Sliding Snow + Falling Impact ASCE 7-05

[SteveGregory]

ASCE 7-05 seems clear how to treat sliding snow for a lower roof. However, it seems that the sliding load in ASCE 7-05 does not account for the impact of a sliding snow load. This might not make much difference if the the snow only falls 3 or 4 feet.

How about if the lower roof is 10 to 15 feet below the upper roof? By the time it slides, it is probably a combination of wet snow, crusty snow and ice. I've had 3 failures to look at recently of this nature.

Does anyone here add an impact factor to their sliding load?

 

[cvg]

add it
I have witnessed this phenomena first hand, sliding snow and ice off the roof, landed on a redwook deck and broke through several 2x10's. The ice landed on the "pointy side". Lucky it didn't hit my foot. Granted the boards were old and had been wet, but this is typical.

 

[SteveGregory]

What kind of impact factor do you use for what height?

Unfortunately, the snow guards did not prevent the sliding. Not the bar type which came loose in one case nor the "flower" type.

 

[cvg]

I can't help you with the impact factor, however I believe this has been discussed previously on the forum. you might try a search

 

[hokie66]

Like designing for a tornado, this would increase building costs dramatically. Probably why ASCE doesn't address it.