linda1'ppt

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Three Second Gust Extreme Wind Speed Map, CP2363
IEEE
Leon Kempner, Jr., P.E. Bonneville Power
Administration February 6, 2000
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Why Change Rule 250C?

• Current Extreme Wind Map, Based on Fastest Mile
  Wind Speed
• New wind data and engineering wind maps are based
  on Three (3) Second Gust extreme Wind Speed
• Fastest Mile wind speed, no longer collected
• Three Second Gust speed collected at more
  stations
• The Three Second Gust Wind Speed was first
  presented in ASCE 7-95

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Why Change Rule 250C? (Continued)

• The Three Second Gust Wind Speed represents a
  better database and analysis of available data
• The NESC should be consistent with other
  Engineering Loading Standards, Guides and Codes
  such as, ASCE-7, EIA/TIA-222, ASCE-74, IBC, etc.
• This change is the right thing to do,
  Standard/Engineering Creditability

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Database/Analysis

• Continental Windsè 485 weather stations, min.
  5 years of dataè Data was assembled from a
  number of stations in state-size areas to
  reduce sampling errorsè Fisher-Tippett Type
  I extreme value distribution, 50 year return
  periodè Insufficient variation in peak gust
  wind speeds to justify contoursè 33 ft. above
  ground, Exposure C

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Database/Analysis

• Hurricane Windsè Based on Monte-Carlo
  simulationsè The coastline was divided into
  discrete points spaced at 50 nautical
  miles.è Hurricane contours over the Atlantic
  are provided for interpolations and represent
  values for Exposure C over land.è Importance
  factors are accounted for in the map wind
  speeds, gt1.0 at the coast to 1.0, 100 miles
  inland.

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Fastest Mile Vs. 3 Sec. Gust
FASTEST MILE SPEED
3 SECOND GUST SPEED
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Gust Response Vs Gust Factors

• GRF - Accounts for Dynamic effects of gusts on
  the response of T-line components- Gusts may
  not envelop the entire span between T-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

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Gust Response Vs. Gust Factors

• Gust Factor - The Ratio of the gust extreme wind
  speed at a specified averaging period,
  e.g. 2 seconds, to the selected mean extreme
  wind 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 (Typ. 1.1 - 1.5)

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NESC Extreme Wind Format
CP2363

• Equations l Figures l Tables

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Gust Response Factor, GRF

• 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, 1995
• IEC 826, Loading and Strength of Transmission
  Lines, 1991

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3 Second Gust Wind Pressure
qZ 0.00256 kZ V2 I GRF Cd

• Where
• kZ - Velocity Pressure Exposure
  Coefficient, C, as defined in Rule
• 250C.1, Table 250-2.V - Basic
  Wind Speed, 3-second gust wind speed, miles per
  hour,
• at 33 ft. above ground, an annual
  probability of 0.02 (50 year
• return period), Figure 250-2.sI
  - Importance Factor, 1.0 for structures, wires,
  and their support facilities.GRF -
  Gust Response Factor, as defined in Rule
  250C.2.Cd - Shape Factor, as defined in Rule
  252B.

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KZ Parameter
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GRF Parameter
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Table Values Represent a Uniform Change
3
7
7
4
3
4
3
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Tables of Factors
Structure/Wire Gust Response Factors
1. Recommend General Equations for tower height,
h, greater than 250 ft. and span length, L,
greater than 2000 ft.
Velocity Pressure Exposure Coefficients
1. Recommend General Equation for height greater
than 250 ft.
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General Equations
Structure/Wire Gust Response Factors
Structure GRF (1 2.7ES (BS)0.5)/kV2 Wire GRF
(1 2.7 EW (BW)0.5)/kV2 ES
0.346(33/(0.67h))1/7 EW
0.346(33/h)1/7 BS 1/(10.375h/220)
BW 1/(10.8L/220)
E Exposure Factor B Dimensionless response
term corresponding to the quasi-static background
wind load kV 1.430
Velocity Pressure Exposure Coefficients
Structure kZ 2.01(0.67h/900) (2/9.5) Wire kZ
2.01(h/900) (2/9.5) (for 15
ft. h 900 ft.)
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Example
CP2363
C2-1997
qZ 0.00256kZV2IGRFCd
qZ 0.00256V2Cd
qZ 0.00256kZV2 GRF (ICd)
qZ 0.00256V2C
qZ 0.00256kZV2 GRF C
7Ì
Wind in Tower (Height 141 ft.)
70 mph À 12.54C 80 mph À 16.38C 90 mph À 20.74C
25Ì
Kz Twr GRF
141 ft. 1.30 0.859
25Ì
Wind in Wire (1000 Ft. Span)
10Ì
Kz Wire GRF
75Ì
Note No Gust Factor
74 ft. 1.20 0.712 99 ft. 1.30
0.698 124 ft. 1.40 0.687 141 ft.
1.40 0.687
9Ì
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Example (Continued)
CP2363
Tower Wind Pressure
_at_ 141 ft. 0.002561.3(90mph)20.859C 23.2C
psf
Wire Wind Pressure
_at_ 74 ft. 0.002561.2(90mph)20.712C
17.7C psf _at_ 99 ft. 0.002561.3(90mph)20.6
98C 18.8C psf _at_ 124 ft.
0.002561.4(90mph)20.687C 19.9C psf _at_ 141
ft. 0.002561.4(90mph)20.687C 19.9C psf
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Example (Continued)
C2 20.7C á
CP 19.9C á
C2 psf
CP psf
C2 20.7C á
CP 19.9C á
C2 20.7C á
CP 18.8C á
C2 20.7C á
CP 23.2C á
C2 20.7C á
CP 17.7C á
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Review of CP2363

• Presents the 3 second gust extreme wind speed
  approach
• Updates the NESC extreme wind speed map to
  current wind engineering knowledge
• Presents statistically better wind speed data
• Technically correct/consistent approach for
  determining wind loading