ASCE 7-05 Section 6.5.15.1 Roof top structures and equipment 3

[Jeng79]

I came accross this section determinining a wind load for a duct that we will be mounting on roof top. There is no discussion in the commentary for this section.

Simply reading the section it appears that we need to bassically double the wind load for the duct. Our project is in a 130 mph zone on the coast of texas. My preliminary calculations show that I need to design the duct support framing and check the existing roof joists for a wind load on the duct of 75 psf, which seam a little high.

I have discussed it with a couple of engineers in our office and no one seams to have heard of the section. I was wondering if anyone has used it or has any thoughts on why it is necessary.

Thanks for help in advance and I am looking forward for any kind of justification for the factor.

 

[jike]

There is some discussion on page 300 Commentary ASCE 7-05. Perhaps Ref C6-99 has more info.

 

[UcfSE]

This is because the gust factor for the equipment is more like 1.6 instead of 0.85, based on tests for pressures on roof equipment. To account for the use of 0.85 earlier in the pressure calculation, you multiply the result by 1.9 (1.6/0.85 is about 1.9).

 

[structuresguy]

Yes, the 2005 ASCE7 wind pressure for rooftop equipment is much higher than before, basically double as you said. This has really made a big impact on the selection of rooftop mechanical units, as many of the "off the shelf" units are not rated for such high wind pressures. Our mechanical engineers really have dropped the ball on this, as they had no idea until we told them, and now they are scrambling to find suppliers that can meet these criteria. We show a special line item on our wind loading just for rooftop equipment, and also get our mechanical guys to show the same wind pressure on their drawings, just to make sure no one can claim that they didn't see it.