CP 2766
Submitter
Donald Heald
Proposed Change
Remove the 60-ft exclusion from Grade B and Grade C construction and show a
maximum wind load for Grade B and Grade C construction under 60 ft for Tables 253-1
and 253-2. Minor formatting changes of these tables is also suggested so the wording
“Vertical Loads” will line up with “Transverse Loads” and “Longitudinal Loads.” This
change proposal is based a previously submitted change proposal which provides
appropriate load factors for Grade C construction from Grade B construction under 250C
wind loads (extreme wind). The proposed changes are shown below:
Refer to PDF page 390-391 for tables listed below.


Table 253-1—Overload Factors for Structures,1 Crossarms, Support
Hardware, Guys, Foundations, and Anchors to Be Used with the
Strength Factors of Table 261-1A
Table 253-2—Alternate Overload Factors for Wood and Reinforced
(Not Prestressed) Concrete Structures1, 5 to Be Used with the
Strength Factors of Table 261-1B
Additional changes:
The following additional rules need to be changed to accommodate the above change:
1. Rule 250A1:
250.
General Loading Requirements and Maps
A.
General
1.
It is necessary to assume the wind and ice loads that may occur on
a line. Two weather loadings are specified in Rules 250B and
250C. Where both rules apply, The required loading shall be the
one that has the greater effect. The removal of the 60-ft exclusion
makes the wording “where both rules apply” no longer applicable.
Both loading conditions apply at all times.
2.
Rule 250C:
C.
Extreme Wind Loading
If no potion of a structure or its supported facilities exceeds 18m (60 ft)
above ground or water level, the provisions of this rule are not required
except as specified in Rule 261A1c or Rule 261A2f. Where a structure of
its supported facilities exceeds 18 m (60 ft) above ground or water level
the Structures and its their supported facilities shall be designed to
withstand the extreme wind load associated with the Basic Wind Speed, as
specified by Figure 250-2.
Reference to the 60-ft exclusion is no longer necessary.
3.
Delete the last sentence to Rule 250C2:
Wire attachment points that are 18m (60 ft) or less above ground or
water level must be considered if the total structure height is
greater than 18m (60 ft) above ground or water.
This sentence is moved to footnote 8 of Table 253-1 and footnote 9
of Table 253-2.
4.
At the bottom of Tables 250-2, in the equations, replace the lower
limits of 18 m and 60 ft with 10 m and 33 ft.
Reference to these heights is no longer needed.
5.
Change Rule 261A1c to read:
All structures including those below 18 m (60 ft) shall be designed
to withstand, without conductors, the extreme wind load in Rule
250C applied in any direction on the structure.
The phrase “including those below 18 m (60 ft)” is no longer
needed.
6.
Change Rule 261A2f to read:
All structures including those below 18 m (60 ft) shall be designed
to withstand, without conductors, the extreme wind load in Rule
250C applied in any direction on the structure.
The phrase “including those below 18 m (60 ft)” is no longer
needed.
Supporting Comment
Subcommittee 5 established task force 5.1.2 of Working Group 5.1 to revisit the 60-ft
height limit for extreme winds in the 2002 NESC. Rule 250C, Extreme Wind Loading,
states:
“If no portion of a structure or its supported facilities exceeds 60 ft above ground or
water level, the provisions of this rule (Extreme Wind Loading) are not required, except
as specified in Rule 261A1c or Rule 261A2f.”
SC5 established the Working Group to make a recommendation concerning the
disposition of the 60-ft exclusion limit.
The “60-ft exclusion” was added in the 1977 edition of the NESC at the same time that
the extreme winds (50 year) were added. Extreme winds were added primarily for
instances of conductors over .9 inches in diameter used on transmission. Thus, whenever
the conductor diameter exceeded .9 in, the extreme wind case could possibly dictate the
governing transverse load within the heavy loading district. This additional loading case
enhanced the structure safety under transverse loads, particularly on EHV lines where
large diameter conductors are being employed. The 60-ft exclusion limit was added
primarily so distribution lines need only meet Light, Medium, and Heavy District loads
requirements and to keep line design simple but safe.
