Appendix C

1
Appendix C Example Applications of Rule 250C
(Extreme Wind)
Wade Shultz (Alabama Power)
January 9, 2007 Orlando Towers, Poles
Conductors Meeting
2
NESC APPENDIX C

• Intended to clarify NESC requirements for
  calculating extreme wind pressures
• Appendix is new for 2007 to explain application
  and intent of Rule 250C, and use of Tables 250-2
  and 250-3
• Recognizes that sound, practical engineering
  judgment is appropriate and acceptable

3
2002 NESC

• 2002 rule was unclear (not incorrect)
• Need recognized for more details in Rule 250C and
  for explanation of application methodology
• Lacked details
• How to treat different loading exposures?
• How to treat multiple concentrated loads?
• How to treat loads due to wind on supported
  components?

4
How are extreme wind pressures determined for
different loading situations?
5
2007 NESC

• Rule 250C revised
• "Center-of-pressure" concept introduced
• Center-of-component wind exposure
• Load resultant
• Corresponding kz and GRF (by calculation), or
• kz and GRF for height and span length ranges (by
  precalculated values from Tables 250-2 and 250-3,
  respectively)
• Appendix C added to explain intent

6
Wind Speed-to-Pressure(Customary units)

• Pressure 0.00256 V2 kz GRF I Cf
• Where
• V Wind Velocity (mph)
• kz Velocity Pressure Exposure Coefficient
• GRF Gust Response Factor
• I Importance Factor (1.0 default value)
• Cf Force Coefficient (Shape Factor) - 1.0
  for curved surfaces 1.6 for flat surfaces 3.2
  for lattice structures
• 0.00256 Velocity-Pressure Numerical Coefficient
  (reflects mass density of air for standard
  atmosphere)

7
kZ and GRF for Wires

• Formulas from Tables 250-2 and 250-3 (ft)

8
kZ and GRF for Structures

• Formulas from Tables 250-2 and 250-3 (ft)

9
kZ and GRF Variables

• EW - Wire exposure factor
• ES - Structure exposure factor
• BW - Dimensionless response term corresponding
  to the quasi-static background wind loads on the
  wire
• BS - Dimensionless response term corresponding
  to the quasi-static background wind loads on the
  structure
• kV - 1.43
• h - Structure or wire height per Rule 250C2
• L - Design wind span

10
kZ and GRF for Components, Supported Facilities
and Equipment

• kZ for height to center-of-pressure
• GRF for structure applies to components,
  supported facilities (not wires) and equipment

11
NESC Tables 250-2 and 250-3

• Values of kZ and GRF for selected ranges of
  heights and span lengths for structures and wires
  
• Values of kZ based on highest value for height
  range (top of range)
• Values of GRF based on both
• Highest value for height range (bottom of range)
• Highest value for span range (shortest span of
  range)

12
Appendix C

• Developed to illustrate application of revised
  Rule 250C
• Does not preclude other engineering methods
• Presents typical wind pressure calculations
• (Note For this presentation customary units are
  used for illustration purposes)

13
Appendix C Examples

• 1) Uniform wind on structure phases at same
  height, groundwires
• 2) Center-of-pressure of phases at different
  heights
• 3) Tall (gt75 m or 250 ft) structure and irregular
  exposure
• 4) Pole structure with phases at different
  heights, groundwire, underbuilt communication,
  and supported equipment

14

• Example 1 Uniform Wind on Phases, Groundwires,
  Structure

15
Wind Pressure on Phases

• h 79.4 ft
• L 1310 ft
• V 90 mph

16
(No Transcript)
17
(No Transcript)
18
kZ and GRF for Phases

• Formulas from Tables 250-2 and 250-3 (ft)

19
Velocity-to-Pressure Conversion
Pressure 0.00256 V2 kz GRF I Cf

• For tabulated values
• From formula values

Pressure, P 0.00256 (90)2 1.2 0.69 1
1 P 17.17 psf
Pressure, P 0.00256 (90)2 1.205 0.657 1
1 P 16.42 psf
20
(No Transcript)
21
Wind Pressure on Groundwire

