability
indices based on 50 year recurrence loads that exceed the threshold value of 0.62 which
approximates a reliability index of 2.5 based on annual loads.A threshold reliability index of
2.5 (one year recurrence)has been selected by IPCo for previous reliability assessments
performed on this line.The structures in this line should therefore provide adequate
performance with regards to avalanche loading.
While all the structures evaluated exhibit adequate reliability for service in the avalanche hazard
areas,structure #511 exhibits a significantly lower reliability than the other structures which
were evaluated.To further increase line reliability for avalanche loadings,IPCo may elect to
upgrade this structure.
Analyses of the proposed new structures for the golf course re-route indicate that all of the
new structures should provide adequate performance with regard to avalanche loading.
PEI-HLY 55-0864 148029-01 (09/94)ab 25
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L "V ¦REFERENCESy«•.
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7.REFERENCES
Engineering Data Management Inc."Structural Reliability and Risk Assessment of the Wood
River -Ketchum 138 kV Line #433."Fort Collins,Colorado.December 1993.
American National Standards Institute,Inc."For Wood Poles-Specifications and Dimensions."
ANSI 05.1 -1992 New York,New York,1992.
American National Standards Institute,Inc."National Electrical Safety Code."Institute of
Electrical and Electronics Engineers,Inc.ANSI C2,New York,New York,1993.
Bodig,J.,J R.Goodman,G.E.Phillips and G.B.Fagan."Wood Pole Properties."Palo Alto,
California.El-4109.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.El-6-5063.
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.
Criswell,M.E.and M.D.Vanderbilt."Reliability-Based Design of Transmission Line
Structures."Palo Alto,California.March,1987,EL-4793.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.
Engineering Data Management,Inc."Comparative Design and Economic Analysis of
Transmission Line Structures for Lifetime Performance."Fort Collins,Colorado.Report No.
WWPI002.February 1987.
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.
TMEngineeringDataManagement,Inc."PoleTest
Version 1.20)"TechWare Division,March 1988.
Technical Supplement (Model PT101,
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
PEI-HLY 55-0864 148029-01 (09/94)afa 26
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.
Hasenoehrl,P.,"Nondestructive Evaluation in Wood-Pole Management."Transmission and
Distribution.December,1987,pp.38-43.
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.
American Society of Civil Engineers "Design of Steel Transmission Pole Structures,2nd
Edition."ASCE Manual No.72,New York,New York,1990.
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27PEI-HLY 55-0864 148029-01 (09/94)ab
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