Extreme Wind Loading CP2766

2. NESC 2007 Rule 250 C Extreme Wind Loading If no portion of a structure or its supported facilities exceeds 18 m (60 ft) above ground or water level, the provisions of this rule are not required, except as specified by the addition in Rule 261 A1c, 261A2e, or 261 A3d .....

3. NESC 2007 Section 261 Grades B and C Construction All structures including those below 18 m (60 ft) shall be designed to withstand, without conductor, the extreme wind load in Rule 250 C.....

4. NESC 2007 on Extreme Wind Above Ground Height Over 60 ft. All structures are required to design for Extreme Wind Load. Above Ground Height Under 60 ft. All structures are required to design for Extreme Wind Load on structure only, no conductor.

5. NESC Change Proposal 2766 Remove the 60 ft. Exemption. Delete �If no portion of a structure or its supported facilities exceeds 18 m (60 ft) above ground or water level, the provisions of this rule are not required, except ......� All structures are required (under CP 2766) to check �Extreme Wind�, �Combined Ice and Wind�, and �Extreme Ice & Concurrent Wind� loading criteria.

6. NESC Change Proposal 2766 For structures less than 60 ft. high, the extreme wind pressure needs not be designed above a set of pre-determined limits. There are a few other change proposals discussed related issues CP 2673 Elimination of 60 ft exemption CP 2798 Elimination of 60 ft exemption CP 2707 Maximum Conductor Tension CP 2802 Extreme Ice & concurrent wind CP 2739 Load Factors

7. CP 2739 Proposed Extreme Wind Load Factors for Grade B and Grade C

8. History of 60 ft Exclusion Limits It was first included in the 1977 edition of the NESC when high winds (fastest-mile, 2% annual probability) were added to the loads in Rule 250 SC 5 considered a Change Proposal to remove this exclusion for the 2002 edition of the code SC 5 decided to keep the exclusion for 2002 but established a task force to revisit the issue

9. Why did SC 5 want to revisit the 60 foot exclusion limits issue ? The Code needs to recognize that wind blows below 60 feet The Code is moving more to actual loads rather than legislative loads (light, medium, heavy loading districts) The Code sacrifices some credibility when other national codes which recognize that wind blows below 60 feet Other �

10. 4th Edition (1926) of the NESC HEAVY LOADING DISTRICT1/2 inch ice, 8 psf wind MEDIUM LOADING DISTRICT1/4 inch ice, 8 psf wind LIGHT LOADING DISTRICTno ice, 12 psf wind No constant is be added to conductor loads for calculating sag and tension

11. WHAT DOES THIS MEAN? GRADE B wood��50 % of MOR, the equivalent wind pressure for light loading is 12/.5 or 24 psf GRADE C wood��75 % of MOR, the equivalent wind pressure for light loading is 12/.75 or 15 psf ** Ultimate fiber stress of wood pine poles is 6500 psi vs. today�s 8000 psi 4th Edition of the NESC

12. Recall in the 4th Edition of the NESC Light Loading District GRADE B wood��50 % of MOR, the equivalent wind pressure for light loading is 12/.5 or 24 psf GRADE C wood��75 % of MOR, the equivalent wind pressure for light loading is 12/.75 or 15 psf Medium Loading District GRADE B wood��50 % of MOR the equivalent wind pressure for medium loading varies with conductor size between 36 and 20 psf GRADE C wood��75 % of MOR the equivalent wind pressure for medium loading varies with conductor size between 24 and 14 psf (Ultimate fiber stress of wood pine poles is 6500 psi vs. today�s 8000 psi Heavy Loading District GRADE B wood���50 % of MOR the equivalent wind pressure for heavy loading varies with conductor size between 56 and 25 psf GRADE C wood���75 % of MOR the equivalent wind pressure for heavy loading varies with conductor size between 37.5 and 17 psf (Ultimate fiber stress of wood pine poles is 6500 psi vs. today�s 8000 psi

13. Comparison of the 4th and 2002 Edition of the NESC for Wood

14. CP 2766 (as amended) to NESC 2007 Remove 60 foot exclusion and for wires under 60 feet the wind pressure need not exceed : For Grade B����22.5 psf For Grade C����15.0 psf

15. CP 2798 to NESC 2007 Remove 60 foot exclusion and for wires under 60 feet the wind pressure need not exceed : For Grade B����30 psf For Grade C����15 psf for wind speed locations that are greater than 90 mph For Grade C�..��.10 psf for wind speeds locations that are less than 90 mph

16. For most parts of the country, where extreme wind velocity is below 90 mph, and for structures under 60 feet, the cap of 20 psf has little or no affect for Grade B construction. For areas in which the wind speed is greater than 90 mph, the structures and facilities under 60 feet may be under-designed, but will be safe up to the point that blowing debris and fallen debris cause the line to de-energize or fail. Utilities may design for higher loads for survivability, but the additional strength may not add to the overall safety to the public and to linemen.

17. NESC Change Proposal 2766 Supportive Comments In the past, with smaller wires and relatively small communication cables, the high wind were not much of a problem. The use of larger size conductors and communication cables in recent years makes the high wind a controlling design criteria. Typical structures are taller, usually exceed the height which instruments are mounted to measure wind speed.

18. SC 5 Comments on CP 2766 Supportive Comments All Structures (including structural height less than 60 ft.) experience extreme wind loads. The control loading shall be the one that has the greatest effect. Removal of 60 ft. limit increases structural reliability and public safety. The �not to exceed limits� recommended are in agreement with historical performances and meteorological observations

19. SC 5 Comments on CP 2766 Non-Supportive Comments Most structural fails due to debris and tornadic activity during extreme wind events. The removal of 60 ft. limit does not address these problems and is not likely to increase public safety. Dramatic increase in loading does not appear to be reasonable.

20. Non-Supportive Comments The �not to exceed limits� recommended are too high. Thus, designers are forced to use the more complicated formula. The �not to exceed limits� recommended are too low and would permit unsafe design. A technically sound method should be developed to deal with lower profile wind loads before the removal of 60 ft. limit. SC 5 Comments on CP 2766

21. The major reason why people think the removal of the 60 foot exclusion will not add to safety is the fact that at high winds, blowing debris and overturning trees damage the lines and cause safety problems --- not wind on the conductors and structures.

23. Reasons to Change the Code* Specific information has been provided about accidents where the designer used the code, but an accident occurred because of inadequate safety code values For clarification of the intent of a rule To make the rule simpler and easier to apply and understand by the field user For consistency in safety values from one part of the code to the other that lends credibility so that it makes safety sense to the user *Dana Hale, Maine Public Service Company