Snow Drift for a large AHU

[SteveGregory]

I understand that ASCE 7-05 requires using the "windward" 0.75Hd snow drift against a roof projection 15' or longer. What about the opposite side of the AHU? Should you evalute the loading with 0.75Hd drifts occurring on both sides of the AHU at the SAME time?

The reference is section 7.8. The Figure 7-7 is not refered to in section 7.8. But it shows windward and leeward drifts occurring at "steps".

 

[CNCLLC]

The way I am interpreting to section 7.8 is that the drift loads need to be applied, simultaneously, to all sides that are 15' or greater.

I believe that the intent is to say that the wind will come from any direction and deposit the snow on all sides, however, due to the 15' width dimension, the snow on the leeward side will be shielded and not be removed by the wind coming from the opposite direction.

 

[SteveGregory]

CNCLLC
If I understand your post, you are saying that there are potential windward drifts being accumulated on each side. One day the wind blows snow from the west and the next day it blows from the east. Is this what you meant?

 

[sshields]

When I do snow drift calculations for AHU's, I'll develop four load scenarios (one for each direction of the wind), and dump all four into Ram Steel. Each seperate scenario will have a windward and leeward componenet.

 

[ARLORD]

If it is possible and conceivable for snow to drift on all sides with L >15', then you have to design for that case, as being the worst load case.

 

[SteveGregory]

Assuming that only the North and South sides are >15' and the distance from the far edge of the roof to the AHU is 350' and 30' respectively, would you compute different windward drifts (0.75hd) based on the different upwind distances and apply these loads simultaneously.

I am trying to justify placing an AHU on an existing roof if I can.

 

[DaveAtkins]

SteveGregory,

What you are proposing is how I would do it.

DaveAtkins