Snow drift on very small lower roof 1

[structurebeton]

What is your opinion on a very long Lu and very tall upper roof, against a very small lower roof and in which the calculated drift width W ends up being a LOT larger than the lower roof width?

Would you then use the drift width W the same as the lower roof itself or would use a truncated snow drift shape? (Trapezoidal shape) Doing that would increase the snow drift load by a factor 6 or 7.

 

[PEinVA]

COEngineer,
There is no real clear definition, but it does say canopy in the title. How could justify NOT using it? You can't just dismiss the fact that canopy is in the description of the LL required. Although marquees is by itself in the IBC LL Table. My boss has always made us design to 75 psf, and didn't really care for what the code also states in 1607.11.2.4 that Awnings and canopies shall be designed for uniform live load of 5 psf + snow and wind loads. Then in section 3105 and 3106 it tries to define a canopy, awning and marquee, but I think it just makes things worse.

But I actually called ICC and they couldn't answer me for how to define a canopy, awning and marquee.

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[COEngineeer]

Because I go to that table under roof. They have a section about canopies and awnings.

Never, but never question engineer's judgement

 

[structurebeton]

What the LL has to do versus the snow load? If the snow load is higher it controls...that's it, isn't it?

 

[structurebeton]

Now, if the upper roof has a parapet, then this applies:

ASCE7 commentary: "C7.8 ROOF PROJECTIONS
Drifts around penthouses, roof obstructions, and parapetwalls are
also of the “windward step” type because the length of the upper
roof is small or no upper roof exists. Solar panels, mechanical
equipment, parapet walls, and penthouses are examples of roof
projections that may cause “windward” drifts on the roof around
them."

Therefore, lower roof Lu applies and the 10 ft tall snow drift disappears...

 

[miecz]

We get all kinds of snow here, and there are these little canopies all over town, facing every direction. There's never any snow on any of them. One feature of them though, is they all are plexiglass (slippery looking) and are curved downward or they slope away from the building (say, 5 on 12?). I don't think the code allows for any reduction in drift load for a sloped or curved surface.

 

[msquared48]

As I stated before, I would consider impact (1.25+-X) on the lower roof too if the snow from the upper roof can fall onto it, thus adding up to the 75 psf load. Falling wet now is nothing to take lightly (no pun intended).

Mike McCann
MMC Engineering

 

[miecz]

msquared48-

I agree, if the upper roof is steeply sloped. In this case, I got the feeling the op was dealing with a flat upper roof.