ASCE Practice 74-Rev 2006

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 October 18, 2006

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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 Guide

Î

Î

First edition was published in 1984 “Design Guidelines”

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

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Transmission Line Structural Loading Guide

Î Î Î Î Î Î Î

Forward Section 1 - Introduction to Load Criteria Section 2 - Weather Related Loads Section 3 - Additional Load Considerations Section 4 - Wire System Section 5 - Examples Appendices

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Transmission Line Structural Loading Guide Appendices Î Reference Î Definitions, Notations and SI Conversion Factors Î Limitations of Reliability Based Design Î Numerical Coefficient Q Î Conversion of Wind Speed Averaging Time Î Supplemental Information on Structure Vibration Î Equations for Gust Response Factors Î Supplemental Information on Force Coefficients Î Supplemental Information on Ice Loading Î Supplemental Information on Special Loads Î Investigation of Transmission Line Failures October 18, 2006

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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. October 18, 2006

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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. October 18, 2006

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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). October 18, 2006

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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 50year periods with 2 or more events equaling or exceeding RP50 values. October 18, 2006

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Probability Density Function of Load Effect

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Probability of RPN Events in 50 Years Load Return Period RP (years)

Exceedance Probability of RP Event in 50 Years = 1-(1-1/RP)50

25

0.87

50

0.64

100

0.39

200

0.22

500

0.12

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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)

Probability of failure for a RP50 load event RRF ≅ Probability of failure for a RPN load event

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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. October 18, 2006

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Extreme Wind Load Factors (Table 1.2-1) Relative Reliability Factor (RRF) 0.5 1 2 4 8 October 18, 2006

Load RP (years)

Wind Load Factor (γw)

25 50 100 200 400

0.85 1.00 1.15 1.30 1.45

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Extreme Ice Factors (Table 1.2-2) Relative Reliability Factor (RRF) 0.5 1 2 4 8 October 18, 2006

Load RP (years)

25 50 100 200 400

Ice Concurrent Thickness Wind Load Factor Factor (γi) (γw) 0.80 1.0 1.00 1.0 1.25 1.0 1.50 1.0 1.85 1.0

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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. October 18, 2006

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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. October 18, 2006

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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. October 18, 2006

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LRFD Format

φRn ≥ Effect of [DL + γQ ]

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

LEL, e%, of the Nominal Strength Value

0.05

0.10

0.20

0.1

1.00

1.16

1.48

1

0.97

1.07

1.27

2

0.95

1.04

1.21

5

0.93

1.00

1.12

10

0.92

0.96

1.04

20

0.90

0.92

0.95

mean

0.86

0.85

0.79

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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. October 18, 2006

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Summary of LRFD Method I - SELECT RELATIVE RELIABILITY FACTOR (RRF) OR MINIMUM DESIGN LOAD RETURN PERIOD DEPENDING OF TYPE OF LINE (TABLE 1.2-1) II - OBTAIN FACTORS, γ , from Tables 1.2-1 and 1.2-2 III - DETERMINE DESIGN LOAD EFFECT QD IN EACH COMPONENT:

Weather or

QD = EFFECT OF [DL and γ Q50 ] QD = EFFECT OF [DL and QRP ]

Failure Containment

QD = EFFECT OF [ DL & FC ]

Construct & Maint. Legislated Loads

QD = EFFECT OF [DL and γCM (C&M)] QD = EFFECT OF [ LL ]

IV - OBTAIN STRENGTH FACTOR, φ, FROM TABLE 1.4-2 V - DESIGN COMPONENT for NOMINAL STRENGTH, Rn SUCH THAT:

φ Rn > QD

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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) October 18, 2006

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3 Second Gust Wind Force

(Section 2.1.1)

F = γw * Q * kZ * kzt * (V50)2 * G * Cf * A Where: F - Wind Force γw - 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. October 18, 2006

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Numerical Coefficient •

(Section 2.1.2)

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

Q = 1/2 ρ where ρ = mass density of air. Appendix D

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Basic Wind Speed Map

(Section 2.1.3)

