ASCE Practice 74-Rev 2006
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Transcript of ASCE Practice 74-Rev 2006
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 1
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 2
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 3
Transmission Line Structural Loading GuideTransmission Line Structural Loading Guide
First edition was published in 1984First edition was published in 1984Design 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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 4
Transmission Line Structural Loading GuideTransmission Line Structural Loading Guide
ForwardForward Section 1 Section 1 -- Introduction to Load CriteriaIntroduction to Load Criteria Section 2 Section 2 -- Weather Related LoadsWeather Related Loads Section 3 Section 3 -- Additional Load ConsiderationsAdditional Load Considerations Section 4 Section 4 -- Wire SystemWire System Section 5 Section 5 -- ExamplesExamples AppendicesAppendices
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 5
Transmission Line Structural Loading GuideTransmission Line Structural Loading Guide
AppendicesAppendices ReferenceReference Definitions, Notations and SI Conversion FactorsDefinitions, Notations and SI Conversion Factors Limitations of Reliability Based DesignLimitations of Reliability Based Design Numerical Coefficient QNumerical Coefficient Q Conversion of Wind Speed Averaging TimeConversion of Wind Speed Averaging Time Supplemental Information on Structure VibrationSupplemental Information on Structure Vibration Equations for Gust Response FactorsEquations for Gust Response Factors Supplemental Information on Force CoefficientsSupplemental Information on Force Coefficients Supplemental Information on Ice LoadingSupplemental Information on Ice Loading Supplemental Information on Special LoadsSupplemental Information on Special Loads Investigation of Transmission Line FailuresInvestigation of Transmission Line Failures
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 6
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 7
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 8
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 9
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 10
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 11
Weather-Related Events (1.2.1)
Extreme wind. Extreme ice with accompanying wind. High intensity winds
Microbursts Tornados
Coincident temperature
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 12
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 13
Probability Density Function of Load Effect
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 14
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)50Load Return Period RP
(years)
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 15
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 16
Probability Density Function of R
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 17
Probability Density Functions of Q & R
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 18
Relative Reliability Factor (RRF)
event load RP afor failure ofy Probabilitevent load RP afor failure ofy Probabilit
N
50RRF
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 19
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 dont 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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 20
Extreme Wind Load Factors (Table 1.2-1)
1.4540081.3020041.1510021.005010.85250.5
Wind Load Factor(w)
Load RP (years)
Relative Reliability
Factor (RRF)
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 21
Extreme Ice Factors (Table 1.2-2)
1.01.8540081.01.5020041.01.2510021.01.005011.00.80250.5
Concurrent Wind Load
Factor(w)
Ice Thickness
Factor(i)
Load RP (years)
Relative Reliability
Factor(RRF)
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 22
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 23
Additional Load Considerations (1.2.2)
Failure containment Construction and maintenance loads Legislated loads
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 24
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 25
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 26
LRFD Format
[ ]QDLRn + ofEffect
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 27
Strength Factor 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, , for COVR =LEL, e%, of
the Nominal Strength
Value
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 28
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 29
Summary of LRFD Method
V - DESIGN COMPONENT for NOMINAL STRENGTH, Rn SUCH THAT: Rn > QD
IV - OBTAIN STRENGTH FACTOR, , FROM TABLE 1.4-2
III - DETERMINE DESIGN LOAD EFFECT QD IN EACH COMPONENT:Weather QD = EFFECT OF [DL and Q50 ]
or QD = EFFECT OF [DL and QRP ]
Failure Containment QD = EFFECT OF [ DL & FC ]
Construct & Maint. QD = EFFECT OF [DL and CM (C&M)]Legislated Loads QD = EFFECT OF [ LL ]
II - OBTAIN FACTORS, , 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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 30
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 31
3 Second Gust Wind Force (Section 2.1.1)
Where:F - Wind Forcew - 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 = w * Q * kZ * kzt * (V50)2 * G * Cf * A
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 32
Numerical Coefficient (Section 2.1.2)
Converts kinetic energy of moving air into potential energy of pressure.
Q = 1/2 where = mass density of air.
