Rule 250D

1
New Rule 250D Extreme Ice with Concurrent Wind
Clayton Clem Tennessee Valley Authority
January 9, 2007 Orlando Towers, Poles
Conductors Meeting
2
Extreme Ice Concurrent Wind History

• Past Industry Ice Loading Criteria
• NESC District Map put in place with the 2nd ed.
  (1916) and used in the 3rd ed. (1920)
• classed some areas as heavy because of high wind
  velocities even though ice did not accumulate to
  any greater extent than in the medium area
• recognized the need for local authority to fix
  the loadings based upon local knowledge and
  weather records.

3
Extreme Ice Concurrent Wind History

• NESC 2nd ed., 1916 and 3rd ed., 1920

4
Extreme Ice Concurrent Wind History

• Past Industry Ice Loading Criteria
• NESC District Map modified with the 4th ed.
  (1926) with substantial discussion published in
  (1928)
• revised the map in the western states stating
  that the lines have been made to follow more
  closely the natural physical dividing lines and
  also the lines dividing the political
  subdivision,

5
Extreme Ice Concurrent Wind History

• Past Industry Ice Loading Criteria
• NESC District Map modified with the 4th ed.
  (1926) with substantial discussion published in
  (1928)
• utilized information from the U.S. Weather Bureau
  and from numerous answers to a circular sent to
  power and telephone companies,
• the chosen values do not represent the most
  severe cases recorded, but do represent
  conditions that occur more or less frequently

6
Extreme Ice Concurrent Wind History

• NESC 4th ed., 1926

7
Extreme Ice Concurrent Wind History

• Past Industry Ice Loading Criteria
• NESC District Map modified with the 5th ed. of
  Part II (1941) discussion published in 1944
• continued to revise the original map boundaries
  in the western states along political
  subdivisions and made extensive changes in the
  southeast along political subdivisions based upon
  weather data and experiences

8
Extreme Ice Concurrent Wind History

• Past Industry Ice Loading Criteria
• NESC District Map modified with the 5th ed. of
  Part II (1941) discussion published in 1944
• provided general boundaries for the states of
  California and Nevada recognizing that the
  general orders of the state provided the details
• recognized that heavy-loading intensity storms
  occurred in the local areas of Washington and
  Oregon and left the definition of these locations
  to the states

9
Extreme Ice Concurrent Wind History

• NESC 5th ed. Part II, 1941

10
Extreme Ice Concurrent Wind History

• Past Industry Ice Loading Criteria
• NESC District Map put in place with the 7th ed.
  of Part II (1977) and continued through the 1977,
  1981, 1984, 1987, 1990, 1993, 1997 ed.
• revised the medium load boundary established in
  the 1941edition extending it northward from
  Kentucky to include parts of Ohio

11
Extreme Ice Concurrent Wind History

• NESC 7th ed. Part II, 1977 to present

12
Extreme Ice Concurrent Wind History

• Past Industry Ice Loading Criteria
• In the early 1980s, TL engineers from across the
  country began working on new criteria for line
  design based on research and benchmarking.
• this resulted in the ASCE Committee report on
  loadings (1984)
• recognized that most utilities had design
  guidelines for ultimate loadings in addition to
  the NESC district loadings

13
Extreme Ice Concurrent Wind History

• Past Industry Ice Loading Criteria
• Local Utility Maps Practices
• Duke Power Map
• BPA Map
• Many Others Had Detailed Written Criteria

14
Extreme Ice Concurrent Wind History

• Past Industry Ice Loading Criteria
• ASCE Manual 74 published in 1991
• 50-year return interval ice based on 9 years of
  data collected by Bennet
• 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

15
Extreme Ice Concurrent Wind History

• ASCE Manual 74, 1991

16
Extreme Ice Concurrent Wind History

• Increased interest in transmission line icing
  started as a result of catastrophic line failures
  in 1990 and 1991.
• On December 26, 1991, President George H. W. Bush
  declared 43 Iowa counties disaster areas as a
  result of severe icing.
• FEMA recommended that greater ice loads that
  exceeded current code levels be used

17
Extreme Ice Concurrent Wind History

• In February 1992, EPRI and Omaha Public Power
  District hosted a workshop for utilities and
  icing experts.
• Ten utilities, represented by 27 participants,
  discussed recent ice related line failures.
• The consensus was that an industry study on icing
  was needed.

