Gust effect factor - flexible structure on a building 1

[Kom3]

Hello all,

I have a flexible sign that will go on top of a building. I need to determine the wind force on the sign to design the sign supports and anchorage. Please correct me if I am wrong but in such a case, I would compute the gust effect factor of the sign structure and not that of the combined structure "building+sign". So my question is, how do I go about determining the gust effect factor of only the sign? I have RISA 3D software but do I only model the sign? Should I somehow account for the building's flexibility?

Note that the sign is very small when compared to the building.

Thanks,

 

[Rabbit12]

I'd look at the sign as if the building isn't there. Use the flexibility of the sign support structure to calculate your gust factor. For the height, use the actual height from grade of the sign as that will effect the pressures. Big difference being 10' off the ground vs. 100'.

 

[RWW0002]

For what its worth, the AASHTO signs and Lunimaries specification recommends a gust effect factor (G) of 1.14 unless a more rigorous approach is used to determine G. (Wind loads in this AASHTO guide are similar to ASCE7 loading). I would recommend this as a lower bound for your case.

 

[azcats]

While we weren't given much (any) info about the building or the sign, I disagree with using 1.14 as a lower bound for the gust effect. It is highly unlikely that a sign atop a building is anywhere near as flexible as a light fixture atop a slender pole. If the system is rigid, that's a 1/3 bump in your wind loads over the G=0.85.

Agree with Rabbit12 on the height. Although I'd consider the building flexibility using ASCE 12.8.2 as a rough guide although it tends to under-estimate periods for seismic consideration.

Really need some sense of scale here. If this is a 8' high sign atop a 15' high CMU building I'd call it rigid and use G=0.85.

I will ETA to ask how you know it's flexible as stated in the title. Does the sign structure really have a fundamental period over 1? Seems odd for a 'tiny' sign.

 

[RWW0002]

This is a bit of an aside, but I think many engineers simply uses G of 0.85 across the board simply because G is "hard" to compute. I think it is a mistake for code writers to make a more rigorous computation result in a more conservative design. In general the code should yield more rigor = less conservative (i.e. if you sharpen your pencil a bit you can whittle a bit more out of your structure)

In the case of the gust effect factor it goes something like this for most engineers.
Hmm is my structure seems flexible, lets look at the equations..
-OK so G=0.925*[(1+1.7*Iz*SQRT(gQ2Q2+gR2R2))/(1+1.7gvIz)].. and
gR = SQRT(2*ln(3600n1))+0.577/SQRT(2*ln(3600n1))]..
OK this is getting too complicated.
Do we have a software that does this??
No?
What do you guys usually do?
You just use G=0.85? Even for something pretty flexible?
OK well if that's how we've always done it so be it

G=0.85 by inspection

For the case above we are told it is a flexible sign. Who knows how "flexible" but G=1.14 seems like a better "wild guess" than G=0.85. At least this way the engineer is rewarded for plowing through the G equations if it ends up being less.

 

[JStephen]

Some of the parameters in that G calculation deal with the length of the structure in the direction of the wind, so neglecting the building won't necessarily give you a better answer.
Meanwhile, there's a 1.9 increase factor for rooftop equipment, and using that would seem more like the right approach.

 

[azcats]

Yeah - my post was distracted and bouncing around. More than anything I question the assumption that a rooftop sign is flexible - certainly not asserting that if flexible he should use 0.85. I didn't even notice that it was defined as flexible until I edited it later - my mistake for being hasty and having poor reading comprehension and if my disagreement with your post came across poorly.

I find this part of the code frustrating. Some of my structures bounce between the code definition of 'rigid' and 'flexible' and the code formulas for G don't converge anywhere near 0.85. Agree with: 'why the penalty for working thru those formulae?'

 

[RWW0002]

I agree with you az, both in questioning whether a sign structure is in fact flexible and in frustration on that portion of the code. No offense taken on the disagreement, I apologize for the rant. this issue is a pet peeve of mine.

The 1.9 for Rooftop structures has been modified a bit from ASCE7-05, but still may be a good solution. In ASCE7-05 is was a 1.9 factor of the calculated wind pressure. In ASCE7-10 GCr=1.9 instead of a 1.9 factor. As long as it is not a huge really flexible sign, I think Gcr=1.9 could be justified in which case G would not need to be calculated.

Again, it really comes down to the actual proportions of the sign and the building. Except for overall height of structure for qh, I do not see the building (or its flexibility) making much of a difference in the sign wind calculation for attachment.