Snow Load - Exposure Factor (Ce)

[Struct-007]

In ASCE7-16, Section 7.3.1, it is mentioned that "An exposure factor (Ce) shall be determined for each roof of a structure."

Can anyone please give some examples of such conditions?

Thank you.

 

[Simba13]

Isn't it just covered in table 7.3-1?

Basically the exposure factor (from my understanding) is the likelihood that snow will collect and remain on the roof. So if a building is in a very exposed environment (like a plain) and has no cover from trees or other buildings, the snow is much more likely to be blown off by the wind. This is why the largest Ce reduction to the snow loads is for Surface Roughness D (open) under the fully exposed condition.

 

[bones206]

Some roofs can be sheltered by other roofs or other buildings.

 

[Struct-007]

Thank you.

What confuses me here is the term "each roof of a structure". This implies that one structure may have several roofs.

Ce is determined based on 1- [surface roughness category] and 2- [Exposure of roof].

The [surface roughness category] will not change. So, Ce will change for "each roof in the same structure" based on [Exposure of roof].

I am not sure how the exposure of each roof in the same structure will vary?

 

[phamENG]

Consider the picture below. The taller portions of the building are almost always very exposed. The roof of the lobby down in the center, on the other hand, is very sheltered. The wind isn't going to remove that snow, and you'll get drifts from the upper roofs to boot.

 

[Struct-007]

Ok got it, thanks! This picture worth 1000 words. It should be added to the upcoming version of ASCE7

 

[phamENG]

You're most welcome. Glad I could help.