ASCE Practice 74-Rev 2006

October 18, 2006Revised ASCE Manual No. 74 - Section 2 -

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ASCE Manuals and Reports on

Engineering Practice #74

Guidelines for Electrical Transmission Lines Structural Loads

Frank W. Agnew

Terry Burley

Michael D. Miller

John D. Mozer

Mark Ostendorp

Alain Peyrot

C. Jerry Wong


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ASCE Manuals and Reports on

Engineering Practice #74

Frank W. Agnew Richard F. Aichinger Carl W. Austin

Jim Andersen Terry Burley Ron J. Carrington

Mike S. Cheung Habib J. Dagher Nicholas J. DeSantis

Harry V. Durden William Y. Ford Bruce Freimark

Jim Hogan Magdi F. Ishac Kathleen Jones

James M. McGuire Kishor C. Mehta Michael D. Miller

John D. Mozer Robert E. Nickerson Wesley J. Oliphant

Mark Ostendorp Alain Peyrot David Tennent

George T. Watson C. Jerry Wong


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Transmission Line Structural Loading GuideTransmission Line Structural Loading Guide

First edition was published in 1984First edition was published in 1984““Design GuidelinesDesign Guidelines””

Second edition was published in 1991 Second edition was published in 1991 ““Manual and Reports on Engineering Manual and Reports on Engineering PracticePractice””


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ForwardForwardSection 1 Section 1 -- Introduction to Load CriteriaIntroduction to Load CriteriaSection 2 Section 2 -- Weather Related LoadsWeather Related LoadsSection 3 Section 3 -- Additional Load ConsiderationsAdditional Load ConsiderationsSection 4 Section 4 -- Wire SystemWire SystemSection 5 Section 5 -- ExamplesExamplesAppendicesAppendices


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AppendicesAppendicesReferenceReferenceDefinitions, Notations and SI Conversion FactorsDefinitions, Notations and SI Conversion FactorsLimitations of Reliability Based DesignLimitations of Reliability Based DesignNumerical Coefficient QNumerical Coefficient QConversion of Wind Speed Averaging TimeConversion of Wind Speed Averaging TimeSupplemental Information on Structure VibrationSupplemental Information on Structure VibrationEquations for Gust Response FactorsEquations for Gust Response FactorsSupplemental Information on Force CoefficientsSupplemental Information on Force CoefficientsSupplemental Information on Ice LoadingSupplemental Information on Ice LoadingSupplemental Information on Special LoadsSupplemental Information on Special LoadsInvestigation of Transmission Line FailuresInvestigation of Transmission Line Failures


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OVERVIEW OF LOAD CRITERIA – Section 1

• Introduction (1.0)• Principal Systems of a Transmisison Line (1.1)• Loads and Relative Reliability (1.2)

– Weather Related Events– Additional Load Considerations– Loads and Load Effects

• Wire Systems (1.3)• Limit States (1.4)

– Component Strength– Relative Reliability of Components and Failure Containment– Considerations for Special Structures– Load and Resistance Factor Design


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Introduction (1.0)

• This manual addresses transmission line structure design issues that must be considered to provide:– Cost effective structures– Reliable structures


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Key Issues Addressed by the Manual

• Uniform procedures and definitions across the industry for calculation of loads.

• Structure designs with acceptable minimum reliability.

• Design loads and load factors that are independent of structure materials.

• Adjustments of load criteria to reduce occurrence of cascading failures.

• Incentives for developing better local data for weather related phenomena.

• Inclusion of legislated load.


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Principal Systems of a T-Line (1.2)

• The Structural Support System.– Towers, poles and foundations.– Primary task of supporting the wire system.

• The Wire System.– Conductors, ground wires, insulators and

attachment hardware.– Much of the unusual behavior and most of

the problems in a line start on, or are generated by, the wire system.


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Loads and Relative Reliability (1.2)

• Convenient to distinguish between events that produce loads and the resulting loads in the line components.

• Load events can be classified as:– Weather-Related Loads.– Construction and Maintenance Loads.– Secondary Loads.

• Loads causing damage to a line component, due to:– Vehicle or aircraft accidents– Lightning– Ice and/or wind overload– Vandalism

• May result in a cascading failure.• Falls within the designation of Failure Containment (FC).


