Very much so. There is also the vertical impact from wet, sliding snow to consider.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

They are probably rationalizing that there is only so much snow - if it is drifted from the high roof, there is no snow to slide. They also may be reaching for the fact that the drift may stop snow from sliding off the roof, but it would have to be full height so i doubt it.

I would think in certain situations, where the roof height differential is substantial, you would want to combine the two effects. At worst its conservative versus the code, so it shouldnt be a problem.

A source you could try for an answer is the snow load guide ASCE publishes. I think you can even email the author of the guide (roark? I think he wrote the majority of the code too) for questions.

Thanks for the replies.

After thinking about it there probably is some engineering judgment that needs to take place...imagine that.
I may try to reach out to O'Rourke. I post back if I do.

Thanks,

EIT
www.HowToEngineer.com

Canadian code requires a combination of both, but in their case the sliding snow is also a tapered load unlike the uniform load in ASCE.

I have a question similar, so I'll just put it here:

In general, should different snow loads be combined in general? For instance, should drift loads be applied with partial loading? Or should they be considered separate cases?

- Aaron

Good question. I will try to look into that. It seems though that they are meant to be separate.

Also note that when combining snow with wind and live or EQ and live, you would only use the 'flat/normal' roof snow load.

EIT
www.HowToEngineer.com

Interestingly enough - while I believe ASCE7 does not add these, IBC 2009 figure 1608.1(5) shows sliding show imposed on top of drifting snow. So maybe these do need to be added.

EIT
www.HowToEngineer.com

What version of IBC 2009 are you looking at? My copy of the code shows only one paragraph for Section 1608.1 that simply says looks at Chapter 7 of ASCE 7. No figures, tables, charts, etc.

It says August 2010 2nd printing. Also I should mention that is code/commentary side by side and this appears to be in the commentary.
I will try to make a scan of the figure (don't tell IBC :) ).

EIT
www.HowToEngineer.com