18
Extreme Ice Concurrent Wind History

• In 1993, EPRI, in conjunction with several
  utilities (Illinois Power, Midwest Power, WAPA,
  TVA, Mid America, and Northern States Power) and
  the US Army Cold Regions Research Engineering
  Laboratory (CRREL), developed a plan to study
  map icing with the following key elements

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Extreme Ice Concurrent Wind History

• Severe ice storms were identified using
  historical meteorological data
• Ice accretion modeling using historical
  meteorological data would be performed
• The ice accretion modeling would be validated
  using utility records for the events
• Validation would also be performed using Storm
  Data (NOAA 1959-1995) and newspaper reports

20
Extreme Ice Concurrent Wind History

• In 1994 the EPRI efforts got underway with storm
  data, newspaper reports, and a staged approach to
  detailed meteorological study and ice accretion
  mapping.
• Nationwide Ice Severity Event Index Maps -
  completed 1996
• Region 1 made up of Iowa and portions of
  Illinois, Wisconsin, Indiana, Missouri, Kansas,
  Nebraska, and South Dakota - completed 1996

21
Extreme Ice Concurrent Wind History

• 1994 efforts continued
• Region 2 made up of Tennessee, Kentucky, and
  portions of N. Carolina, S. Carolina, Georgia,
  Alabama, Mississippi, Louisiana, Arkansas,
  Missouri, Illinois, and Indiana - completed 1997
• Region 3 made up of Minnesota and portions of
  Wisconsin, N. Dakota, S. Dakota, and Iowa -
  completed 1997

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Extreme Ice Concurrent Wind History

• 1994 efforts continued
• Region 4 consisting of the rest of North and
  South Dakota - completed 1997

23
Extreme Ice Concurrent Wind History

• In 1993 an ASCE task committee on icing was
  formed to begin the revision of ASCE 7-95
  Minimum Design Loads for Buildings and Other
  Structures, which had used the 1991 Manual 74
  transmission line ice map.
• Several members of the initial EPRI, utility,
  CRREL team participated.

24
Extreme Ice Concurrent Wind History

• 1993 ASCE task committee continued
• The group used the research being performed for
  the utilities as a starting basis for the
  nationwide map
• CRREL and others began the effort of completing
  the map for the rest of the United States.

25
Extreme Ice Concurrent Wind History

• ASCE 7-98 Map proposed in CP2309 for the 2002
  edition of the NESC

26
Extreme Ice Concurrent Wind History

• ASCE 7-98 Map proposed in CP2309 for the 2002
  edition of the NESC

27
Extreme Ice Concurrent Wind History

• ASCE 7-98 Map proposed in CP2309 for the 2002
  edition of the NESC

28
Extreme Ice Concurrent Wind History

• ASCE 7-98 Map proposed in CP2309 for the 2002
  edition of the NESC

29
Extreme Ice Concurrent Wind History

• NESC - 2002, Strength and Loadings ASCE
  Combined Ice Wind Map, Rule 250B, CP2309, p.250
• The committee recommended the new map as an
  alternate to the Rule 250B district map

30
Extreme Ice Concurrent Wind History

• NESC - 2002 CP2309 continued
• Present rule 250B would become Rule 250B1
  Combined Ice and Wind Loading Method 1
• New Rule 250B2 would be added Combined Ice
  Wind Loading Method 2
• All references to existing rule 250B will be
  updated to reference both methods

31
Extreme Ice Concurrent Wind History

• NESC - 2002 CP2309 continued
• Ultimate ice wind loads of Rule 250B2 will be
  treated like the ultimate wind loads of rule 250C
• Where references to rule 250C are in the code,
  references to Rule 250B2 will be added

32
Extreme Ice Concurrent Wind History

• NESC 2002 CP2309 Rejected
• Subcommittee 5 established Task Force 5.1.6 to
  prepare a change proposal for the 2007 code
  cycle.
• NESC 2007 CP2802 utilized the ASCE 7-05 map and
  was adopted by the committee as a new Rule 250D.

