ASCE Practice 74 Rev 2006

ASCE A SCE Manu Man u al alss an andd Rep Repoo r t s o n Eng ngin ineeeri ring ng Practic Pract icee #74 Guid uideeliline ness for f or Ele lectr ctrical ical Transmis Transmissi sion on Lines L ines Str truct uctural ural Loads Loads Frank W. Agnew Terry Burley Michael D. Miller John D. Mozer Mark Ostendorp Alain Peyrot C. Jerry Wong

October 18, 2006

Revised ASCE Ma Manual nual No. 74 - Section 2 Ice and Wind

1

ASCE A SCE Manu Man u al alss an andd Rep Repoo r t s o n Eng ngin ineeeri ring ng Practic Pract icee #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

October 18, 2006


2

Transmission Line Structural Loading Guide

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First edition was published in 1984 “Design Guidelines”

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

October 18, 2006


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Forward Section 1 - Introduction to Load Criteria Section 2 - Weather Related Loads Section 3 - Additional Load Considerations

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Section 4 - Wire System Section 5 - Examples

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Appendices

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October 18, 2006

Revised ASCE Manual No. 74 - Section 2 Ice and Wind

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Appendices Reference   

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    

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


8

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


9

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.

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Probability Density Function of Load Effect

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13

Probability of RPN Events in 50 Years

Load Return Period RP (years)

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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 Revised ASCE Manual No. 74 - Section 2 Ice and Wind

14

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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17

Relative Reliability Factor (RRF)

RRF ≅

Probability of failure for a RP50 load event Probability of failure for a RP N 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


19

Extreme Wind Load Factors (Table 1.2-1) Relative Reliability Factor

Load RP (years)

Wind Load Factor

(γw)

(RRF)

October 18, 2006

0.5

25

0.85

1

50

1.00

2

100

1.15

4

200

1.30

8

400

1.45


20

Extreme Ice Factors (Table 1.2-2) Relative Reliability Factor

Load RP (years)

(RRF)

Ice Concurrent Thickness Wind Load Factor Factor

(γi)

(γw)

0.5

25

0.80

1.0

1

50

1.00

1.0

2

100

1.25

1.0

4

200

1.50

1.0

8

400

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

φ Rn

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≥ Effect of [ DL + γ Q ]


26

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 – Steel prestr prestresse essed d concre concrete te poles. poles. – Reinforced – Reinforced concrete. – Wood – Wood poles. – Foundations. – Foundations. – Conductors – Conductors and ground wires. October 18, 2006


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Summary of LRFD Method I - SELECT RELATIVE RELATIVE RELIABILITY FACTOR (RRF) OR MINIMUM DESIGN LOAD RETURN PERIOD DEPENDING OF TYPE OF LINE (TABLE 1.2-1) II - OBTAIN OBTAIN FACTOR FACTORS, S,

, from Tables 1.2-1 and 1.2-2

III - DETERMINE DETERMINE DESIGN DESIGN LOAD EFFECT 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, , FROM TABLE 1.4-2 V - DESIGN DESIGN COMPON COMPONENT ENT for NOMINAL STRENGTH, R n SUCH THAT:

φ R n >

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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

F =

w

(Section 2.1.1)

* Q * kZ * kzt * (V50)2 * G * Cf * A

Where: F - Wind Force - Load Factor. w 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

• where

= 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: •



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

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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 for Exposure C over land. Importance factors are accounted for in the map wind 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 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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TABLE 2.1.4-1 Power Law Constants Exposure category

zg (feet)

B

7.0

1200

C

9.5

900

D

11.5

700

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37


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 Profi Profille e Statistical based

•

Not a significant signif icant factor in typical buildings – seldom 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. Structure GT = (1 + 2.7*E (BT)1/2)/kV2

Appendix G

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

(κ)1/2*(33/zh)1/αfm

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


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 October 18, 2006


42

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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43

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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44

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


46

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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49

Categories of Icing (2.3.2) • • • •

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

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50

Design Assumptions for Ice Loading (2.3.3) • Equivalent uniform radial thickness

Radial Ice

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51

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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53

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


54

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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55

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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56


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.

October 18, 2006


57


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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58


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.

October 18, 2006


59

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.