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Weather-Related Events (1.2.1)

• Extreme wind.• Extreme ice with accompanying wind.• High intensity winds

– Microbursts– Tornados

• Coincident temperature


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Return Period (RPN)

• For example, an event with a 50-year return period (RP50) represents an extreme event that is reached or exceeded with a probability of 1/50 or 2% every year.

• Because extreme events are not evenly spaced over time, there will be some 50-year periods with no RP50 events and other 50-year periods with 2 or more events equaling or exceeding RP50 values.


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Probability Density Function of Load Effect


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Probability of RPN Events in 50 Years

0.12500

0.22200

0.39100

0.6450

0.8725

Exceedance Probability of RP Event in 50 Years

= 1-(1-1/RP)50

Load Return Period RP(years)


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Return Period Adjustments (1.2.1.1)

• Can adjust the relative reliability of a design by changing the RP of the design load.

• The higher the RP of the design load, the more reliable (lower probability of failure) the design.

• Using a consistent nominal design strength, the relative probability of failure of two components is inversely proportional to the design load RP.

• Thus, doubling the design load RP reduces the relative probability of failure by a factor of approximately 2.


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Probability Density Function of R


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Probability Density Functions of Q & R


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Relative Reliability Factor (RRF)

event load RP afor failure ofy Probabilitevent load RP afor failure ofy Probabilit

N

50≅RRF


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Why Use Relative Reliability?

• Useful tool to approximately adjust design reliability.

• Currently very difficult to accurately calculate probability of failure.

• Powerful mathematical tools are available, but we don’t have all of the data necessary to carry out the analysis.

• For example, consider the uncertainty in predicting the Force Coefficients.


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Extreme Wind Load Factors (Table 1.2-1)

1.4540081.3020041.1510021.005010.85250.5

Wind Load Factor(γw)

Load RP (years)

Relative Reliability

Factor (RRF)


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Extreme Ice Factors (Table 1.2-2)

1.01.8540081.01.5020041.01.2510021.01.005011.00.80250.5

Concurrent Wind Load

Factor(γw)

Ice Thickness

Factor(γi)

Load RP (years)

Relative Reliability

Factor(RRF)


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Spatial Influences on Weather-Related Events (1.2.1.2)

• Data for the wind and ice maps were collected at points.

• Appropriate for the design of point structures.• A transmission line is a linear system that is

exposed to a larger number of extreme load events than a single point structure.

• Difficult to select load criteria based on length of the line.

• Result would be structure designs suitable for a line of given length, but not suitable for another line of different length.


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Additional Load Considerations (1.2.2)

• Failure containment• Construction and maintenance loads• Legislated loads


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Limit States Design (1.4)

• Failure limit state– Condition where component can no longer

sustain the load.– May lead to failure of the line.

• Damage limit state– Condition where the component and line

will still function, but permanent damage has been done.

– Serviceability and performance of line may be compromised.


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Load and Resistance Factor Design (1.4.4)

• Manual provides suggested load factors and load combinations for transmission line design.

• Load factors can be based on the selected Relative Reliability Factor, load combination, safety requirements and legislated standards.

• Strength factors account for the variability of component strength and are applied to nominal strength equations for the components based on strength guides and standards.


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LRFD Format

[ ]QDLRn γφ +≥ ofEffect


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Strength Factor φ to convert to a 5% LEL with 10% COVR (Table 1.4-2)

0.790.850.86mean0.950.920.90201.040.960.92101.121.000.9351.211.040.9521.271.070.9711.481.161.000.1

0.200.100.05Strength Factor, φ, for COVR =LEL, e%, of

the Nominal Strength

Value


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Selection of Strength Factor (1.4.4.4)

• Manual provides typical values of the LEL and COVR for different components used in a line.– Steel components and steel and

prestressed concrete poles.– Reinforced concrete.– Wood poles.– Foundations.– Conductors and ground wires.