3-SECOND GUST SPEED October 18, 2006

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Database/Analysis • Continental Winds: • Î 485 weather stations, minimum 5 years of data Î Data assembled from a number of stations in state-size areas to reduce sampling errors Î Fisher-Tippett Type I extreme value distribution, annual probability of 2% Î Insufficient variation in peak gust wind speeds to justify contours Î 33 ft. above ground, Exposure C

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Database/Analysis • Hurricane Winds: • Î Based on simulations and hurricane model Î The Atlantic Coastline was divided into discrete points spaced at 50 nautical miles. Î Hurricane contours over the Atlantic are provided for interpolations and represent values for Exposure C over land. Î Importance factors are accounted for in the map wind speeds • >1.0 at the coast • 1.0 at 100 miles inland. October 18, 2006

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Velocity Pressure Exposure Coefficients (Section 2.1.4) Exposure B Urban and suburban Terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger Exposure C Open terrain Open 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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Velocity Pressure Exposure Coefficients (Section 2.1.4)

TABLE 2.1.4-1 Power Law Constants Exposure category

α

zg (feet)

B

7.0

1200

C

9.5

900

D

11.5

700

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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/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 Response Factor • Structural Responses • Wind Characteristics

• Horizontal Wind Profile • Statistical based • Not a significant factor in typical buildings – seldom been studied October 18, 2006

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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 G

Structure GT = (1 + 2.7*E (BT)1/2)/kV2 Wire GW = (1 +2.7 *E (BW)1/2)/kV2 E = 4.9

(κ)1/2*(33/z

1/α h) fm

BT = 1/(1+0.56*zh/Ls) BW = 1/(1+0.8*L/ Ls) October 18, 2006

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E = Exposure Factor B = Dimensionless response term corresponding to the quasi-static background wind load kV = 1.430 40

Gust Response Factor

(Section 2.1.5)

• Conversion Factor, kV. (Durst Curve) • Relationship between 3-second gust wind and 10-minute average wind Appendix 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 October 18, 2006

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Force Coefficient

(Section 2.1.6)

Appendix H

• • • • •

Shape and Size Aspect Ratio Yawed Wind Solidity Shielding

•

Not a precise science

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Topography Effects

(Section 2.1.7)

• Funneling of Winds • Mountains • Wind Speed-up

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Extreme Wind Loads – Section 2.1 Wind is a Random Event • Equations are not exact • Equations are not intended to cover all potential conditions • Load factor is generally applied to cover uncertainty • With today’s technology, these equations are more scientific than most people think October 18, 2006

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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 October 18, 2006

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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 October 18, 2006

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Using Ice Map (2.3.4.2) •

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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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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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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Using Ice Map (2.3.4.2) •

ASCE 74 New Maps

Figure 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 3sec. wind speed.

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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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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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Model for the accretion of ice in freezing rain (App. I) N 1/2 1 2 2⎤ ⎡ t = ∑⎢(Pj ρo ) + (3.6VjWj ) ⎥ , ⎦ ρi π j =1⎣

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 rainfilled 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 Octoberice 18, 2006 storms

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Extreme value analysis (App. I) Peaks-over-threshold method with generalized Pareto distribution 1/ k

⎡ k(x − u) ⎤ F ( x ) = 1 − ⎢1 − ⎥ α ⎣ ⎦ ⎡ −(x - u) ⎤ = 1 − exp ⎢ ⎥ ⎣ α ⎦

k ≠0 k =0

Determine parameters using Probability Weighted Moments shape parameter k =

4b1 − 3b0 + u b0 − 2b1

scale parameter α = (b0 − u)( 1 + k) 1 b0 = n 1 b1 = n

n

∑x

(i )

i =1 n

∑ i =1

i −1 x( i ) n −1

Equivalent ice thickness for return period T: xT = u + α ⎡1 − ( λT )− k ⎤ October 18, 2006

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k⎣

⎦

63

Ice Load on Wires due to Freezing Rain (2.3.4) •

Combined Wind and Ice Loads – Ice Load 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)

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

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Ice Buildup on Structural Members (2.3.5)

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Ice Buildup on Structural Members (2.3.5)

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Ice Buildup on Structural Members (2.3.5) • 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 October 18, 2006

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Tornados Scale

Tornado Wind Speed F (mph)

Path Length P (miles)

Path Width P (feet)

0

≤72