Drift Load Calculation CAN. code 2

[palk7 EIT]

Hi,

It would be great to have your view on which method to use to calculate drift load on roof with the parapet box at the top. I see there are two methods in NBCC
1). Snow drift load step (Multi-level roof). <From Jabacus NBCC2010>
2). Snow drift load obstruction (like roof projection).<Jabacus NBCC2010>

Why I ask is because I get a different accumulation factor in each of the method, it's safer to take the conservative value, but in general when you Engineers calc. the drift load behind a parapet do you consider it as a projection (less conservative) or like two level roof (more conservative). Attached an image for this particular example.

Thank you & really appreciate your time.

 

[dik]

Don't understand the question... it depends if you have a step in the roof, or have a rooftop projection like a RTU or something of that ilk... If you have a clear case of two roofs at different levels then 1 or if you have a small projection (could be 8'x8'x30') then 2. It won't let me upload the *.png file.


Dik

 

[BAretired]

It's a projection or obstruction. If the 4' dimension was 40', I would consider two roof levels, but if the snow on a 4' width of roof blows onto the low roof, it has little effect on the snow load.

BA

 

[dik]

Try again... wouldn't let me upload the *.pdf, either...

Dik

 

[skeletron]

It really comes down to the beta-factor in the Ca0 equation. Obstructions press a hard limit on the accumulation so the beta=0.67. Roof-steps maintain beta=1.0. If you read in the commentaries, there is some information that relates to wind direction and the drift not actually filling the step up the full amount. The commentary gives the example as parapet walls.

Your snow accumulation is probably around 18-20". Your obstruction is 30". Theoretically, there will be accumulation. Also keep in mind that the drift will only extend about 48" out past the parapet.

All this considered, I would run the calculation with the increased accumulation factor as per NBCC 2015 and size my member with less reserve capacity since my load will be at the code maximum.