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Summary of LRFD Method

V - DESIGN COMPONENT for NOMINAL STRENGTH, Rn SUCH THAT:φ Rn > QD

IV - OBTAIN STRENGTH FACTOR, φ, FROM TABLE 1.4-2

III - DETERMINE DESIGN LOAD EFFECT QD IN EACH COMPONENT:Weather QD = EFFECT OF [DL and γ Q50 ]

or QD = EFFECT OF [DL and QRP ]

Failure Containment QD = EFFECT OF [ DL & FC ]

Construct & Maint. QD = EFFECT OF [DL and γCM (C&M)]Legislated Loads QD = EFFECT OF [ LL ]

II - OBTAIN FACTORS, γ , from Tables 1.2-1 and 1.2-2

I - SELECT RELATIVE RELIABILITY FACTOR (RRF)OR MINIMUM DESIGN LOAD RETURN PERIODDEPENDING OF TYPE OF LINE (TABLE 1.2-1)


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Extreme Wind Loads – Section 2.1

• Based on 2% annual probability, 3-second gust wind speed– Wind force equation (Section 2.1.1)

– Numerical coefficient (Section 2.1.2)

– Basic wind speed (Section 2.1.3)

– Velocity pressure exposure coefficient (Section 2.1.4)

– Gust response factor (Section 2.1.5)

– Force coefficient (Section 2.1.6)

– Topography effects (Section 2.1.7)

– Wind load applications on latticed towers (Section 2.1.8)


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3 Second Gust Wind Force (Section 2.1.1)

Where:F - Wind Forceγw - Load Factor.Q - Numerical Coefficient.kzt - Topographic Factor.kZ - Velocity Pressure Exposure Coefficient.V50 - Basic Wind Speed, 3-second gust wind speed, miles per

hour, at 33 ft. above ground, an annual probability of 2%. G - Gust Response Factor.Cf - Force (Drag) Coefficient.A - Projected Surface Area.

F = γw * Q * kZ * kzt * (V50)2 * G * Cf * A


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Numerical Coefficient (Section 2.1.2)

• Converts kinetic energy of moving air into potential energy of pressure.

• Q = 1/2 ρwhere ρ = mass density of air.

Appendix D


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Basic Wind Speed Map (Section 2.1.3)

3-SECOND GUST SPEED


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• Continental Winds:

• 485 weather stations, minimum 5 years of dataData assembled from a number of stations in state-size areas to reduce sampling errors

Fisher-Tippett Type I extreme value distribution, annual probability of 2%

Insufficient variation in peak gust wind speeds tojustify contours

33 ft. above ground, Exposure C

Database/Analysis


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Database/Analysis

• Hurricane Winds:

• Based on simulations and hurricane modelThe Atlantic Coastline was divided into discretepoints spaced at 50 nautical miles.

Hurricane contours over the Atlantic are providedfor interpolations and represent values forExposure C over land.

Importance factors are accounted for in the mapwind speeds• >1.0 at the coast • 1.0 at 100 miles inland.


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Velocity Pressure Exposure Coefficients(Section 2.1.4)

Exposure B Urban and suburbanTerrain with numerous closely spaced obstructions having the size of single-family dwellings or larger

Exposure C Open terrainOpen terrain with scattered obstructions having heights generally less than 30 ft

Exposure D Coastal Flat unobstructed areas directly exposed to wind flowing over open water


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70011.5D

9009.5C

12007.0B

zg (feet)αExposure category

TABLE 2.1.4-1 Power Law Constants


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Velocity Pressure Exposure Coefficient, kZ, modifies the basic wind speed to account for terrain and height effects.

Structure or Wire

kZ = 2.01*( zh / zg ) (2/α)

(for 15 ft. ≤ h ≤ 900 ft.)

Effective Height, zh, the height above ground to the center of wind pressure (Section 2.1.4.3).


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Gust Response Factor (Section 2.1.5)

•• Gust Gust ResponseResponse FactorFactor•• Structural ResponsesStructural Responses•• Wind CharacteristicsWind Characteristics

•• Horizontal Wind ProfileHorizontal Wind Profile•• Statistical basedStatistical based•• Not a significant factor in typical Not a significant factor in typical

buildings buildings –– seldom been studiedseldom been studied


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Structure / Wire Gust Response Factors(Section 2.1.5.1)

Gust Response Factor, G, accounts for the dynamic effects of wind and lack of gust correlation on the transmission line components.

Appendix GStructure GT = (1 + 2.7*E (BT)1/2)/kV

2

Wire GW = (1 +2.7 *E (BW)1/2)/kV

2

E = 4.9 (κ)1/2*(33/zh)1/αfm

BT = 1/(1+0.56*zh/Ls)BW = 1/(1+0.8*L/ Ls)

E = Exposure Factor

B = Dimensionless response term corresponding to the quasi-static background wind load

kV = 1.430


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Gust Response Factor (Section 2.1.5)

•• Conversion Factor, Conversion Factor, kV. (Durst Curve). (Durst Curve)•• Relationship between 3Relationship between 3--second gust wind and 10second gust wind and 10--minute minute

average windaverage windAppendix E


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Gust Response Vs Gust Factors

• Gust Response Factor– Accounts for dynamic effects of gusts on the response of

transmission line components– Gusts may not envelop the entire span between transmission line

structures– Values can be greater than or less than 1.0– Represents the ratio of peak gust load effect to the selected mean

extreme load effect

• Gust Factor– The ratio of the gust wind speed at a specified average period, e.g.

