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Strength & Loading Of Wood Utility Poles
Webinar
Overview
• Benefits of Wood as a Utility Pole Material
• Defining Wood Pole Strength
• Loading Criteria
• In-Service Analysis
• Upcoming NESC 2017 Changes
Pole Material Choices
Wood
Pole Material Choices
Metal (Steel, Aluminum)
Concrete
Composite (Fiber Reinforced Polymer)
Benefits of Wood as a Utility Pole Material
• Long-Life Span o ~45 years national average without remedial treatment
• Lowest Cost o Both initial and full life-cycle costs
• Proven Performance o “Go to” overhead line construction material since the early 1900’s
• Climb-ability o Ability to service attachments without heavy equipment
Benefits of Wood as a Utility Pole Material
• Supply Chain is Proven o Even in natural disaster events where demand is high, it has met the
challenge of providing the necessary poles in required timeline.
• Beneficial Physical Properties o Good electrical insulator, resilience to wind and mechanical impacts
• Easy Maintenance and Modification in Service
• “Green” o a treated wood pole have a reduced environmental impact when
competed to other utility pole materials.
o A renewable and plentiful resource
“10 Features Often Overlooked About the Extraordinary Wood Pole.” North American Wood Pole Council. www.woodpoles.org
Bending Load Bending Capacity >
Strength and Loading of Wood Poles
Wire with Ice
7
Defines:
• Loading Criteria
• Strength Requirements
Defines:
• Wood Strength
• Wood Quality
Strength and Loading of Wood Poles
Wire with Ice
8
ASC O5
Accredited Standards
Committee O5:
Standards for Wood
Utility Structures
• Secretariat: AWPA
• Revised: 5 year cycle
• Founded in 1924
ANSI O5.1 Wood Pole Specification
9
http://asco5.org/standards/
ASC O5 Standards http://asco5.org/standards/
O5.4 - 2009 Naturally Durable Hardwood Poles
O5.5 - 2010 Wood Ground Wire Moulding
O5.6 - 2010 Solid Sawn Naturally Durable Hardwood Crossarms & Braces
O5.TR.01-2004 Photographic Manual of Wood Pole Characteristics
Poles Glu-Lam Crossarms
10
Scope
Single Pole
Simple Cantilever
Transverse
Groundline
11
Maximum Stress Point
Max Stress @ 1.5 Diameter Load Point
Solid, Round, Tapered, Cantilever
Distribution Usually Groundline
Load (Wind Force on Wires, Equip., etc.)
12
ANSI O5.1 – Wood Poles
Wood
Quality
Class
Loads
Pole
Dimensions
Fiber
Strength
13
Wood Quality
• Allowable knots
14
Wood Quality
• Sweep
15
Wood Quality
• Growth Rings
16
Class Loads
2 ft Lc
Horizontal
Class Load (lb)
10 370
9 740
7 1,200
6 1,500
5 1,900
4 2,400
3 3,000
2 3,700
1 4,500
H1 5,400
H2 6,400
H3 7,500
H4 8,700
H5 10,000
H6 11,400
17
Class Loads
2 ft Lc
Telco
Distribution
Transmission
Horizontal
Class Load (lb)
10 370
9 740
7 1,200
6 1,500
5 1,900
4 2,400
3 3,000
2 3,700
1 4,500
H1 5,400
H2 6,400
H3 7,500
H4 8,700
H5 10,000
H6 11,400
2 ft Lc
18
Lc
Fiber Strength
Compression
(psi)
Tension
(psi) Fiber Strength
Bending Capacity =
k x fiber strength x C3 (ft-lb)
19
Designated Fiber Strength: Pole Species
20
Distribution: Southern Yellow Pine
Transmission: Douglas fir
Western red cedar
Southern Pine
Distribution: Douglas fir
Transmission Douglas fir
Western red cedar
Southern Yellow Pine 8,000 psi
Douglas fir 8,000 psi
Western red cedar 6,000 psi
#40
COV
Strengths are Average Values
COV = Coefficient of Variance COV= Approx. 0.20 (20%)
for Southern Pine, Douglas
Fir and Western Red Cedar
Wood Poles
Steel Poles
22
Lc
D
2 ft
Class 1 4,500 lb
Class 2 3,700 lb
Class 3 3,000 lb
Class 4 2,400 lb
Class 5 1,900 lb
Applied Bending Load
Applied Bending Load =
Lc x D (ft-lb)
23
L x D = Bending Moment (ft-lb)
76,800 ft-lb
2400 lb
32 ft
40 ft Class 4
41 ft
98,400 ft-lb
2400 lb
50 ft Class 4
24
Pole Dimensions: Circumference
TIP
6ft
G/L
Bending Capacity =
k x fiber strength x C3 (ft-lb)
25
Pole Dimension Table
(in) 1. Circumference 6ft from butt
Southern Pine and Douglas Fir
26
1) The figures in this column are not recommended embedment depths; rather,
these values are intended for use only when a definition of groundline is necessary
in order to apply requirements relating to scars, straightness, etc.
