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Snow drift on very small lower roof

Snow drift on very small lower roof

Snow drift on very small lower roof

(OP)
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.

 

RE: Snow drift on very small lower roof

I would consider impact!

Mike McCann
MMC Engineering

RE: Snow drift on very small lower roof

Per Code you must use the truncated shape.

DaveAtkins

RE: Snow drift on very small lower roof

Yes, ASCE 7 is pretty clear on this.  You MUST use the truncated shape.

RE: Snow drift on very small lower roof

The code says you're stuck with the truncated shape.  Seems silly to me, though.  I've never seen six feet of snow on a four foot wide roof.  Perhaps this is a place for engineering judgement to overrule the code.

RE: Snow drift on very small lower roof

We see this situation a lot on warehouse type buildings with small canopies and you'd be amazed how the snow actually does pile up on these small roofs.

Use the truncated drift.

RE: Snow drift on very small lower roof

I don't know why you find it so hard to imagine 6' of snow on a 4' wide roof.  Additionally, if you ignored it and it ended up failing you will have a lot to answer for.
Use the truncated shape.  ASCE is very clear on this and I don't believe it's open to much interpretation.

RE: Snow drift on very small lower roof

I agree.. but I think ASCE needs to improve it somehow.  Lets say you have a skyrise and you have a small canopy on the 1st floor, I dont see how snow from the penthouse can drift to the lower roof.  Only leeward drift makes sense.

Never, but never question engineer's judgement

RE: Snow drift on very small lower roof

I've lived for 58 years in New England, and I've never once seen it.  It's something that I look for, as I once designed a canopy to attach to a parking garage.  I used the truncated load to design the support.  The city put up some cheap, flimsy canopy instead. Twenty years later, and it's still there.  It's never had even close to the snow drift I had designed for.

RE: Snow drift on very small lower roof

For such short members, scaled to 3' or so, is it going to make that large a difference? You also have a 75 PSF LL you need to apply, so the snow still might not control with the appropriate load factors.

Is there an angle of friction for the snow that you cna use to justify reducing the snow load?

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

 

RE: Snow drift on very small lower roof

75 psf LL from what?

Never, but never question engineer's judgement

RE: Snow drift on very small lower roof

Depends on many variables. I live and work in Wisconsin and have seen drift loads in the 1st year after construction larger than the code specified loads.    

RE: Snow drift on very small lower roof

(OP)
Per ASCE 7 the build-up would be 10 ft high of snow drift...the lower roof is only 3 ft wide...Pretty crazy...

RE: Snow drift on very small lower roof

My mistake. I thought this was a canopy, not a lower roof. Canopy LL is equal to 75 psf in ASCE.  

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

 

RE: Snow drift on very small lower roof

I am still confused with the 75 psf.  I always design canopy with snow+drift load.

Never, but never question engineer's judgement

RE: Snow drift on very small lower roof

If you look at Table 4-1 it states "Marquees and canopies" it is listed as 75 psf design LL.

 

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

 

RE: Snow drift on very small lower roof

Am I missing something?

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

 

RE: Snow drift on very small lower roof

Hmm.. I never really used Marquees for loading.  Isnt that mostly for non permanent cicus type roof structure?

Never, but never question engineer's judgement

RE: Snow drift on very small lower roof

Use the truncated.

I've seen small eyebrow canopies with piles of snow like that many times in the midwest.

The thing is, whether the snow piles up or not depends on a lot of factors (prevailing wind direction, wet vs. dry snow).

For some conditions, it might not ever occur (per miecz's experience stated above), but it can happen (I've seen it) and therefore the code requires you to account for it.

 

RE: Snow drift on very small lower roof

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

 

RE: Snow drift on very small lower roof

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

Never, but never question engineer's judgement

RE: Snow drift on very small lower roof

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

RE: Snow drift on very small lower roof

(OP)
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...

RE: Snow drift on very small lower roof

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.   

RE: Snow drift on very small lower roof

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

RE: Snow drift on very small lower roof

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.

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