Appendix D
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 33
Basic Wind Speed Map (Section 2.1.3)
3-SECOND GUST SPEED
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 34
Continental Winds:
485 weather stations, minimum 5 years of data Data 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 to
justify contours 33 ft. above ground, Exposure C
Database/Analysis
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 35
Database/Analysis
Hurricane Winds:
Based on simulations and hurricane model The Atlantic Coastline was divided into discrete
points spaced at 50 nautical miles. Hurricane contours over the Atlantic are provided
for 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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 36
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 37
Velocity Pressure Exposure Coefficients(Section 2.1.4)
70011.5D
9009.5C
12007.0B
zg (feet)Exposure categoryTABLE 2.1.4-1 Power Law Constants
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 38
Velocity Pressure Exposure Coefficients(Section 2.1.4)
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/)
(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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 39
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 40
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)/kV2
Wire GW = (1 +2.7 *E (BW)1/2)/kV2
E = 4.9 ()1/2*(33/zh)1/fm
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 41
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 42
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 43
Gust Response Factor, G
Davenport Equations, Gust Response Factors for Transmission Line Loading, Proceeding, 5thInternational 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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 44
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 45
Topography Effects (Section 2.1.7)
Funneling of WindsFunneling of Winds MountainsMountains Wind SpeedWind Speed--upup
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 46
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 todays 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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 47
ICE and WIND LOADING Section 2.3
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 48
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 49
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 50
Categories of Icing (2.3.2)
Freezing Rain (Glaze) In-Cloud (Rime or Glaze) Wet Snow Hoarfrost
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 51
Design Assumptions for Ice Loading (2.3.3)
Equivalent uniform radial thickness
Radial Ice
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 52
Design Assumptions for Ice Loading (2.3.3)
Equivalent uniform radial thickness
Radial Ice
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 53
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 54
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 55
Using Ice Map (2.3.4.2)
ASCE 74 Maps (New!) Based on work of Kathy Jones from U.S. Armys 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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 56
Using Ice Map (2.3.4.2)
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 57
Using Ice Map (2.3.4.2)
ASCE 74 New Maps
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 58
Using Ice Map (2.3.4.2)
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 59
Using Ice Map (2.3.4.2)
ASCE 74 New Maps
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 60
Using Ice Map (2.3.4.2)
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 61
Model for the accretion of ice in freezing rain (App. I)
= = +
1/22 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
o = density of water (1 g/cm3) i = density of ice (0.9 g/cm3) N = duration of the freezing rain storm (hr)
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 62
Superstations for extreme value analysis (App. I)
pattern of damaging ice storms
terrainproximity to waterlatitude
frequency of ice storms
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 63
= = =
1/( )( ) 1 1 0
1 exp 0
kk x uF x k
(x - u) k
=
=
+= = +
=
=
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
( ) = + 1 kTx u TkEquivalent ice thickness for return period T:
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 64
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 65
Ice Buildup on Structural Members (2.3.5)
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 66
Ice Buildup on Structural Members (2.3.5)
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Ice and Wind 67
Ice Buildup on Structural Members (2.3.5)
Vertical Loads Concurrent Wind Unbalanced Ice Loading
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 68
Whats 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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 69
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
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Ice and Wind 70National Weather Service
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 71
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 19161978 in the United States of America (Tecson et al. 1979)
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 72
Downbursts
Associated with severe thunderstorm cells
Relatively wide gust fronts
Elliptical damage pattern
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 73
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 74
SoWhat 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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 75
APPENDIX K:
Investigation of Transmission Line Failures
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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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 77
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 78
FAILURE INVESTIGATIONS
Our Goal is to improve future designs, if necessary, or validate existing design based on accurate failure analysis.
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 79
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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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 80
Causes of Failure
Natural load conditions that exceed the design criteria
Manmade causes Structure deficiencies Wire system deficiencies Construction causes
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
Ice and Wind 81
Post Failure Containment
Longitudinal Cascade
Transverse Cascade
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October 18, 2006Revised ASCE Manual No. 74 - Section 2 -
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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
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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
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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
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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 (
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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)
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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
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Identify Tension Section (4.1)
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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
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Cable condition After Creep
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Cable condition After Load
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Wire Tension Limits of Use (4.3)
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The Ruling Span Approximation (4.4)
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Wire Tension Loads (4.5)
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Need for alternate to Ruling Span (4.6) (also discuss uneven wind on spans of section)