• h 103 ft
• L 1310 ft
• V 90 mph

22
(No Transcript)
23
(No Transcript)
24
(No Transcript)
25
Wind Pressure on Structure

• h 104 ft
• V 90 mph

26
(No Transcript)
27
(No Transcript)
28
kZ and GRF for Structure

• Formulas from Tables 250-2 and 250-3 (ft)

29
(No Transcript)
30
Example 2 Center of Pressure of Phases at
Different Heights

• L 900 ft
• V 85 mph

31

• Delta phase arrangement
• One bundle at 87 ft
• Two bundles at 58 ft
• Center of pressure determined to be midway
  between phase heights - 72.5 ft

32
(No Transcript)
33
(No Transcript)
34
(No Transcript)
35
Example 3 Tall (gt75 m or 250 ft) structure
and/or irregular exposure
36
Example 3 Wind Pressures on Structure, by
Sections

• Identify structure sections to evaluate
• One GRF value - based on entire structure height
  (by formulas)
• Determine kZ values for each section based on
• Height of WS 1
• Height to centers of pressure for WS 2, WS 3,
  and WS 4
• Wind velocity 90 mph

37
Wind Sections

• Total structure height 348 ft
• Section WS 1 height 228 ft
• Section WS 2 center-of-pressure 248 ft
• Section WS 3 center-of-pressure 290 ft
• Section WS 4 center-of-pressure 329 ft

38
WS 1
Section height, h 228 ft
39
(No Transcript)
40
(No Transcript)
41
kZ for Wind Section WS 1

• Formula from Table 250-2 (based on height to
  center-of-pressure of section)

42
GRF for Structure

• Formulas from Table 250-3(ft)
• For h 348 ft, GRF 0.762

43
GRF for Overall Structure
44
WS 2
Section center of pressure 248 ft
45
kZ for Wind Section WS 2

• Formula from Table 250-2 (based on height to
  center-of-pressure of section)

46
GRF for Overall Structure
47
WS 3
Section center of pressure 290 ft
48
GRF for Overall Structure
49
WS 4
Section center of pressure 329 ft
50
GRF for Overall Structure
51
Example 4 Distribution Wood Structure

• Pole structure with phases at different heights,
  groundwire, underbuilt communication, and
  supported equipment

• L 500 ft
• V 90 mph

52
Velocity Pressure Coefficient, kZ

• For each load component
• Phases (height to center-of-pressure of
  resultant)
• Structure (tabulated values or formulas based on
  height of structure)
• Shieldwire and communication wire (heights of
  attachment)
• Transformer (height to center-of-pressure)

53
Gust Response Factors, GRF

• Same GRF value for structure and transformer -
  based on entire structure height
• GRF values for phases - based on height of center
  of pressure
• GRF values for groundwire and communication wire
  based on attachment height at structure

54

• L 500 ft
• h 54.5 ft
• V 90 mph

55

• L 500 ft
• h 64.5 ft
• V 90 mph

56

• h 65.5 ft
• V 90 mph

57

• L 500 ft
• h 35.5 ft
• V 90 mph

58

• h 45.5 ft
• V 90 mph

GRF for Overall Structure
59
Appendix C Summary

• Appendix C offers examples of wind pressure
  calculation methodology
• Center-of-pressure approach is permissible and
  expected
• Either tabulated or calculated values of kZ and
  GRF may be used
• Tabulated values normally yield conservative
  results
• KZ is height dependent (increases with height)

60
Summary (cont.)

• GRF for structure is height dependent (values
  decrease with height)
• GRF for wires is height and span length dependent
• Values decrease as height increases, and
• Values decrease as span length increases
• Structure GRF applies to supported components
  (not wires) and structure sections

End