Drift Load + Unbalanced ?

[BSVBD]

I have a 12:12 gable roof addition adjacent to a 62' vertical wall. The gable roof will receive a drift load.

ps + pd = 30 + 74 = 104 PSF. <<<--- This does NOT include unbalanced snow load prior to adding the drift.

I do not see in ASCE 7-05 (our state's current reference) if drift should be added to unbalanced or if the drift could (reasonably) cancel or omit the unbalanced.

In the past, I've never included the unbalanced in such a situation.

In the pre-IBC past, 90 PSF drift (for this region) was typically the max drift ever used - with this firm.

Suggestions?

Thank you!

 

[flight7]

7.7.1 - Drift loads shall be superimposed on the balanced snow load.

 

[Bala0404]

See attached. its a good example. that I have been using. I would use drift loads + unbalanced loads to be conservative. I also could not find any documentation of the load combinations or reduction. may be other Engineers can shed some light.

 

[BSVBD]

flight7, your statement, "basically", and literally answers and ends the discussion, "if" there are no other implications.

My inquiry was intended to discover if ASCE implied, anywhere, that i could not see, that drift should be superimposed on unbalanced. I simply want to avoid the potential that a few of the respectfully analytical state plan reviewers may decide that the drift need be imposed to unbalanced, thus, requiring me to recalc the entire project.

Bala0404, you state that you apply the conservative drift + unbalanced. However, according to your attachment, (which is beneficial - thank you!), the applicable calculation for Area C1 implies only drift + balanced.

Thank you both for your input.

 

[jimstructures]

I am not aware of any statement in ASCE 7 - Minimum Design Loads, going back to 1988, that states or suggests one should combine drift loads with unbalanced loads only with the balanced snow load from the minimum county Ground Snow Load. The unbalanced snow loads are made up loads which seek to replicate other kinds of snow loads which have been observed. It may be conservative to use engineering judgment and to combine drifts with unbalanced snow loads but they are normally separate conditions that do not need to be combined, unless you feel that you have an unusual case that may warrant the combination.

Jim