NBCC Commentary I: Light tower wind loads

[wadavis]

Greeting eng-tips forums,

I am working on designing a 18m cantilever pole light tower according the the National Building Code of Canada, using Commentary I and the dynamic procedures.

The details are as follows:
Wind pressure, q: 0.28 kPa
Height, H: 18.25 m
Width ave, w: 0.4 m
Natural Freq, fn: 0.045 Hz (EI~ 67*10^12 Nmm2, p~ 105 kg/m)
Critical Damping, B: 0.5%

Working through paragraphs 46 to 56 of Commentary I, I have come up with a Cg (gust factor) of 9.4. This value struck me as being very high, is anyone familiar with this piece of code and willing to chip in two cents?

Also, if you are familiar with the code, is figure I-24 of the Commentaries intended for this type of design and can you point out any recommended reading to understand their use of Cf and Cp separably?

Regards,
wadavis

 

[dik]

Can you plug the numbers in to this SMathStudio file? I'll attach a *.pdf so you can give me the values if you don't have SMath...

Dik

 

[dik]

and pdf...

 

[dik]

I've used the numbers in the example... SMathStudio is a freeware program similar to MathCAD... not quite as many bells and whistles..

Dik

 

[JedClampett]

I just took a RedVector course on pole design to pad my pdhs for license renewal. In an example for a 45 ft tall pole, the Gust Response Factor was .948. They use NESC design criteria, but I suspect they're pretty close to Canadian Codes. The calculation is a pain in the posterior, and I think I need more information than you supplied to use the NESC method. But it seems that you made a mistake if it's that high.

 

[wadavis]

dik,
Attached is the SMathStudio file with the numbers plugged.
For Y I'm using =39.2*SQRT(0.28 kPa)
wadavis

 

[dik]

I just looked at your last number and if the other data is correct, it may be that the approach is not correct for some applications. I couldn't see any exclusions last night, but will take a gander at the commentary this evening. A gust factor of 9 seems to be excessive and the commentary may have to have some exclusions...

Dik

 

[TLHS]

I'm pretty sure there's some stuff about how to calculate loads on light standards in the highway bridge code, CSA S6. I think it was just a couple of paragraphs though, so it might not be worth it if you don't already have it.

 

[wadavis]

Thanks TLHS,
I see that CSA S6 recommends a Cg of 2.5 for lighter structures / slender structural elements / light standards. That looks like the similar approach Commentary I took with the static analysis of structural components.

It is a degree of confidence for using the less conservative method.