Carga Hielo

Loading Trees for Structure & Foundation Design
IEEE Overhead Lines Sub-Committee Winter Meeting Tampa, FL February 5, 2012

1

A Definition
“A transmission line is a system of interrelated components whose combined purpose is to safely support electric conductors above the ground between two end points separated a long distance.”

2

Weather Constraints
 Average Annual MinimumTemperature  Lowest Averaged Over the Years  Not Lowest Recorded  Average Annual High Temperature  Highest Averaged Over the Years  Not Record High  Maximum Wind  Highest Over the Years  This can be confusing
3

Loading Districts
 Loading districts  Light


30°F, 9psf wind, 0” ice 15°F, 4psf wind, 0.25” ice 0°F, 4psf wind, 0.5” ice

 Medium


 Heavy


NOT A DESIGNSPECIFICATION!
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Structural Loading on Overhead Lines
 Transference of loads from conductors to Structures  Transference of loads from Structures to Foundations  Transference of loads from Foundations to Soil  Vertical Loads
 Conductor Weight and Dead Loads  Uplift on Foundations  Ice Load  Construction

 Transverse Loads
 Wind Load on Conductors  Line Tension at Angles

Longitudinal Loads
 Line Tension  Imbalances  Broken Conductor
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Overload Factors
 National Electrical Safety Code
 Wind – 2.5  Tension – 1.65  Vertical – 1.50

 Railroads - Varies  Local and State Jurisdictions – Usually NESC  Others
 DOT  Canals

 Inclusion in loading table requires note

6

Design Load Factors
 Wind Gusts  Varies  Microburst on structure only Height Factors  NESC  ASCE  Span Factors  Reduces load for long spans  NESC
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Wind Gusts
 Weather Services  Meteorological Stations  Gust Factors  Multiplier of wind speed  Commonly used to evaluate gusts  Historical Data  Life of Line - Probability of occurrence  Gust durations

8

Gust Factor Study
Ranges from 1.2 – 1.5

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Load Factors
 Based on desired reliabilitylevel  Factor applied to source of load before calculating point load  1.15 common for 100 year event return

10

Span Reduction Factors
 Gust front probability  Less than 1.0  Long Spans – greater reduction  Short spans – 1.0

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Mechanical Loading
 Wind Loading  Transverse  Quartering (Lattice Towers)  Microbursts  Conductor Tension  Transverse and Longitudinal  IceLoading  Transverse, Longitudinal, and Vertical  Construction Loads  Vertical
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Wind Loading
 Transverse on Conductor and Shieldwire  Point loads  Specify in Loading Table  Transverse and Quartering on Structures  Not a point load  Specify in Specifications  Ice Thickness  Separate Load case  Applies to structure also
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Microburst Wave Front

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Microbursts
 High WindSpeeds  As high as 170mph!  150 mph has been used  Relatively Narrow Wind Front  Action on Conductors not subject to burst  Ambient wind is High wind case  Applied to Structure Only

15

Tension Loading
 Longitudinal  Point Load  May have transverse and longitudinal components in structure space  Major Component at Line Angles  Point Load  Consider wind from both directions

16Wind Loading on Structure
 Addressed in the Specification  Not a point load  Include footnote on Loading Table  Quartering Wind for Lattice Towers  Not a point load  Include footnote on Loading Table

17

Transverse Loads at Point of Attachment (Point Loads)
 Wind Load  Line Tension  Line Angle  Application of OLF’s

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Ice Loading on Structure
 Address in theSpecification  Not a point load  Include footnote on Loading Table  May effect redundant members  Increases wind load area

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Vertical Loading
 Dead Weight  Construction  Ice and Snow  Personnel

20

Wind Loading on Conductor
 Transverse

Lh=lb/sqft V=wind speed d=diameter in inches Sh=Wind Span in feet

21

Ice Loading on Conductor
 Transverse, Longitudinal, and Vertical...