Summary of comments to CP 2151 for the 1997 NESC: This change proposed to
remove the 60-ft exclusion from Rule 250C. Comments from the public and from
members of the committee seem to indicate that removal of the 60-ft exemption would
not necessarily increase safety and reliability. During extreme wind events, debris is
blown into overhead line facilities (especially those under 60 ft), which has a more
dramatic affect on the line than does extreme wind. Removal of this exemption ignores
this problem while imposing a possible costly solution. Darr of Virginia Power, further
explained “...how unnecessary it is to require extreme wind load calculations for
structures of low height that are typically shielded by buildings and trees. Multi-pole
structures and structures above 60 ft in height are much more likely to be affected by
high wind loads and should be covered by this rule. The combined ice and wind loads for
each conductor plus the required overload factors provide the protection required to the
public and utility workers.” The change proposal to remove the 60-ft exclusion was
eventually voted down.
Task force 5.1.2: The task force discussed two options: 1) do not require high wind for
structures below 60 ft and Grade C construction, or 2) to require high wind but at a
reduced amount for structures below 60 ft and Grade C construction. The task force
decided on option 2.
The task force debated as to what this designated maximum wind load should be for
structures under 60 ft. The Saffir-Simpson Hurricane Scale and the Fujita Tornado
Damage scale were reviewed to help determine a designated wind load at which point
debris and objects are blown into the line. The task force realizes that this is only a rough
estimate.
These two scales seem to indicate that the wind speed that debris and trees would blow
into the line or fall on the line happens between 75 and 110 mph for hurricane loads and
73 to 112 mph for tornado loads. If one considers that the hurricane scale closely
approximates the fastest mile wind, the wind load (fastest mile) would be between 13.7
psf and 32 psf.
Because the range of the wind load is considerable, the task force decided to calibrate the
maximum load using existing requirements associated with the light loading district.
Existing light loading district loads for Grade C is the product of the load factor of 1.75
and 9 psf (2002 NESC, Table 253-1) and the load factor of 2 and 9 psf (2002 NESC,
Table 253-2). The load in the light loading district is between 15.8 for Table 253-1
and 18 psf for Table 253-2.
The task force decided to designate 15 psf as the maximum load as defined by 0.00256
V2 kz GRF for structures and facilities under 60 ft that need to be taken into account.
This 15 psf wind becomes 16.1 psf and 17.4 psf when considering the kzGRF factor for
spans less than 250 ft and between 250 and 500 ft, typical spans one might find for lines
under 60 ft in height. The wind velocity for these loads is approximately 80 mph,
within the range of 73 mph and 110 mph.
The task force considered maximum load limits for Grade B construction under 60 ft. For
Grade B construction, the task force decided to designate 30 psf as the maximum load as
defined by 0.00256 V2 kz GRF for structures and facilities under 60 ft that need to be
taken into account. This maximum load approximately corresponds to a class 3 hurricane.
The task force recognizes that there exists an inconsistency in relative strengths between
Grade B and Grade C for structures under 60 ft when considering extreme winds.
The Saffir-Simpson Hurricane Scale
http://www.nhc.noaa.gov/aboutsshs.shtml
The Saffir-Simpson Hurricane Scale is a 1–5 rating based on the hurricane's present
intensity. This is used to give an estimate of the potential property damage and flooding
expected along the coast from a hurricane landfall. Wind speed is the determining factor
in the scale, as storm surge values are highly dependent on the slope of the continental
shelf in the landfall region. Note that all winds are using the U.S. 1-minute average.
Category One Hurricane
Winds 74–95 mph (64–82 kt or 119–153 km/hr). Storm surge generally 4–5 ft above
normal. No real damage to building structures. Damage primarily to unanchored mobile
homes, shrubbery, and trees. Some damage to poorly constructed signs. Also, some
coastal road flooding and minor pier damage. Hurricanes Allison of 1995 and Danny of
1997 were Category One hurricanes at peak intensity.
