Strengths and Loadings, Sections 25-27
NESC Subcommittee 5, Strengths and Loadings, WG 5.2, is continuing its efforts to develop a complete re-write of the strength and loading sections (Sections 25–27). The corresponding change proposal (CP 2372-revised) addresses the 2007 Edition of the NESC. The change proposal will attempt to introduce this rewrite as (1) a replacement to the respective sections in the 2002 Edition, or (2) an alternative method in the 2007 Edition. Since an important part of this revision would incorporate an ice (plus concurrent wind) map based upon a 50-year recurrence interval (ASCE 7 map), the radial ice indicated would be significantly greater than that presently specified by the Loading Districts in the 2002 and earlier editions. Furthermore, since the clearance rules of Section 23 are presently based upon conductor sag and tension under the same ice loads specified in the loading map in Section 25, the resultant sags may be considerably increased with the inclusion of the ASCE 7 ice/wind map, severely aggravating the ability of the utilities to meet minimum clearance requirements.
Subcommittee 4, Overhead lines – Clearances, established WG 4.10, to work in cooperation with WG 5.2, to investigate the corresponding impact on sag and clearance issues, and to develop appropriate change proposals compatible with the possible introduction of the new method. It is noted that it is likely that other change proposals will be submitted to Subcommittee 5 that will independently propose the incorporation of the new ASCE 7 ice/wind map.
Incorporation of the new ASCE 7 ice/wind map would have a major impact on sag and clearance issues for both distribution and transmission facilities, depending upon the geographic area, span length, and conductor. Several possible options were discussed for addressing this critical issue within WG 4.10 and Subcommittee 4.
60-Foot Exclusion
Subcommittee 5, Strengths and Loadings, established working group 5.8 to revisit the 60-foot 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."
The original change proposal to the 1997 edition of the code was to remove the 60-foot exclusion from Rule 250C. Comments from the public and from members of the committee seem to indicate that removal of the 60-foot exemption would not necessarily increase safety and reliability. During extreme wind events, debris is blown into overhead line facilities (especially those under 60 feet), which has a more dramatic effect on the line than does extreme wind. Removal of this exemption ignores this problem while imposing a possible costly solution. However, the subcommittee recognizes that wind blows below 60 feet and has asked this working group to develop a position that would accommodate both opinions for the 2007 edition of the NESC.
If you would like more information or have any questions, please contact Donald Heald, Structural Engineer, Transmission Branch, at 202-720-9102 or at dheald@rus.usda.gov.