Steel OMF in SDC D

[karicow]

ASCE 7-10 table 12.2-1 says steel ordinary moment frames are not permitted in seismic design category 'D' and higher, with the footnote 'i' leading you to section 12.2.5.6. That section does allow the use of OMF in SDC 'D' for a single story building up to a height of 65 ft "where the dead load supported by and tributary to the roof does not exceed 20 psf...". I am working on a small, rectangular building addition to an existing structure (structurally independent) that is only 24' tall in SDC 'D'. In general, my roof dead load is lower than 20 psf (typically around 15 psf). However, I know we will be supporting a few rooftop units to serve the space below. I am waiting on some information from the mechanical engineer regarding preliminary unit sizes and weights, but am wondering if I might be able to use this exception. Even if each unit weighs 10,000 lbs, the total weight tributary to the moment frames still averages to lower than 20 psf (less than 18 psf), even though the weight per sq ft at the individual unit locations would be higher than 20 psf.

I'm not sure how literally I need to interpret this exception. Any ideas? Do you think that because my weight at small, localized areas exceeds 20 psf I'm automatically out of luck, or do you think this is more of an average load requirement and I can disperse the weight over a larger area, specifically the area tributary to each moment frame? I lean toward the latter, which of course is why I'm asking for viewpoints from other engineers. For lateral load purposes, this seismic weight flows to the moment frames the same way it would if it were a distributed area load. I did check the commentary, but it was not very helpful.

Thanks so much!

 

[MrHershey]

I'm assuming you're using a flexible diaphragm. In that case I would argue that it could be averaged over the tributary width of the frame the unit(s) is (are) sitting over. I don't think I'd average over the whole roof unless the units are pretty well spread out over most of the roof.

Would also tend to argue that 20 psf should include whatever weight of exterior wall or interior partition is tributary to the roof, if you haven't done so already. Would think that any dead weight whose mass ends up in the roof during an earthquake should be counted.

 

[KootK]

or do you think this is more of an average load requirement and I can disperse the weight over a larger area, specifically the area tributary to each moment frame?

I'm on board with that interpretation. Seismic design, and especially the height/system limitations, is rough stuff. I wouldn't take the 20 psf limit as anything other than the code saying "a generally lightweight building". It would take a whole lot of RTU to convince me that you're not in that category.

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.