Category Two Hurricane
Winds 96–110 mph (83–95 kt or 154–177 km/hr). Storm surge generally 6–8 ft above
normal. Some roofing material, door, and window damage of buildings. Considerable
damage to shrubbery and trees with some trees blown down. Considerable damage to
mobile homes, poorly constructed signs, and piers. Coastal and low-lying escape routes
flood 2–4 h before arrival of the hurricane center. Small craft in unprotected anchorages
break moorings. Hurricane Bonnie of 1998 was a Category Two hurricane when it hit the
North Carolina coast, while Hurricane Georges of 1998 was a Category Two Hurricane
when it hit the Florida Keys and the Mississippi Gulf Coast.
Category Three Hurricane
Winds 111–130 mph (96–113 kt or 178–209 km/hr). Storm surge generally 9–12 ft above
normal. Some structural damage to small residences and utility buildings with a minor
amount of curtain-wall failures. Damage to shrubbery and trees with foliage blown off
trees and large trees blown down. Mobile homes and poorly constructed signs are
destroyed. Low-lying escape routes are cut by rising water 3–5 h before arrival of the
center of the hurricane. Flooding near the coast destroys smaller structures with larger
structures damaged by battering from floating debris. Terrain continuously lower than 5
ft above mean sea level may be flooded inland 8 mi (13 km) or more. Evacuation of lowlying residences with several blocks of the shoreline may be required. Hurricanes
Roxanne of 1995 and Fran of 1996 were Category Three hurricanes at landfall on the
Yucatan Peninsula of Mexico and in North Carolina, respectively.
Category Four Hurricane
Winds 131–155 mph (114–135 kt or 210–249 km/hr). Storm surge generally 13–18 ft
above normal. More extensive curtain-wall failures with some complete roof structure
failures on small residences. Shrubs, trees, and all signs are blown down. Complete
destruction of mobile homes. Extensive damage to doors and windows. Low-lying escape
routes may be cut by rising water 3-5 h before arrival of the center of the hurricane.
Major damage to lower floors of structures near the shore. Terrain lower than 10 ft above
sea level may be flooded requiring massive evacuation of residential areas as far inland as
6 mi (10 km). Hurricane Luis of 1995 was a Category Four hurricane while moving over
the Leeward Islands. Hurricanes Felix and Opal of 1995 also reached Category Four
status at peak intensity.
Category Five Hurricane
Winds greater than 155 mph (135 kt or 249 km/hr). Storm surge generally greater than 18
ft above normal. Complete roof failure on many residences and industrial buildings.
Some complete building failures with small utility buildings blown over or away. All
shrubs, trees, and signs blown down. Complete destruction of mobile homes. Severe and
extensive window and door damage. Low-lying escape routes are cut by rising water 3–5
h before arrival of the center of the hurricane. Major damage to lower floors of all
structures located less than 15 ft above sea level and within 500 yards of the shoreline.
Massive evacuation of residential areas on low ground within 5–10 mi (8–16 km) of the
shoreline may be required. Hurricane Mitch of 1998 was a Category Five hurricane at
peak intensity over the western Caribbean. Hurricane Gilbert of 1988 was a Category
Five hurricane at peak intensity and is one of the strongest Atlantic tropical cyclone of
record.
Fujita Tornado Damage Scale - Refer to PDF page 394-395 for scale
NESC Task Force 5.1.2 - Refer to PDF page 395 for list.
Subcommittee 5 Recommendation
Accept as modified.
Subcommittee 5 Comment
Remove the 60-ft exclusion from Grade B and Grade C construction, and show a
maximum wind load for Grade B and Grade C construction under 60 ft for Tables 253-1
and Tables 253-2. Minor formatting changes of these tables are also suggested so the
wording “Vertical Loads” will line up with “Transverse Loads” and “Longitudinal
Loads.” This change proposal is based on a previously submitted change proposal which
provides appropriate load factors for Grade C construction from Grade B construction
under 250C wind loads (extreme wind). The proposed changes are shown below:
Refer to PDF page 396-397 for tables listed below.


