Windward Drift on Adjacent Structures ASCE7-10

[dsweet1984]

Team,

I am a bit confused as to why Section 7.7.2 in ASCE7-10 requires "windward" drift load. I am picturing the snow blowing off of this location into the area between the buildings. Unless there is some kind of wind stoppage force due to higher structure I am not understanding. Would someone set me straight.

Thanks!
Dustin

 

[ajh1]

You hit it on the last part of your statement. Windward drift occurs when wind blows across a lower roof and causes a snow drift up against a higher area of the building. While there is a 3/4 factor in the calculation this condition may control when the lower roof is substantially longer than the upper roof.

 

[dsweet1984]

Thank you very much for the reply. For others reading I should clarify that 7.7.2 refers to "adjacent structures" so the 2 buildings are not in contact (s<20ft). The windward drift we are discussing is resulting from wind blowing from the lower roof towards the higher structure. The result being a truncated drift at the edge / eave of the lower roof. I did see that in the commentary Figure C7-3 shows a diagram of this situation. They show a parapet, but do not require a parapet in the section.

My thinking is that investigation has shown some sort of wind swirling effect that deposits additional snow in this location. The code provisions make sense. I am just curious to learn more about the background of this requirement.

Thanks!
Dustin