Practical limit to Lu (drifts on lower roofs)

[WWTEng]

Is there any practical limit to the "Lu" length of higher roof? The reason I ask is because I have a situation where the lower roof is about 14' lower then the higher roof and the high roof is well over 300' long. This results in large drift loads for 25 psf ground snow. Reading the ASCE-7 it doesn't appear that there is a cutoff point but I was wondering if I missed anything in my interpretation.

 

[DST148]

No. You have not missed anything. The graph in ASCE 7-05, Fig 7-9 is given for values of lu up to 600 feet and suggest to use the eqn. for lu > 600 feet.
If I remember correctly, the Canadian Code has a provision to account for reduced drift in the presence of a parapet at a higher roof and also an upper bound value on the snow drift.

 

[RHTPE]

lu in ASCE 7-10 controls the maximum height of the drift - in your case about 5.5 feet. The width of the drift effect, w, is controlled by hc & hd (4 * hd^2/hc). Further, w shall not be greater than 8 * hc (2nd paragraph, 7.7.1). For your case, hc = 1.45' and w = minimum of 83.5' or 11.6'. So you wind up with a drift 5.5' high against the wall beteen the upper & lower roofs, tapering to 0' at 11.6' out from that same wall.

That's my understanding of ASCE 7-10 Ch 7.


Ralph
Structures Consulting
Northeast USA

 

[DST148]

@RHTPE - I don't have ASCE 7-10. But per ASCE 7-05, for hd < hc, the drift width = 4 x hd = 4 x 5.5 = 22 feet.
The distribution of drift loading on the lower roof, whether triangular or trapezoidal, will depend on the length of the lower roof. The OP does not give the length of the lower roof.

 

[RHTPE]

DST148, You are correct about the lower roof with wind blowing towards the wall between the low roof and the high roof. Given that WWTEng stated

Is there any practical limit to the "Lu" length of higher roof? The reason I ask is because I have a situation where the lower roof is about 14' lower then the higher roof and the high roof is well over 300' long.

I kinda presumed that the low roof was on the leeward side, and that he was concerned about drifting snow coming from the upper roof onto the lower.

Per ASCE 7-10, with the low roof on the windward side, hd is based on lu for the low roof and hd is limited to 3/4 of that determined from Figure 7-9. The larger hd shall be used for design. I would then conclude that, in either case:

- For hd <= hc, w = 4 * hd
- For hd > hc, w = 4 * hd^2/hc
- w shall not exceed 8 * hc

Thanks for reminding me that the wind can pile up a drift against the wall from either direction.

Which then leads us to Section 7.8 (which covers parapets & roof projections) where a similar condition exists for wind blowing over the roof toward the parapet or projection.


Ralph
Structures Consulting
Northeast USA