October 18, 2006


60

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

1

N

1/2

⎡(P ρ )2 + (3.6V W )2 ⎤ t= j j ⎥⎦ ρi π j =1⎢⎣ j o

∑

,

where t = equivalent radial ice thickness (mm) P j = precipitation amount (mm) in jth hour V j = wind speed (m/s) in jth hour 3

W j = liquid water content (g/m ) of the rain0.846

filled air in jth hour = 0.067P j 3

ρ o =

density of water (1 g/cm )

ρ i =

density of ice (0.9 g/cm )

3

N = duration of the freezing rain storm (hr)

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61

Superstations for extreme value analysis (App. I) pattern of damaging ice storms

•terrain •proximity to water •latitude

frequency of Octoberice 18, 2006 storms


62

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

− 2b1 scale parameter α = (b0 − u)( 1 + k) b0

=

b1 =

b0

n

1 n

∑x

1 n

−1 x( i ) 1 n − i =1

(i )

i =1 n

∑

i

Equivalent ice thickness for return period T: xT = u + October 18, 2006


α⎡ −k ⎤ 1 − ( λT ) ⎣ ⎦ 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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64

Ice Buildup on Structural Members (2.3.5)

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65

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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67

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


68

Tornados Scale

Tornado Wind Speed F (mph)

Path Length P (miles)

Path Width P (feet)

0

≤72

<1.0

≤50

1

73-112

1.0-3.1

51-170

2

113-157

3.2-9.9

171-530

3

158-206

10-31

531-1670

4

207-260

32-99

1671-4750

5

261-318

100-315

4751-6,000

TABLE 2.2.1-1. Ranges of Tornado Wind Speed, Path Length, and Path Width for FPP Scale October 18, 2006


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National Weather Service October 18, 2006


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TABLE 2.2.1-2 Tornado Frequencies and F-Scale Classifications for 1916—1978 in the United States of America (Tecson et al. 1979) 35 30 25 20 Percentage

15 10 5 0 F0

October 18, 2006

F1

F2

F3

F4

F5


71

Downbursts

•Associated with severe thunderstorm cells

•Relatively wide gust fronts

•Elliptical damage pattern

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Micro bursts Micro Burst: A strong localized downdraft from a thunderstorm with peak gusts lasting 2 to 5 minutes. Natio nal Weather Servi ce, Missoula, Mt.

•Intensity levels up to F2 Tornado strength

•Gust width ± 330’ – 660’

•Elliptical and strip damage patterns October 18, 2006


73

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. October 18, 2006


74

APPENDIX K: Investigation of Transmission Line Failures

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75

Why investigate failures?

•Increase understanding of line behavior •Affirmation of existing design and maintenance criteria •Improvement of design criteria and maintenance practices

October 18, 2006

Revised ASCE Manual No. 74 Section 2 - Ice and Wind

76

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. October 18, 2006


77

FAILURE INVESTIGATIONS

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

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78

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. October 18, 2006


79

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 October 18, 2006


83

THE INVESTIGATION • The Field Checklist • The Office Checklist • Report Preparation

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84

October 18, 2006


85

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 LoadRevised (3.6)ASCE Manual No. 74 - Section 2 -

October 18, 2006

Ice and Wind

86

Introduction (3.0) Section 3 does not address: • • • • •

Landslides Ice Flows Frost Heave Flooding Other Special Loading Scenarios

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87

Construction & Maintenance Loads (3.1) General • Construction Loads are directly related to construction methods • Personnel Safety is the paramount factor Construction 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) Revised ASCE Manual No. 74 - Section 2 – Load Factor for vertical loads from intact condition (2.0) October 18, 2006 Ice and Wind 88

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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89

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 Reviseddevices ASCE Manual No. 74 - Section 2 Number of climbing

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Ice and Wind

90

Longitudinal Loads (3.3) •

Structures may be required to resist longitudinal loads – Loads resulting from inequalities of wind and/or ice on adjacent spans – 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 October 18, 2006


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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 adjacent spans – 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 October 18, 2006


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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 spoilersRevised ASCE Manual No. 74 - Section 2 October 18, and Wind – 2006 Modification of conductor Ice 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) • • • • •

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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)


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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) • • • •

–

Strain Supports (J.2.2) • • •

•

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 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) • • •

–

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) • • •

–

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

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) Revised ASCE Manual No. 74 - Section 2 – Basic Assumption (J.4.2) October 18, Ice and Wind – 2006 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 October 18, 2006


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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 October 18, 2006


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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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ASCE Practice 74 Rev 2006

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