Annex B: Groundline Stresses
Minimum circumferences specified at 6 feet from the butt
Were calculated so each species in a given class
Can support the class horizontal load applied 2 ft from the tip
Bending Capacity =
k x fiber strength x C3 (ft-lb)
Applied Bending Load =
Lc x D (ft-lb)
27
Pole Dimension Table
(in)
Southern Pine and Douglas Fir
28
Applied Bending Load=
Class Load * Distance
Bending Capacity =
k x fiber strength x C3
79,401 ft-lbs=
.000264 x 8000x 33.53
76,800 ft-lbs=
2,400 lbs* 32ft
40 ft Class 4 Poles
Douglas fir
(8000 psi)
36 1/2”
Western Red Cedar
(6000 psi)
33 1/2”
2400 lb
29
Circumference3 Effect
MG/L = .000264 x Fiber Stress x Circumference 3
26” 34”
37,120 ft-lb 83,010 ft-lb
Circumference Increase - 30%
Bending Capacity Increase - 123%
80-90%
Pole’s Bending Strength
In The Outer 2-3” Of Shell!
30
ANSI O5.1 Summary
2 ft Lc
Bending
Capacity = k x fiber strength x C3 (ft-lb)
All Species
Same Length & Class
Similar Load Capacity
31
Loading Criteria: National Overhead Line Standard
ANSI C2:
National Electrical
Safety Code
• Secretariat: IEEE (Institute of Electrical and
Electronics Engineers)
• Revised: 5 year cycle
• Founded in 1918
NESC
32
Loading Criteria: CA Overhead Line Standard
CPUC:
California Public Utilities Commission
General Order 95:
Overhead Line Construction
• Revised: As Needed
• Founded in 1941
From GO 64 est. 1922
GO95
33
Basic Safety Standard
Electric Supply and Communication Facilities
National Electric Safety Code (NESC)
34
Topics of the NESC
Topics Covered
Grounding
Substations
Overhead Electric Supply and
Communication Lines
Underground Electric Supply and
Communication Lines
Work Rules
Wood Poles
35
NESC - Committees
Main Committee
SC 1 Sections 1, 2 & 3
(Scope/Purpose, Definitions, References)
SC 2 Grounding
SC 3 Substations
SC 4 Overhead Lines - Clearances
SC 5 Overhead Lines – Strength and Loading
SC 7 Underground Lines
SC 8 Work Rules
36
Section 24 Grades of Construction
Section 25 Loading for Grade B&C
Section 26 Strength requirements
• Grades of construction
• Grades include B, C &
N
(B being the highest)
• Loads to apply
• Rule 250B:
Combined ice and Wind
District loading
• Rule 250C:
Extreme wind Loading
• Rule 250D:
Extreme Ice with concurrent
wind loading
• Strength utilization
Overhead Lines Subcommittee
Section 27 Insulators
• Electrical Strength
• Mechanical Strength
37
Section 24: Grades of Construction
• Grade B: • Crossing Limited Access Highways
• Crossing Railways
• Crossing Navigable Waterways
• Grade C: • All other standard construction
• Grade N: • Mainly used for temporary and emergency construction
• Defined as the strength shall exceed the expected loads
38
Section 25: Loading for Grade B & C
• Rule 250B: Combined Ice and Wind District loading
• Rule 250C: Extreme wind Loading • (Applies only to Structures 60ft above ground and taller)
• Rule 250D: Extreme Ice with concurrent wind loading • (Applies only to Structures 60ft above ground and taller)
Deterministic Loads
Probabilistic Loads
39
NESC District Loading
½” Ice – 40 mph
¼” Ice – 40 mph
0” Ice – 60 mph
40 mph = 4 lbs/sqft
60 mph = 9 lbs/sqft
40
GO95 District Loading
>3000ft elevation
½” Ice - 48 MPH
<3000ft elevation
0” Ice - 56 MPH
Heavy Loading District
Light Loading District
41
Extreme Wind– Rule 250C
Summer Storm
85 mph = 18.5 lbs/sqft
90 mph = 21 lbs/sqft
130 mph = 43 lbs/sqft
150 mph = 58 lbs/sqft
42
Ice with Concurrent Wind –Rule 250D
Winter Storm
Wind Speeds
30 mph
40 mph
50 mph
60 mph
Radial Ice
0”
0.25”
0.5”
0.75”
1.0”
43
Grade B Grade Cx Grade C
Ru
le 2
50
B
Vertical Loads 1.50 1.90 1.90
Transverse Loads
(wind) 2.50 2.20 1.75
Longitudinal
Loads 1.10 No Req. No Req.
