Snow Drift Loads Plus wind loads

[jeffhed]

I design a lot of awnings attached to existing buildings. As anyone knows that has designed any of these in snow country, the drift loads can be huge. I feel like they can be overkill at times, but those are the rules so I design for them and call it good. I include the wind downward pressures as well in the appropriate load combinations. I was talking to another engineer and he told me that the wind only needs to be additive to the uniform snow loads, not the drift snow loads. I found a thread on here earlier tonight that said the same thing with no specific code reference. When I try to search for it again I am not finding it. I thought the S was the snow load at that location and the 0.25 reduction was taking into account the fact that both wind and snow may not be fully experienced at the same time. Some of these drifts loads are huge and any little bit helps. Has anyone else heard of the allowance to only use the uniform snow load in the wind plus snow load combinations? It is supposed to be in ASCE 7.

 

[straub46]

In ASCE 7-10 the exceptions are shown under the basic load combos in 2.3.1 and 2.4.1, and say S is based off flat roof snow or sloped roof snow load.

The Commentary for Sections 2.3.1 and 2.4.1 says the following:

Section 2.3.1: "Exception (2) permits the companion load S appearing in combinations 2,4, and 5 to be the balanced snow load as defined.... Drifting and unbalanced snow loads, as principal loads, are covered by combination (3)".

2.4.1 has similar wording with appropriate load combos.

The wording of both of those pretty much suggests drift loads are not required to be considered in combination with wind.



Go Bucks!

 

[Once20036]

ASCE 7-10?
LRFD load combos or ASD load combos?

I agree that straub46 starts the conversation, with LRFD exception 2, however, this does not apply to load combination 3.
1.2D + 1.6S + 0.5W.
Combination 3 is not exempted and therefore includes drift + (some) wind.

It looks like there is no ASD combination with wind + drift.
Honestly, this seems odd to me. How do you get the snow drifting if you don't have the wind to push it?

 

[straub46]

Yea, it looks like the intent is to design for drift when snow is the primary load being designed for. I agree that it's odd that while there is a primary snow with companion wind for LRFD, that same load combination doesn't exist for ASD (or it does, but has the exception applied).

I generally just calculate my snow load based off drift and use that as the snow load in all combinations. It's only when I really need to look into getting something to work that I'll consider using the exceptions.

Go Bucks!

 

[jeffhed]

I see that exception in ASCE 7 now. And the ASD load combinations do include that same exception for load combinations 4 and 6. But this exception is not included in the IBC. Since ASCE 7 is a referenced standard can you still use those exceptions? I have many times where my drift loads can be around 80 psf with the flat roof snow load around 20 psf. This could make a huge difference in beam sizes in some cases.

 

[birneys]

I see it as a judgement call that can vary widely based on the location of the project. In Nebraska, I had a prefab canopy that was attached to an existing precast building. After a large snowstorm (I think it was 12-18 inches of snow over 2 days and strong winds), there was a snow drift on the canopy that was over 10' tall and ran the entire length of the canopy/building. That drift was very close to exceeding the snow drift calculated using ASCE 7-05. I know it's only one case, but anytime someone says that ASCE 7 is overly conservative for drifting, I have to disagree. Personally, I would design for wind and the full snow drift load for canopies/awnings especially at existing buildings. It might be completely different in other parts of the US, but I would think the farther north you go, the more I would err on the side of conservative loading.