Snow Drift ASCE 7 1

[ronstruc]

When calculating snow drift against a roof projection the UBC had a maximum value of 50' for the winward roof length (97' UBC 1644.5). I am designing with the IBC 2003 which references ASCE 7-02 for snow drift design. Under section 7.8 Roof Projections, there isn't a maximum value for lu (length of roof upwind) like there was in the UBC. Does anyone have any input on this? It makes sense that there should be a maximum value for lu, otherwise the snow drift height and length seem very unreasonable.

 

[mrengineer]

1) I believe UBC 97 (and several other building codes) had several glitches in the snow drift section. Even tho you get the impression from 1644.5 that it is 50' max, I believe intent was 50' min (notice definition of "Wb" on page 2-387). Going all the way back to ASCE 7-88 you will see 25' minimum in Figure 13. I think UBC actually wanted to be more conservative than ASCE.
2) ASCE 7-02, section 7.8 references 7.7.1 and Figure
7-9. Figure 7-9 specifies 25' minimum just above the equation for "hd".

 

[jike]

Here is another thing that I have wondered about....

It seems to be where a parapet exists (of significant height), that should serve to block the blowing of snow off a longer extent of flat roof. I don't think that this is addressed by code.

Any thoughts?

 

[ronstruc]

jike,
I agree with what you are saying especially if the opposing side of the parapet is the leeward side, which this situation happens to be.

 

[DaveAtkins]

jike,

I have also thought about this, but have conservatively assumed the parapet does not block snow (my thinking is that the wind picks the snow up and carries it over the parapet).

DaveAtkins

 

[SperlingPE]

A timely thread considering what I am looking at out my window.

 

[jike]

DaveAtkins,

What you describe, I also do, for the same reasoning.

 

[jike]

DaveAtkins,

What you describe, I also do, for the same reasoning.

Obviously, as the parapet increases in height, it will become more affective in blocking the wind driven snow.

 

[CarlB]

"Obviously, as the parapet increases in height, it will become more affective in blocking the wind driven snow."

Not necessarily, depends if this is new snow or snow re-entrained by the wind from upstream. In the arctic, one way to capture snow is to erect a snow fence, which collects snow in a long drift downwind from it, usually up to the top of the fence. The fence creates a dead air zone where the snow can settle out. Similar to jetties collecting sand at the shoreline, downstream of the tidal current.