Parapet Considerations for ASCE 7 Snow Load Calc

[SixkHz]

Is there any reduction for drift loads against a high roof with a parapet? Referencing Figure 7-8 from ASCE 7-05, I have hb+hc+hp, where hp is the height of a parapet on the high roof.

I'm more familiar with the Canadian National Building Code, which accounts for this parapet height in the equations for snow accumulation on low roofs. You can get lower drift values because the parapet prevents (to a certain extent) snow from blowing off of the high roof and onto the lower one.

I'm looking for any reduction in snow load I can get. I'm designing a space frame supporting a glass roof, and the member sizes need to be optimized.

 

[JAE]

Parapets do not reduce the snow drifting on the roof below. The drifting is caused buy either leeward drifting or windward drifting and both are not straight, laminar flow effects but rather more chaotic.

The codes don't have provisions to reduce drifting with parapets that I'm aware of.

 

[SixkHz]

Thanks for your help, JAE.

 

[vmirat]

mdeegan,
I actually ran into this same situation. Although I work in the US, I found similar research in Canada dealing with the blocking effect of a high parapet or wall on the upper roof. My situation was the design of a lower canopy which had a much smaller length than the main building upper roof but the upper roof had a parapet wall that was 8 feet tall (from the upper roof level). I contacted ASCE and asked them. They stated that they do not recognize the effect of a high parapet or wall to block snow blowing off of the upper roof. However, that doesn't mean it isn't effective. But, I think the parapet or wall would have to be pretty high to block the snow from blowing over it. ASCE doesn't address snow drifting from a lower roof up onto a higher roof, so there must be something to it. I was going to contact CRREL and see what they know, but I forgot about it, until now.

 

[mmillerpe]

I don't have a copy of it anymore so I cannot confirm, but didn't the "Guide to the use of Snow Load provisions based on ASCE 7-" discuss the use of walls in reducing the drift effect on adjacent roofs, or am I mis-remembering?

Personally, I would not count on it since who knows if this item will be VE'd out or removed in the future.

 

[SixkHz]

Thanks for the input vmirat and mmillerpe. I may look into that snow load guide, as I'm starting to work on more projects in the US.

 

[vmirat]

mmillerpe,
I have a copy of the snow guide for ASCE 7-02 (copyright 2004). The guide does discuss this very issue. However, they discount it by saying the height of wall required would be impractical. They also state that the effectiveness of any barrier is less than 100%. Since the guide is not the code, and ASCE 7-05 does not give any consideration to a high parapet wall, there is no code basis from which to design. When I talked to ASCE about this, they basically told me that a high parapet wall could not be used as a snow drift reducer. The snow guide also mentions the use of baffles to reduce the snow drift, but they state that it may be difficult to convince regional building to allow this because it's not codified. I suspect the same would be true with the high parapet.

Although both authors of the snow guide worked at CRREL, I would be interested to know if CRREL has any new info on this subject.