4 Sided Open Roof Top Signage Wind Loading

[rlmhawk]

I have a project that involves placing a 4 sided sign, open top and bottom, onto a building. It forms a 10'x 10' cube, is located at a building corner, and slightly overhangs a 3 foot high parapet on one side. The building it is situated upon is a 5 story hotel (<60 ft mean roof height). The question is: what wind load procedure would you apply to it per ASCE 7-10? Do you analyse it as a roof top structure, a parapet (extension), a free standing sign - something else?

The open nature of it, as well as its overhanging condition is a bit challenging. It is one of those conditions that is rather unusual and not addresses specifically in the ASCE 7 code (no great surprise there). My current approach is to look at it using each ASCE 7 design case, then design it and its supports for the most conservative values, however, that really doesn't answer my question of how it "should be" viewed for design. Its seems pretty subjective. I am curious as to how others would view this?

 

[thaidavid40]

If you want to be most conservative (and I would be tempted to be so, given the conditions you described) you could analyze by the directional method, and treat it as a C&C load.
Dave

Thaidavid

 

[JAE]

Not a sign as I think that is based more on a flat surface near the ground.

For your case I think the rooftop structure or an extended parapet would get you close to "reality".

I would guess that the technically correct answer is that if ASCE 7 doesn't directly address a condition you should look at wind tunnel testing.

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[Thawkes]

We run into this case all the time where I work. We have developed some rules of thumb for when to design as rooftop equipment versus as a parapet. In our collective opinion, the proximity to the edge and the aspect ratio (height vs. width) affect the design approach. In this particular case, I would apply parapet pressure to the two walls at the corner of the building. If the "cube" is designed to behave as a unit, I would also apply some pressure (not sure what, but less) to the other two walls at the same time.

I agree with JAE that this is not a case for sign loading. I disagree that wind tunnel testing should be performed. It certainly could be, but I don't believe that wind tunnel testing is required if a structure doesn't exactly fit into any of the categories defined in ASCE 7. For a small building appurtenance like this, nobody is likely to want to pay for a wind tunnel test. That would be beyond industry standards in my experience.

 

[rlmhawk]

Thanks for the responses everyone. I am still uncertain as to what the "true" load should be. But I will error on the high end.

thaidavid40: I don't want to be too conservative on my approach here, and I think C&C might push me into that area. I didn't mention it, but this is a wood building (4 levels of wood over podium). Too conservative (high) a loading will have unwanted impacts.

Thawkes: I can confirm your thoughts regarding wind tunnel testing. The owner would never pay for it. And i agree the true load is likely somewhere in between a rooftop structure and a parapet, as you mention.

As an aside note here, this particular hotel chain has these cubes on all their building in Europe. However, each example of these hotels, say about a dozen, shown to us by the owner, were all of concrete and steel construction - no wood. I would have no problem really loading up this cube with either steel or concrete construction. The wood part is really the crux of my loading issue.

 

[JAE]

I did say "technically correct way..." so I agree with Thawkes too that a tunnel test is too much for this situation.

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