2 seconds, to the selected mean speed, e.g. 10 minute– Used as a multiplier of the mean extreme wind speed to obtain the

gust wind speed.– Values greater than 1.0


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Gust Response Factor, G

• Davenport Equations, “Gust Response Factors for Transmission Line Loading,” Proceeding, 5th

International Conference on Wind Engineering, 1979• ASCE 74, “Guidelines for Electrical Transmission Line

Structural Loading,” 1991• ASCE 7, “Minimum Design Loads for Buildings and

Other Structures,” 2002• IEC 60826, “Loading and Strength of Transmission

Lines,” 2002


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Force Coefficient (Section 2.1.6)

•• Shape and SizeShape and Size•• Aspect RatioAspect Ratio•• Yawed WindYawed Wind•• SoliditySolidity•• ShieldingShielding

•• Not a precise scienceNot a precise science

Appendix H


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Topography Effects (Section 2.1.7)

•• Funneling of WindsFunneling of Winds•• MountainsMountains•• Wind SpeedWind Speed--upup


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Wind is a Random EventWind is a Random Event

•• Equations are not exactEquations are not exact•• Equations are not intended to cover all Equations are not intended to cover all

potential conditionspotential conditions•• Load factor is generally applied to cover Load factor is generally applied to cover

uncertaintyuncertainty•• With todayWith today’’s technology, these equations s technology, these equations

are more scientific than most people thinkare more scientific than most people think

Extreme Wind Loads – Section 2.1


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ICE and WIND LOADING – Section 2.3


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ICE and WIND LOADING – Section 2.3

• Introduction (2.3.1)• Categories of Icing (2.3.2)• Design Assumptions for Ice Loading (2.3.3• Ice Load on Wires due to Freezing Rain (2.3.4)

– Using Historical Ice Data– Using Ice Map– Combined Wind and Ice Loads

• Ice Buildup on Structural Members (2.3.5)– Vertical Loads– Concurrent Wind Loads– Unbalanced Ice Loading


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Introduction (2.3.1)

• Ice accretion is often a governing loading criterion– Larger Vertical Loads– Larger Exposed Wind Area on Wires– Larger Tensions– Loading Imbalances


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Categories of Icing (2.3.2)

• Freezing Rain (Glaze)• In-Cloud (Rime or Glaze)• Wet Snow• Hoarfrost


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Design Assumptions for Ice Loading (2.3.3)

• Equivalent uniform radial thickness

Radial Ice


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Design Assumptions for Ice Loading (2.3.3)

• Equivalent uniform radial thickness

Radial Ice


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Ice Load on Wires due to Freezing Rain (2.3.4)

• Using Historical Ice Data– (Modeling your own Service Area (App. I.3)) new!

• Using Ice Map new!• Combined Wind and Ice Loads new!


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Using Ice Map (2.3.4.2)

• ASCE 74 – 91 Version

– 50-year return interval ice based on 9 years of data collected by Bennett. Data collected from 1928-1936, and did not differentiate between glaze, rime and accreted snow. Also, did not report the equivalent radial ice thickness.

– Added a wind-on-ice requirement as a percentage of the 50 year basic wind speed intended to represent the extreme wind which could be expected over a 7 day period


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• ASCE 74 Maps (New!)– Based on work of Kathy Jones from U.S. Army’s Cold

Regions Research and Engineering Laboratory (CRREL), funded by EPRI, CRREL, FEMA, CEA and a number of individual utilities

– Same map as presented in ASCE 7-2005 – Maps present 50-year values for icing from freezing rain only

with concurrent gust speed


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• ASCE 74 New Maps

Figure 2.3-1. Extreme Radial Glaze Ice thickness (in.), Western United States 50-year return period with concurrent 3-sec wind speeds


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Figure 2.3-2. Extreme Radial Glaze Ice thickness (in.), Eastern United States, 50-year return period with concurrent 3-sec. wind speed.


