ASCE 7-05, 7.8 Roof Projections

[BSVBD]

ASCE 7-05, 7.8 Roof Projections, says, "If the side of a roof projection is less than 15 ft. (4.6 m) long, a drift load is not required to be applied on that side."

See attached... [URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1480959017/tips/ASCE_7-05_7.8_Roof_Projections_wfubpp.pdf[/url]

I have a 450' x 330' roof with (30) Roof Top Units (RTU's) that are each 7'-4" x 14'-3" in plan dimension; 5'-0" high. The plan dimensions create a 16'-0" diagonal dimension, which, when viewed from a direction normal to the diagonal, would, thus, prompt the greater than 15'-0" requirement.

These RTU's are evenly and uniformly spaced 50', 75' or 100' o/c.

My typical practice would be to employ the conservative, safe route of applying the drift loads to each of these RTU's, even though each "side" is less than 15'-0". HOWEVER, I do NOT want to over-design and unnecessarily raise the cost. YET... Safety!

1. Would any of you consider the 15'-0" diagonal a justifiable "side"?

2. Is there a(n unpublished) concern for the multiple units which might induce the slightest turbulence or tunneling effects as implied by ASCE 7-05, 6.5.15.1 Rooftop Equipment, thereby justifying the snow drift application?

3. Would any of you ignore the drifts around the RTU's altogether?

Suggestions?

Thank you!

 

[KootK]

I haven't been considering the diagonal but, frankly, that's just because I haven't been sharp enough to think of it. I feel that the aerodynamics of wind at the diagonal would reduce deposit some but I can supply no quantitative basis for that.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

The text of the ASCE 7 code refers to the "side" of the projection.

I don't think taking the diagonal is what was intended here.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[BSVBD]

Koot & JAE...

Thank you for your replies.

What about Question 2?

2. Is there a(n unpublished) concern for the multiple units which might induce the slightest turbulence or tunneling effects as implied by ASCE 7-05, 6.5.15.1 Rooftop Equipment, thereby justifying the snow drift application?

Thanks again!

 

[JAE]

Not that I'm aware of.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[KootK]

Spaced very tightly, I'd be concerned that multiple units might start to present like larger single units aerodynamically. Your spacing is nowhere near that close in my opinion however. Heck, some tunnelling might actually reduce drift. It is, after all, the slow moving air that deposits the snow, just like silt in a river.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

You might read through the ASCE 7 commentary on this "roof projection" section - they talk a bit about closely spaced solar panels but it doesn't really directly respond to your question on your RTU's

Check out Eng-Tips Forum's Policies here:
faq731-376