Table 253-1—Overload Factors for Structures,1 Crossarms, Support
Hardware, Guys, Foundations, and Anchors to Be Used with the
Strength Factors of Table 261-1A
Table 253-2—Alternate Overload Factors for Wood and Reinforced
(Not Prestressed) Concrete Structures1, 5 to Be Used with the
Strength Factors of Table 261-1B
Additional changes:
The following additional rules need to be changed to accommodate the above change:
1. Rule 250A.1:
250.
General Loading Requirements and Maps
A.
General
1.
It is necessary to assume the wind and ice loads that may occur on
a line. Two weather loadings are specified in Rules 250B and
250C. Where both rules apply, The required loading shall be the
one that has the greater effect.
The removal of the 60-ft exclusion makes the wording “where both rules apply” no
longer applicable. Both loading conditions apply at all times.
2. Rule 250C:
C.
Extreme Wind Loading
If no potion of a structure or its supported facilities exceeds 18m (60 ft) above
ground or water level,the provisions of this rule are not required except as
specified in Rule 261A1c or Rule 261A2f. Where a structure of its supported
facilities exceeds 18 m (60 ft) above ground or water level the Structures
and its their supported facilities shall be designed to withstand the extreme wind
load associated with the Basic Wind Speed, as specified by Figure 250-2.
Reference to the 60-ft exclusion is no longer necessary.
3. Delete the last sentence to Rule 250C.2:
Wire attachment points that are 18m (60 ft) or less above ground or water level must be
considered if the total structure height is greater than 18m (60 ft) above ground or water.
This sentence is moved to footnote 8 of Table 253-1 and footnote 9 of Table 253-2.
4. At the bottom of Tables 250-2, in the equations, replace the lower limits of 18 m and
60 ft with 10 m and 33 ft.
Reference to these heights are no longer needed.
5. Change Rule 261.A.1.c to read:
All structures including those below 18 m (60 ft) shall be designed to withstand, without
conductors, the extreme wind load in Rule 250C applied in any direction on the structure.
The phrase “including those below 18 m (60 ft)” is no longer needed.
6. Change Rule 261.A.2.f to read:
All structures including those below 18 m (60 ft) shall be designed to withstand, without
conductors, the extreme wind load in Rule 250C applied in any direction on the structure.
The phrase “including those below 18 m (60 ft)” is no longer needed.
Vote on Subcommittee 5 Recommendation
Affirmative: (14) Aichinger, Bingel, Clem, Denbrock, DeSantis, Freimark, Harrel, Heald,
Kempner, Kinghorn, Peters, Rempe, Slavin, Standford
Negative: (7) Amato, Bullinger, Clapp, Hensel, Kluge, Shultz, Wong
Abstention: (0)
Explanation of Vote
Amato: I do not believe there is sufficient technical justification to remove the 60-ft
exclusion.
Bullinger: 60-ft exclusion removal is not justified by data. Anecdotes are insufficient to
justify the possible unintended consequences that could result. It is not clear that the 60-ft
exclusion removal will improve safety. The calibration back to past practice is fraught
with possible unintended consequences as illustrated by the discussions of the committee.
Clapp: I understand the desire to show a relative 2/3 relationship between Grade C and
Grade B, but I think it should be achieved by raising the 15 psf limit to 20 psf (i.e., use 20
psf/30 psf) rather than reducing the 30 psf to 22.5 psf (i.e., 15/22.5).
Hensel: The new RBD Manual will apparently recommend removing the 60-ft exemption
from extreme wind and reducing the load factor for Grade C <= 60 ft to 0.5. I don’t think
we adequately considered the new RBD Manual recommendation. I think we are being
more restrictive in non-hurricane areas than is being called for on the RBD Manual.
Shultz: Removing the 60-ft exemption unilaterally will likely result in considerable
increase in strength requirements in many loading situations, particularly in coastal areas.