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• ASCE 74 New MapsFigure 2.3-3. Extreme Radial Glaze Ice thickness (in.), Lake Superior Detail, 50-year return period with concurrent 3-sec. wind speeds.

Figure 2.3-4. Extreme Radial Glaze Ice thickness (in.), Fraser Valley Detail, 50-year return period with concurrent 3-sec. wind speed.


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Figure 2.3-5. Extreme Radial Glaze Ice thickness (in.), Columbia River Gorge Detail, 50-year return period with concurrent 3-sec. wind speed.

Figure 2.3-6. Extreme Radial Glaze Ice thickness (in.), Alaska, 50-year return period with concurrent 3-sec. wind speed.


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• Modeling ice accretion from weather data (Appendix I)– Very little data on ice accretions on overhead lines are

available; mathematical modeling from weather data is required

Figure I4-1. Locations of weather stations used in preparation of Figures 2.3-1 through 2.3-5.


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Model for the accretion of ice in freezing rain (App. I)

=

⎡ ⎤⎢ ⎥⎣ ⎦

= ρ +ρπ∑1/2

2 2

1,1 ( ) (3.6 )

Noj j j

jit P VW

where

t = equivalent radial ice thickness (mm)

Pj = precipitation amount (mm) in jth hour

Vj = wind speed (m/s) in jth hour

Wj = liquid water content (g/m3) of the rain-

filled air in jth hour = 0.067Pj0.846

ρo = density of water (1 g/cm3)

ρi = density of ice (0.9 g/cm3)

N = duration of the freezing rain storm (hr)


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Superstations for extreme value analysis (App. I)

pattern of damaging ice storms

•terrain•proximity to water•latitude

frequency of ice storms


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−⎡ ⎤= − − ≠⎢ ⎥α⎣ ⎦−⎡ ⎤= − =⎢ ⎥α⎣ ⎦

1/( )( ) 1 1 0

1 exp 0

kk x uF x k

(x - u) k

=

=

− +=

−

α = − +

=

−=

−

∑

∑

1 0

0 1

0

0 ( )1

1 ( )1

4 3shape parameter 2

scale parameter 1

1

1 1 1

n

iin

ii

b b uk

b b(b u)( k)

b xn

ib xn n

Extreme value analysis (App. I)Peaks-over-threshold method with generalized Pareto distribution

Determine parameters using Probability Weighted Moments

( )−α ⎡ ⎤= + − λ⎣ ⎦1 k

Tx u Tk

Equivalent ice thickness for return period T:


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Ice Load on Wires due to Freezing Rain (2.3.4)

• Combined Wind and Ice Loads– Ice Load

– Wind on Ice Covered Wires• Projected Area, force coefficients• 3 sec. gust wind from maps

WI = 1.24(d + Iz)Iz (2.3-3)

Where: WI = weight of glaze ice (pound per foot) d = bare diameter of wire (inches)IZ = design ice thickness (inches)


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Ice Buildup on Structural Members (2.3.5)


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• Vertical Loads• Concurrent Wind• Unbalanced Ice Loading


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What’s the big deal?Why are High Intensity Winds different?

What are the characteristics of High Intensity Winds?

•Narrow front winds

•Wind speeds are greater than “extreme wind” loads

•Affected by local topography


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Tornados

4751-6,000100-315261-3185

1671-475032-99207-2604

531-167010-31158-2063

171-5303.2-9.9113-1572

51-1701.0-3.173-1121

≤50<1.0≤720

Path WidthP

(feet)

Path LengthP(miles)

Tornado Wind SpeedF(mph)

Scale

TABLE 2.2.1-1. Ranges of Tornado Wind Speed, Path Length, and Path Width for FPP Scale


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0

5

10

15

20

25

30

35

F0 F1 F2 F3 F4 F5

Percentage

TABLE 2.2.1-2 Tornado Frequencies and F-Scale Classifications for 1916—1978 in the United States of America (Tecson et al. 1979)


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Downbursts

•Associated with severe thunderstorm cells

•Relatively wide gust fronts

•Elliptical damage pattern


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Micro bursts

•Intensity levels up to F2 Tornado strength

•Gust width ± 330’ – 660’

•Elliptical and strip damage patterns

Micro Burst: A strong localized downdraft from a thunderstorm with peak gusts lasting 2 to 5 minutes. National Weather Service, Missoula, Mt.


