Transmission Line Structures

[abusementpark]

I have a couple questions about transmission line structures:

ASCE publishes a load manual for transmission line structures, ASCE 74, which from what I gather, is not a legal minimum code document. In ASCE 74, it alludes to the National Electric Safety Code (NESC) as being more of a code book. Does anyone know what is considered to be the legal minimum for loads on transmission line structures (particularly wind loads)? Do different jurisdictions have different requirements? Or is it left up to the utility company since transmission line structures don't impose life safety concerns on civilians in the event of their failure?

For the wind loads on the trussed towers that are typical of transmission line structures, ASCE 74 references use of the provisions on Fig 6-23 in ASCE 7-05. However, Mr. Leon Kempner Jr. recently published a paper comparing the wind loads on transmission line towers from both NESC and ASCE 74, and he showed that the NESC provisions yield much greater forces on the towers . I don't understand this. If NESC is the legal minimum (which I suspect it is, but I'm not sure), than wouldn't the provisions of ASCE 74 be moot since NESC will always yield more stringent results?

Any insight would be greatly appreciated. Thanks.

 

[paddingtongreen]

Doesn't IBC list incorporated codes and specifications?

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Ron]

The only one that I know of that is referenced in the IBC is ANSI/TIA/EIA 222G. The whole 222 series is being revised, I believe. ASCE 74 references this.

I don't know of a code mandate for specific loading on the structures other than to use ASCE 7 as applied to any structure.

 

[paddingtongreen]

Back when I was working transmission lines, BOCA listed NESC as the code for transmission and distribution lines.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[westheimer1234]

NESC got overload factors for concurrent wind and ice, extreme wind, extreme ice, nesc heavy

 

[SlideRuleEra]

...is it left up to the utility company...

Typically, yes. The structures are specialized and often site-specific. The utility company will have it's own requirements, based on good engineering practice and accepted code standards.

Do different jurisdictions have different requirements?

Again, in most cases each utility company will have it's own requirements, based on conditions in their service territory.

...what is considered to be the legal minimum for loads on transmission line structures (particularly wind loads)?

The utility may have a small number of generic requirement if the conditions in their territory vary. In SC, for example, design wind speed (per ASCE 7) drops dramatically as distance from the coast increases. Higher rated structures are used, on the same transmission line, as it approaches the coast. Economics plays a big factor. A utility company has hundreds or thousands of these structures. Significant cost savings result by not over designing them. With that said, the fact that they are geographically dispersed minimizes the the probability that ALL or MOST be taken down by a single event.

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