25
0C
Wind Loads 1.00 1.00 1.00
25
0D
Ice and Wind
loads 1.00 1.00 1.00
Section 25: Table 253.1-Load Factors
44
Section 26: Strength Factors
Fiber Strength (ANSI)
× Strength Factor (NESC)=
Allowable Stress of Pole
Grade B Grade C R
ule
25
0B
Metal Structures 1.0 1.0
Wood Structures 0.65 0.85
25
0C
& 2
50
D
Metal Structures 1.00 1.00
Wood Structures 0.75 0.75
Table 261-1
45
Restore or Replace Thresholds
Table 261-1: ②Wood and reinforced structures shall be
replaced or rehabilitated when deterioration
reduces the structure strength to 2/3 of that
required when installed.
Strength required when installed, NOT original strength of wood pole used.
When to restore or replace: NESC
When to restore or replace: GO95 Section 44.3: Lines or parts thereof shall be replaced or
reinforced before safety factors have been reduced (due to
factors such as deterioration and/or installation of additional
facilities) … to less than two-thirds of the safety factors
specified…
46
Load and Strength Factors
Grade B Grade C
Load Factor 2.5 1.75
Strength Factor 0.65 0.85 Effective Overall
Safety Factor 3.85 2.06
Rule 250B: District Loading
Effective Restore
or Replace
Threshold (2/3 ) 2.57 1.37
47
Grade A Grade B Grade C A
t
Inst
all
ati
on
Wood Pole Safety
Factor 4 3 2
At
Rep
lacem
en
t
Wood Pole Safety
Factor 2.67 2 1.33
GO95: Table 4- Safety Factors
48
Bending Load Bending Capacity >
In Service Analysis
Wire with Ice
49
Bending Capacity
In Service Analysis
•Class is determined by the load on the pole
• Length is determined by required clearance
50
Bending Load
In Service Analysis
Wire with Ice
Load types acting on the pole:
o Wind
o Ice
o Line Tension
o Guy Tension
o Weight (equipment, conductors, etc)
51
TRANSVERSE
V
E
R
T
I
C
A
L
Loading Directions
52
Balanced
Unequal
Longitudinal Loading
53
Balanced
Longitudinal Loading - Guys
54
NESC Guying Requirements
261.A.2
c. Strength of guyed poles
Guyed poles shall be designed as columns, resisting the vertical component of the tension in
the guy plus any other vertical loads.
261.A.5.C
2. Wood structures
When guys are used to meet the strength requirements, they shall be considered as taking the
entire load in the direction in which they act, the structure acting as a strut only, …
Guys add to VERTICAL loads,
Balance LONGITUDINAL loads/ wire
tensions
55
56
57
V
E
R
T
I
C
A
L
Vertical Pole Loading
59
•Weight of Conductor
(+ice @ 57lb/ft3)
•Equipment Weight
•Pole Weight
•Down Guy Vertical Load
STRUCTURE MUST RESIST BUCKLING
Vertical Pole Loading
60
Transverse Load Dictates Design
Wire with Ice
61
TRANSVERSE
Wind Bending Loads On:
Wires
Ice
Pole
Equipment
Offset Bending Loads
Wire Tension
Calculating Transverse Loads
62
Height (ft)
DiameterC
x Load Factor
x Span x Wind Pressure
x Height = ft-lb
Groundline Bending Moment
Transverse Wind Load on Conductors
63
0.75” 1.50” 3.00”
Wind Effects on Wire Sizes
2X 2X
double wire diameter, double the load
64
0.75” 1.50” 3.00”
1.25” 2.00” 3.50”
+67% +33% +17%
Wind Effects: Wire Sizes + 0.25” Radial Ice
.25” Ice
.75” Wire
65
All Moments Added Together
66
Moment from Wind on Conductors
+ Moment from Wind on Pole
+ Moment from Wind on Equipment
+ Moment from Equipment Offset
= Total Groundline bending Moment
Evaluating Existing Structures
67
Existing Structures – Efficient Pole Loading
Pole Strength Characteristics:
•Pole Length and Class
•Groundline Circumference
Pole Construction Details:
•Span Lengths
•Span Bearings
•Attachment Heights
•Wire Diameters
Additional Details:
•Equipment Details
•Guying Details
68
Moment from Wind on Conductors
+ Moment from Wind on Pole
+ Moment from Wind on Equipment
+ Moment from Equipment Offset
= Total Groundline bending Moment
Evaluating an In-Service Pole - By Hand
69
Pole Loading Analysis Software Reports
70
Corrective Actions After Evaluation
Proper clearances not maintained
o Suggested corrective actions:
– Re-route attachment
– Move attachments to satisfy clearance needs
– Re-tension attachments
– Replace with appropriate pole length
Load exceeds pole’s rated capacity
o Suggested corrective actions:
– Re-route attachment
– Replace with appropriate pole Class
– Increase pole’s load carrying capacity
71
Determining In-Service Strength
72
Restore or Replace Thresholds
Table 261-1: ②Wood and reinforced structures shall be
replaced or rehabilitated when deterioration
reduces the structure strength to 2/3 of that
required when installed.