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So…What should I do now?

• Tornado F2 wind speeds (157 mph) result in little additional tower structure weights. Tower designs may require additional shear capacity due to lowering of resultant wind loads.

• Tornado F2 wind speeds (157 mph) may have no effect on pole type transmission class structures.


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APPENDIX K:

Investigation of Transmission Line Failures

October 18, 2006Revised ASCE Manual No. 74 -

Section 2 - Ice and Wind 76

Why investigate failures?

•Increase understanding of line behavior

•Affirmation of existing design and maintenance criteria

•Improvement of design criteria and maintenance practices


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Why address failure investigations in a “Loading Manual”?

• Most likely, a utility focuses on restoring power rather than investigating a structural failure.

• “High Load” explanation may not be acceptable.

• A loading case, previously not considered, may be the limiting design condition.

• Information presented is seldom addressed in other publications.


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FAILURE INVESTIGATIONS

Our Goal is to improve future designs, if necessary, or validate existing design based on accurate failure analysis.


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FAILURE INVESTIGATIONS

• Our Plan is to establish and separate the failure mechanisms for the various failed structure pieces.

• Determine the initial failure regardless of cause (ice, narrow or broad front wind, missing structure members or connections, etc.).

• Determine secondary failures caused by load shift from the initial failure.


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Causes of Failure

• Natural load conditions that exceed the design criteria

• Manmade causes• Structure deficiencies • Wire system deficiencies• Construction causes


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Post Failure Containment

• Longitudinal Cascade

• Transverse Cascade


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Failure Investigation Preparation

• Equipment (a.k.a. bug-out bag)

• A Plan for priorities

• Technical preparation


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Failure Investigation Procedure

• Photography survey

• Gather evidence from witnesses and those arriving earlier.

• Develop image of sequence of events

• Safety first


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THE INVESTIGATION

• The Field Checklist

• The Office Checklist

• Report Preparation


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Additional Load Considerations – Section 3

• Introduction• Construction & Maintenance Loads (3.1)

– General (3.1.1)– Construction Loads (3.1.2)

• Structure Erection (3.1.2.1)• Ground Wire & Conductor Installation (3.1.2.2)• Recommended Minimum Loads for Wire Installation (3.1.2.3)

– Maintenance Loads (3.1.3)• Fall Protection (3.2)• Longitudinal Loads (3.3)

– Longitudinal Loads on Intact Systems (3.3.1)– Longitudinal Loads & Failure Containment (3.3.2)

• Design all Structures for Longitudinal Loads (3.3.2.1)• Install Stop Structures at Specified Intervals (3.3.2.2)• Install Release Mechanism (3.3.2.3)

• Structure Vibration (3.4)• Conductor Galloping (3.5)• Earthquake Load (3.6)


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Introduction (3.0)Section 3 does not address:• Landslides• Ice Flows• Frost Heave• Flooding• Other Special Loading Scenarios


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Construction & Maintenance Loads (3.1)

General• Construction Loads are directly related to construction methods• Personnel Safety is the paramount factorConstruction Loads• Loads acting on the structure due to the assembly and erection and

the installation of ground wires, insulators, conductors & hardware• Lifting of Structures

– Tilting of ground assembled structure to vertical alignment– Pick up of structural section by helicopter– Worker Loading (Point Loading on Lattice Members, Etc)

• Ground Wire & Conductor Installation– Recognizes IEEE Std. 524-03 as leading standard– Addresses common stringing load scenarios– Provides recommended minimum installation loads and load factors for

ground wires and conductors (3 psf, no ice on wires and structures)– Load Factor for transverse wind loading (1.5)– Load Factor for vertical loads from dead end condition (1.5)– Load Factor for vertical loads from intact condition (2.0)


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Construction & Maintenance Loads (3.1)

Maintenance Loads• Weight of Workers on structure, structural elements and wires• Load effects resulting from temporary modifications

– Member replacements– Guying

• Load effects resulting from adjustment or replacement of ground wires, conductors, insulators and hardware

• Each maintenance operation is recommended to be analyzed in sequence by engineer

• Load factors not provided


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Fall Protection Loads (3.2)• Dynamic load effects that are created as the result of the fall of a

worker from an elevated position• Dynamic load effects act on the worker anchorage point• Anchorage points are points that provide a secure attachment for a

fall protection system• Fall protection systems assumed to meet all applicable OSHA and

