Wind Load on Sign on Roof (~170ft High)

[caseypoi23]

hey guys. i'm having to check an existing roof top sign. the sign is anchored on the roof structure. via a braced frame type of system. the height of the roof is about 170', and the sign is an addition 30' high or so. i'm trying to nail down the wind loads from asce 7-10, but having a hard time finding specifically what i can use. chapter 30 is of no use as the only items there are for 1) parapets, 2) overhangs, or 3) structures with a height less than 60'

any thoughts? thanks in advance.

 

[azcats]

Chapter 29 & Figure 29.4-1?

 

[caseypoi23]

hi azcats.

thanks for the response, but my understanding is that figure is for a 'free standing' sign, which i don't think is my case. i could be very wrong however. my understanding of free stand is as though the sign is anchored by itself to grade or otherwise isolated. i figure my sign in question is a component of the building, which makes me think of using the C&C values, but i'm not sure how to apply them.

 

[azcats]

Fair enough. It's certainly a rooftop structure, so how about Figure 29.5-1 using square and wind normal to face?

I'd consider looking at both and using the more conservative. The Cf values appear fairly similar.

 

[caseypoi23]

ah, that one looks viable too. i only worry that chapter 29 is specifically related to designs for the MWFRS (i.e., my building is of concrete shear walls), and believe loads from figures within are not for C&C designs (i.e, anchorages, elements, etc.). but that's a good point to just compare the two. thanks again, by the way.

 

[azcats]

I see your concern, but to be fair, this is the only Chapter that really references signs. Maybe the C&C police haven't caught up with signs yet?

If you take a closer look at the freestanding sign figure, you'll see some pretty high Cf values in the 'Case C' design area of the figure that are probably in line with C&C loading. Those are the loads I use when designing individual components on signs.

 

[thaidavid40]

Maybe I am misunderstanding you situation, and your question. However, I was under the impression that "freestanding" simply meant supported as an individual structure (such as a sign mounted on a vertical frame attached at its base to a rooftop), as opposed to a sign mounted on a wall, or other flat surface, of which it is not an integral part. I think that the "freestanding" sign provisions would apply in your case.
Dave

Thaidavid

 

[SAIL3]

I would use a drag coef of somewhere between a free standing sign & parapet...probably a Cf = 2.5....I assume the values for a parapet take into account the affects of the oscilation of the bldg..

 

[JedClampett]

I wouldn't be that worried about using the MWFRS wind loads. After all, for the sign the frame is the Main Wind Force Resisting System. For the remaining items, if you're nervous, just double the coefficients or something.

 

[Hokie93]

I think it is appropriate to use Section 29.5 and Equation (29.5-1) for your situation. Basically, the rooftop sign would be treated as "rooftop equipment". Section 29.5 specifically applies for rooftop equipment where the mean roof height (h) is greater than 60 feet. Alternatively, I think you could apply the freestanding sign provisions of Section 29.4.

 

[caseypoi23]

thanks all for the feedback and thoughts. i think i will compare the values from 29.4 and 29.5 and go with the more conservative values. thanks again.