Strength required when installed, NOT original strength of wood pole used.
When to restore or replace: NESC
When to restore or replace: GO95
Section 44.3: Lines or parts thereof shall be replaced or
reinforced before safety factors have been reduced (due to
factors such as deterioration and/or installation of additional
facilities) … to less than two-thirds of the safety factors
specified…
73
Circumference3 Effect
75
MG/L = .000264 x Fiber Stress x Circumference 3
26” 34”
37,120 ft-lb 83,010 ft-lb
Circumference Increase - 30%
Bending Capacity Increase - 123%
80-90%
Pole’s Bending Strength
In The Outer 2-3” Of Shell!
Circumference3 Effect
76
MG/L = .000264 x Fiber Stress x Circumference 3
26” 34”
37,120 ft-lb 83,010 ft-lb
Circumference Decrease - 24%
Bending Capacity Reduction - 55%
SYP – Southern Yellow Pine
77
Thick Sapwood Species
Southern Yellow Pine
Advanced Shell Rot
78
External Decay Pockets
79
Df - Douglas fir
WC - Western red cedar
www.osmoseutilities.com 80
Thin Sapwood Species
Douglas Fir
Western Red Cedar
Early Stage of Decay/
Enclosed Decay Pocket
81
Advanced Internal Decay
82
Circumference Calculator
• Limited Variables
• Measured Decay Circumference Reduction
• Results in Effective Circumference
• Effective Circumference Allowable Table
Reject Decision
Effective Circumference
Slide Rule Reject Criteria
• External Decay & Internal Pockets o Effective circumference
• Hollow Heart o Minimum average shell
84
Bending Stress
Line of
Lead
Wind on Wires 83%
99%
85
88%
2005 – Electronic Strength Calculator
86
Electronic Strength Calculator
87
Inspector no longer averages measurements
Reject criteria consistent for all circumferences
Reject decision based on remaining strength
Reject criteria can be adjusted
Orientation of decay is considered
Common Reject Threshold
Bending Strength
Remaining Effective Circumference
13%
50% 33% 100%
Reject
87%
67%
Serviceable
4% 100% 33%
69%
40 Class 4 34in Circumference = Reject @ 29.5in
¾” Reduction of shell
100 Years Old
Inaugural NESC Summit
90
Keynote Speakers & Bios
91
High Powered Speakers
92
Bob W. Bradish AEP
Vice President – Transmission Grid Development
Daniel K. Glover Southern Company
Vice President – Power Delivery - Distributioin
Robert Woods Southern California Edison
Managing Director of Asset Management and Operations Support
Stephen A. Cauffman NIST-National Institute of Standards & Tech
Manager, Community Resilience Program
Jorge A. Camacho,PE PSC – District of Columbia
Chief, Infrastructure and System Planning
NESC 100 Years Old
presented by Don Hooper
93
94
95
96
97
98
Schedule for 2017 NESC
Submit change proposals: Jan 2012 - July 2013
First Subcommittees Votes: Sept-Oct 2013
Preprint Distributed: September 2014
Public Comments Until: May 2015
Subcommittees Vote on Comments: Sept-Oct 2015
Draft Submitted for Letter Ballot: January 2016
Revisions Submitted to ANSI: May 2016
Published: August 2016
Effective: January 2017
99
19 Months
13 Months
9 Months
6 Months
3 Months
4 Months
3 Months
2017 NESC
100
Re-format Rule 241.C. At Crossings
101
241. Application of grades of construction to different situations
2012 version
Re-format Rule 241.C. At Crossings
102
241. Application of grades of construction to different situations
2012 version
2017 version
Re-format Rule 241.C. At Crossings
103
241. Application of grades of construction to different situations
2012 version
2017 version
Same Words
Easier to Comprehend
Revised Table 242-1
104
Intention:
Improve logical layout and format
Columns sequence from low to high voltage
Rows sequence from low to high voltage
105
2012
Table 242-1
106
2012
Table 242-1
107
2017 Table 242-1
108
2017 Table 242-1
Revise Table 242-1
Grades of Construction Applications
109
CP Intention:
Improve logical layout and format
Columns sequence from low to high voltage
Rows sequence from low to high voltage
FN 3 reversed; higher grade in the table
3 Grade B C construction shall may be used if the supply circuits
will not be promptly de-energized, both initially and following
subsequent breaker operations, in the event of a contact with
lower supply conductors or other grounded objects.