Government requirements• Recognizes IEEE Std. 1307-04 as Governing Standard

– IEEE Std. provides guidance regarding loads and criteria for anchorages and step bolts

• Anchorage locations and climbing devices recommended to be coordinated with operation and maintenance personnel– Number of anchorages– Location of anchorages– Maximum number of attachments at each anchorage– Maximum expected arresting force– Type of climbing devices– Number of climbing devices


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Longitudinal Loads (3.3)• Structures may be required to resist longitudinal loads

– Loads resulting from inequalities of wind and/or ice on adjacentspans

– Loads resulting from ground wire, conductor, insulator, or structural and component failure

– Inability to resist longitudinal loads may result in a cascading failure of a transmission line

• Types of Longitudinal Loading– Longitudinal Loads on Intact Systems

• Differential loadings on adjacent spans resulting from different wind and ice loading and temperature extremes

• Unequal wire tensions• Wind driven debris and materials

– Longitudinal Loads and Failure Containment• Severe load imbalances caused by breakage of ground wires,

conductors, insulators, hardware and structural components• Addresses designing all structures for longitudinal loads• Addresses installation of stop structures at specified intervals• Addresses installation of release mechanisms


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Longitudinal Loads (3.3)• Structures may be required to resist longitudinal loads

– Loads resulting from inequalities of wind and/or ice on adjacentspans

– Loads resulting from ground wire, conductor, insulator, or structural and component failure

– Inability to resist longitudinal loads may result in a cascading failure of a transmission line

• Types of Longitudinal Loading– Longitudinal Loads on Intact Systems

• Differential loadings on adjacent spans resulting from different wind and ice loading and temperature extremes

• Unequal wire tensions• Wind driven debris and materials

– Longitudinal Loads and Failure Containment• Severe load imbalances caused by breakage of ground wires,

conductors, insulators, hardware and structural components• Addresses designing all structures for longitudinal loads• Addresses installation of stop structures at specified intervals• Addresses installation of release mechanisms


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Design all Structures (3.3.2.1)


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Design all Structures (3.3.2.1)


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Structure Vibration (3.4)• Dynamic forces such as wind, conductor motion and earthquakes

may in isolated cases cause structure vibrations• Majority of problems associated with wind induced vibration of

individual structural elements (tubular and structural shapes)• In isolated cases wind induced vibration can cause:

– Fatigue failures of the member or connection bolts– Loosening of bolted connection– Vibration of members can be eliminated using recommended design

and detailing practices– Tubular arms likely to be susceptible to vibration prior to the stringing

of the ground wire and/or conductor– Use temporary weights on tubular arms to eliminate vibration at or near

the resonant frequency


Ice and Wind 96

Conductor Galloping (3.5)• Galloping (the large amplitude motion) of ground wires and

conductors may occur with moderate winds blowing across ice coated wires

• Galloping of wires is a dynamic event that is random in nature and is capable of producing significant wire tension increases

• Galloping causes mainly vertical large amplitude motions with amplitudes that may reach values approaching the sag of the wires

• Galloping may cause electrical, structural and mechanical problems including:– Flashovers among wires leading to temporary outages– Clashing of wires leading to damaged conductors– Permanent increases in ground wire and conductor sag– Excessive wear, fatiguing and failure of ground wires, conductors,

insulators and hardware (particularly at dead end assemblies)– Collapse of structural systems and components

• Mitigation alternatives include the use of:– Detuning pendulums and inter-phase spacers– Airflow spoilers– Modification of conductor designs


Ice and Wind 97

Earthquake Load (3.6)• Transmission structures need not be designed for ground induced

vibrations caused by earthquake motion because:– Historically, transmission structures have performed well in earthquake

events (only isolated instances of failures have been recorded)– Structural loads caused by wind and/or ice loading combinations and

longitudinal loads exceed earthquake loads• Experience has shown that infrequent failures of transmission

structures are generally related to soil liquefaction and/or earth fractures


Ice and Wind 98

Structure Vibration – Appendix F• Introduction (F.1)

– Caused by Environmental and Geographic Exposure– Potential for Occurrence Higher than for Typical Civil Engineering Structures

• Structure Vibrations (F.2)– Causes of Structural Vibrations

• Aeolian Vibration• Sub-Conductor Oscillation• Galloping• Induced Ground Motion (Earthquakes)