The public comments on a similar proposal for the 2002 revision cycle overwhelmingly
cited collateral loading as the predominant cause of extreme wind failures. This proposal
offers nothing to refute those statements of experience. In addition, in the 2002 cycle,
several utilities in the South reviewed their records to see what claims for injury could be
attributed to extreme wind failures. Those utilities could not identify any such claims for
the previous several years. If this experience is anywhere near typical, the safety record is
very good, and no support is provided in this proposal to demonstrate that removal of the
exemption will improve on this safety performance.
Wong: There is a difference in the wind behaviors between continental wind and
hurricane wind. I support CP 2798 over CP 2766.
Kluge: This CP should be rejected for the following reasons:
1. The “not to exceed” limits are set too high as illustrated in the following example for a
typical distribution pole. By having the limits set so high, the proposal inadequately
responds to the comments from the users that distribution structures are not damaged by
wind directly but by wind blown debris. To require structures to be designed to wind
speeds that surrounding buildings and trees cannot withstand is not justified.
2. If the “not to exceed” limits are set too high, the designer is frequently directed to use
the formulas. This is especially true for communication companies that occupy the lowest
attachment height on a pole. However, most distribution and communication designers
will find the NESC formulas burdensome.
The following table further illustrates why I find CP 2766 as modified unacceptable. The
following table compares the wind pressures calculated using NESC Rule 250C wind
formulas to the “not to exceed” limits of CP 2766 as modified. Note that the “need not
exceed” limits seldom apply—controlling values are shown in bold text. Note also the
numerous pressure values for the phase, neutral, and communication wires and the
structure. These calculations were done for a typical 40-ft distribution pole, buried 6 ft,
with phase conductors at the top, neutral conductors 8 ft below the phases, and
communication cables located 3-1/2 ft below the neutral.
Wind Pressure (psf) on Conductors as proposed in CP 2766 – see pdf pg
400-401
If the subcommittee is determined to eliminate the 60-ft exemption, here are two
modifications that would make this CP more acceptable—either reduce the “not to
exceed” limit or offer a single constant to use in the formula to convert wind speed to
pressure. Both would accomplish the objective of making the design easier and could be
calibrated to recognize the experience of the utilities.
The concept of a single constant to use in the wind speed-pressure formulas has the
advantage of providing a custom wind-pressure for each wind speed location as explained
below. (See also CP 2718.) For structures 60 ft tall or shorter, the following values for kz
· GRF · I would be appropriate. (These values were determined from the NESC formulas
for a typical distribution pole as defined above.)
(The above factors are intended for Grade B. To obtain Grade C factors, CP 2766
recommends using 87% of the Grade B factors.)
Using the above factors for kz · GRF · I defines two wind pressure values (one for the
wires and a second one for the pole) for structures 60 ft or less in height, within a
particular wind speed region.
(The values listed are only for Grade B. To obtain Grade C factors, CP 2766 recommends
using 87% of the Grade B factors. Not to exceed limits could still be applied.)
For short structures, it may be reasonable to use a single kz · GRF · I value for wires and
structures of 0.80. This produces only one pressure value to consider for each wind
speed region or grade of construction, as follows:
These are the calibrated design wind pressures CP 2766 should target for structures 60 ft
or less in height. CP 2766 does not achieve these target wind pressures. Furthermore,
because CP 2766 sets the not to exceed limits so high, it would burden the utilities to use
complex formulas to calculate specific pressures for each wire at attachment height.
Therefore, I have voted to reject CP 2766 as modified.
If you would like to perform calibrations for your typical structures and geographic
location, I would gladly e-mail you a copy of an Excel spreadsheet with the wind speedpressure formulas embedded. Just send me an e-mail at rkluge@atcllc.com and ask for
the NESC wind computation sheet. Your opinions of these proposals are extremely
valuable to the NESC Subcommittee.
For your reference, there are several change proposals addressing wind load—CPs 2673,
2739, 2766, 2798, 2718, 2787.