Revise Table 242-1
Grades of Construction Applications
110
CP Intention:
Improve logical layout and format
Columns sequence from low to high voltage
Rows sequence from low to high voltage
FN 3 reversed; higher grade in the table
Add FN 11 – Grade N for dielectric fiber-optic
supply cables
Revise Table 242-1
Grades of Construction Applications
111
CP Intention:
Improve logical layout and format
Columns sequence from low to high voltage
Rows sequence from low to high voltage
FN 3 reversed; higher grade in the table
Add FN 11 – Grade N for dielectric fiber-optic
supply cables
Meet Rule 230F1b – Insulated Comm Cables in
Supply Space
and
Supported by effectively grounded messenger
or
Bonded at intervals specified in Rule 092C to
Supply messengers supporting cable meeting
Rule 230C1
Revise Table 242-1
Grades of Construction Applications
112
CP Intention:
Improve logical layout and format
Columns sequence from low to high voltage
Rows sequence from low to high voltage
FN 3 reversed; higher grade in the table
Add FN 11 – Grade N for dielectric fiber-optic
supply cables
Clarify when ice is applied
Rule 250D
113
…………
Clarify when ice is applied
Rule 250D
114
…………
Add ice to:
Wires
Conductors
Cables
Messengers
Do not add ice to:
Structure
Other supported facilities
Aeolian Vibration – Rule 261H.1.b
115
Aeolian Vibration – Rule 261H.1.b
116
Aeolian Vibration – Rule 261H.1.b
117
Final Action: Accept
Insulators – New Rating System
118
Old Line Post ratings:
Rating equal to average
Lowest not less than 85% of average
New Line Post ratings:
Rating = Minimum of all insulators
Insulators – New Rating System
119
Old Transmission Suspension ratings:
1.2 standard deviations
New Transmission Suspension ratings:
3.0 standard deviations
Insulators
120
CP Intention:
Introduce Load factors (LRFD)
Adjust allowable stresses
Mostly equivalent insulator applications
Introduce Classes: Distribution & Trans
Different allowables for Rule 250B vs 250C, D
Insulators Table 277-1
121
Insulators Table 277-1
122
Insulators Table 277-1
123
Insulators Table 277-1
124
Nonceramic Table 277-1 cont’d
125
Table 277-1 cont’d
126
Table 277-1 cont’d
127
Table 277-1 cont’d
128
Table 277-1 cont’d
129
FN 3: This percentage shall be supplied by the manufacturer.
Final Action: Accept
Table 277-1 cont’d
130
FN 3: This percentage shall be supplied by the manufacturer.
Final Action: Accept
NESC Visioning Sessions
131
Future of the NESC
Safety
Reliability
Resiliency
NIST-National Institute of
Standards & Technology
132
NIST – Disaster Resilience Framework
133
NIST - NESC
134
While this is truly a safety code, it is applied for use as a
design code in lieu of other guidance.
…the question that exists is whether the baseline set forth in
the NESC addresses the performance desired for resiliency
when considering all hazards (flood, wind, seismic, ice …….)
NIST – NESC Rule 250C
135
Rule 250C
The ASCE 7-10 wind maps were revised to better represent
the wind hazard. …. However, these maps are currently not
used by the NESC based on a decision by their code
committee to retain the use of the ASCE 7-05 wind maps.
Rule 250C
Most distribution structures are lower than the 60 ft height
limitation, therefore, most utilities will not design their
distribution lines to the ASCE 7 criteria (something that may
want to be reconsidered depending upon performance of
these systems during hurricanes and tornadoes over the past
2 decades).
NESC Workshop
October 18-19, 2016
San Antonio, TX
2017 NESC Changes
The Future of the NESC
Thank you Name Nelson Bingel
Phone 770-632-6703
Email nbingel@osmose.com
Chad Newton
770-632-6777
cnewton@osmose.com
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