– Natural Frequencies (Conductor & Wires)• 3 to 150Hz (Aeolian Vibration)• 0.15 to 10Hz (Sub-Conductor Oscillation)• 0.08 to 3Hz (Galloping)

– Mitigation Alternatives (Conductor & Wires)• Dampers & Spacer Dampers• Air Foils & Spoilers• Sag & Tension Adjustments• Specialized Conductor Designs

– Mitigation Alternatives (Structure & Members)• KL/r Ratio (<200 for Double Angle Members)• Identifying Critical Vortex Induced Wind Speed• Identifying Natural Frequencies (Structure & Cross Arms)• Change Mass, Stiffness or Damping (Structure & Cross Arms)• ‘Ballasting’ Tubular Members (Cross Arms)


Ice and Wind 99

Special Loads – Appendix J• Introduction (J.1)

– Caused by Load Inequalities Resulting from the Disturbance or Disruption of the Wire System– Affects the Magnitude of the Unbalanced Loads at each Support Structure

• Weather Related Longitudinal Loads (J.2)– Suspension Supports (J.2.1)

• Unequal Wind and/or Ice Loads Cause Differential Tensions• Conductor Temperature Variation in Unequal Spans Cause Differential Tensions• Unbalanced Loads Generally do not Exceed 10 to 20 Percent of Bare Wire Tension• In Cloud Icing can Produce Unbalanced Loads in Excess of 20 Percent of Bare Wire Tension

– Strain Supports (J.2.2)• Must Resist Differential Tensions of Adjacent Spans• Ground Wire Differential Tensions may be Higher than Comparable Conductor Values• Mitigation Alternatives Include Ground Wire Suspension Links, Slip and Release Clamps, Removing the Ground Wire and Designing

Ground Wire Supports to Collapse at a Defined Load to Act as a Fuse

• Failure Related Longitudinal Loads (J.3)– Residual Static Load (J.3.1)

• Design each Structure for Bare, Broken Wire Residual Static Load (RSL)• RSL Values Approximately Approach 60 to 70% of Everyday Wire Tension• RSL Applied to 1/3 of Conductor Support Points or to 1 or All Ground Wire Support Points

– EPRI Method (J.3.2)• Provides Unbalanced Loads as a Function of Horizontal Wire Tension for each Design Load Case, Span/Sag Ratio, Span/Insulator Ratio,

and Support Flexibility• Provides Unbalanced Loads at each Structure Away from Failure• Provides Unbalanced Loads in Relation to Risk of Failure

– Failure Containment (BPA Method) (J.3.3)• Assumes Breakage of a Single Wire or Phase at any one Time• Suspension Conductor (67% of EDT for Light, 133% of EDT for Standard & Heavy Suspension Structures, Everyday Loading, No Ice or

Wind)• Strain Deadend Conductor (Transverse Wind Load (40mph), No Ice, LTV Overload Factor of 1.5, 125% of EDT)

– Percent of Everyday Wire Tension (J.3.4)• Broken Wire Load (70% of EDT

• Failure Containment Requirements (J.4)– General Rules (J.4.1)– Basic Assumption (J.4.2)– Special Resistance Structures (J.4.3)– Failure Containment for Icing Events (J.4.4)


Ice and Wind 100

THE WIRE SYSTEM – Section 4

• Identify Tension Sections (4.1)• Wire conditions (4.2)

– Initial, After Creep and After Heavy Load• Wire limits of use (4.3)

– Tension limits• The Ruling Span approximation (4.4)• Wire tension loads (4.5)

– At horizontal line angles– At vertical line angles


Ice and Wind 101

Identify Tension Section (4.1)


Ice and Wind 102

Wire Conditions (4.2)

• Initial (at sagging time)• Final After Creep (after several years

under ordinary mechanical tension)Wire will see something close to this

condition most of its life unless stretched by an unlikely heavy load

• Final After Heavy Load (after severe loading causing very high tension)

Wire may never see this condition


Ice and Wind 103

Cable condition “After Creep”


Ice and Wind 104

Cable condition “After Load”


Ice and Wind 105

Wire Tension Limits of Use (4.3)


Ice and Wind 106

The Ruling Span Approximation (4.4)


Ice and Wind 107

Wire Tension Loads (4.5)


Ice and Wind 108

Need for alternate to Ruling Span (4.6) (also discuss uneven wind on spans of section)

Documents

ASCE Practice 74-Rev 2006