ASCE 7-22 drift guidelines are unconservative?

[Jdub Engineer]

Just realizing, that now in the ASCE 7-22 all the snow loads are scaled up significantly. We're in Colorado, most of our buildings are here.
Pg = 40psf was common for ASCE 7-16 load combinations (1.0 * Snow).
Now, Pg = 55psf will be common for ASCE 7-22 combinations (.7 * Snow)

the higher Pg results in a higher hb (balanced snow)
since the drift snow is truncated at the height of the projection, this means for some roofs you'd get the same total max drift load (Pd + Pf) no matter if it's using the 55psf or the 40psf number.
The concern I have, is, then with the ASCE 7-22 you would multiply that total snow *.7. This means for some areas of some roofs, we're designing for 70% of the drift snow that we would have designed for with previous codes.
In 1 quick example, with a H= 3', Lu=60', leeward, I end up 57psf with ASCE 7-16 and 63psftotal with ASCE 7-22 (for use with .7*S combinations). This means the drift snow is 77% (63*.7 / 57) of the ASCE 7-16 drift. Seems like a big accidental redux.

I don't see any commentary that says this is being done on purpose.
It feels like a oversight.

Anyone have any thoughts on this? I don't really want to start switching to the new code, and this is just 1 more unclarity that keeps from doing it.
Jonathan

 

[lexpatrie]

I'd look for anything by Michael O'Rourke on the subject, as he's kind of "the one guy" in this part of the code writing it.

Does the density of the snow change that much between 40 and 55, because that's where the h[sub]b[/sub] comes from.

I'm not sure your example is quite on the money, if you end up with a smaller drift, the drift gets longer, so when the framing is perpendicular to the drift there may be some effects, ... would they be that significant?

 

[Jdub Engineer]

YES, density goes up from 19.3 to 21. but not very minor compared to the .7 load factor.
the drift width equations get more complicated, but in this example it'd be 10' wide for the ASCE 7-16 example and 6.5' wide for the ASCE 7-22 code. the width is a function of the drift height, and the drift height is truncated by the height of the projection.

I read some articles out of Strucuture Mag by O'rourke, they're all more theoretical and I didn't notice anything that addresses this which made me think it's more of a oversight.
JRW