33
New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind

• Incorporates the 50-year return period ice plus
  wind map into the code similarly to how the
  extreme 50-year return period wind map was done
  calling it rule 250D.
• Preserves the District Map rule 250B and the
  Extreme Wind of Rule 250C.
• Changes in rules and tables have been made as
  required for Rule 250D similar to those for the
  existing extreme wind Rule 250C.
• Ultimate ice wind loads of Rule 250D will be
  treated some what like the ultimate wind loads of
  rule 250C.
• Where references to rule 250C are in the code,
  references to Rule 250D have been added.

34
New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind

• Applies where a structure or its supported
  facilities exceeds18m (60Ft) above ground or
  water.
• The simplified wind pressures for the concurrent
  wind speed are given in Table 250-4.
• Wind pressures are applied to the iced diameter
  of the wire.
• Wind pressures are applied to the entire
  supported structure without ice.
• The radial thickness of ice from Figure 250-3
  shall be multiplied by 1.0 for grade B and 0.80
  for grade C.

35
New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind

36
New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind
37
New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind
38
New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind

• Rule 250A1 Three weather loads are specified in
  Rules 250B, 250C, and 250D. Where all three
  apply, the required loading shall be the one with
  greater effect.

39
New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind

• Note the simplified wind pressures used with Rule
  250D as shown in Table 250-4.

40
New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind

• Rule 251.B.3 Total load shall be the resultant
  of the horizontal and vertical components
  calculated at the applicable temperature in Table
  251-1.

41
New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind

• Rule 253 Loads due to the district loads in Rule
  250B, the extreme wind loading condition in Rule
  250C, and the extreme ice with concurrent wind
  condition Rule 250D shall be multiplied by the
  load factors in Table 253-1

42
New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind

• or the alternate load factors in Table 253-2.

43
New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind

• Structures shall be designed to withstand the
  loads in Rule 252 ( uses Rule 250 and 251)
  multiplied by the appropriate factors in Table
  253-1 or 253-2 without exceeding the permitted
  strength.
• Rule 260.B.2 Unless otherwise specified, a
  strength factor of 0.80 shall be used for the
  extreme loading condition specified in Rule 250C
  and the extreme ice with concurrent wind
  specified in Rule 250D for all supported
  facilities.

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New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind

• The permitted strength shall be the strength
  multiplied by the strength factors in Tables
  261-1A..

45
New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind

• or similarly 261-1B.

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New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind

• Rule 261.H.2 a c (splices, taps, dead-ends,
  etc.), Rule 261.I (supply cables) and Rule 261.K
  (Communications cables) limit the stress to 80
  of the rated breaking strength under the
  conditions specified in Rules 250C and 250D.
  Note Remember for comparison Rule 250B loads
  were multiplied by a load factor of 1.65.
• Rule 277 (Mechanical strength of insulators)
  requires that proper allowance be made for the
  loads in Rules 250C and 250D. Note For the
  District Loads Rule 250B 40-50 of the insulators
  strength rating is used. For the ultimate loads
  of 250C and 250 D practice has varied. Many
  utilities have used strengths greater than 40-50
  ( REA 70). Consensus between utilities and
  insulator manufacturers has not been reached on
  this issue.

47
New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind
48
New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind

• Referring to the Rule 250D equivalent conditions
  shown on the previous table, you will realize
  that smaller wire sizes have not had the same
  reliability against failure due to loading as
  larger wires in past editions of the code when
  using Rule 250B
• Calculating tensions with a smaller ice load and
  applying an overload factor is not the same as
  making the calculation with the true ice load
• Rule 250D will require all wire sizes to have a
  consistent reliability level with respect to
  loadings

49
New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind

• Much of the medium loading area in the southeast
  will see an increase in vertical load and a
  reduction of transverse wind load when compared
  to Rule 250B
• Some areas of heavy loading in the mid-west will
  also see an increase in vertical load and a
  reduction of transverse wind load when compared
  to Rule 250B

50
New NESC 2007 Rule 250D, Extreme Ice Concurrent
Wind

• In summary,
• Rule 250D implements an ultimate ice and wind
  speed approach which will be kept up to date just
  like the ultimate wind speeds are adjusted as
  updates to ASCE-7 are developed.
• Offers the advantage of being able to have a
  consistent requirement for all wire sizes on the
  same structure leading to improved safety and
  reliability