Lateral Load from Snow Drifted at a parapet

[druminman]

I am working on a job where there are narrow cavities created at the roof between a parapet and a higher tower-like structure. The snow drift in this cavity is around 70 psf. I did not do the design, but am working with a specialty material supplier. The design calls for VERY large members in the wall of the tower at this location. I thought the design is just overly conservative as the lateral capacity of this wall is in excess of 200 psf.

I asked the engineer's representative why this was so and they said it was because of the snow drift. Having worked with a lot of snow, I never considered that piled snow imparted a significant lateral load onto a wall.

Does anyone know of any studies or reports of lateral loading resulting from drifted snow?

 

[MiketheEngineer]

When it starts to melt and turn to water?? But 200 psf seems a bit high unless the water is about 3' high!!

 

[druminman]

I thought of that too, Mike, but 70 psf snow is just over a foot deep IF it all melts and doesn't run off. That equates to 70 psf lateral load (max) at the base of the wall decreasing to zero one foot up. On an 18' wall I'm sure this condition won't control.

I know the structure is over-designed, It just got me curious about lateral loading from snow drift.

 

[MiketheEngineer]

I think I would make sure the scuppers and drains are clear and redundant!!

 

[a2mfk]

I could be way off, and this is purely anecdotal. But when I was a kid in Wisconsin, on our snow days off we'd start the day by digging out of our front door or garage door. You could open the door and there would be a vertical 5' drift. If the snow imparted any lateral load it would impossible to open a door, or if you opened a garage door the snow would immediately fall into your garage. And wouldn't snow then collapse hundreds of garage doors every winter if this were the case?

Come to think of it, all the ski resorts and pictures I have seen of huge snow drifts and now structural failures because of lateral loading. I think on a microscopic level snow flakes must interlock as they pile up on each other, they do not behave like a frictionless soil against a retaining wall.

 

[MiketheEngineer]

Pure snow has little lateral load as suggested. But an avalanche sure as hell does.

 

[msquared48]

If you cut a x section at the snow drift, initially the snow is in contact with the parapert wall or whatever. But with time, settling and melting, the snow gradually pulls away from the wall, leaving a gaaaaaap and no pressure laterally to worry about.

Mike McCann
MMC Engineering

 

[JAE]

ASCE 7 doesn't suggest lateral load from snow that I recall and I think the points above about leaving a gap and small lateral loads (if any) is correct.

 

[peal724]

the drift triangle is a simplied shape for ease of calculation. the actual shape, that I've observed, did increase as it approached the wall but there was a small space between the drift and the wall possibly caused by melting due to the heat of the structure.

 

[BAretired]

I agree that in time, snow positioned against a heated structure will form a gap, but I'm not so sure about an unheated parapet. When fresh snow falls, would it not tend to fill in the gap?

I don't think it would hurt to provide for some lateral pressure, but the amount cited in this thread seems a bit excessive.

BA

 

[druminman]

Thanks to all for the responses.

BA, I agree that there must be some load, but I'm a little surprised at the apparently complete lack of guidance in the literature.

One would think if the affects were minimal, that the codes would at least claim this if no rational procedure existed for finding the load.

Patrick

 

[port125]

I was thinking about this (as our AC was broken this morning and it is getting towards 95 out.)
Could the design engineer have been considering this load to be an impact load from the snow shedding off a high roof?
Personally I don't ever recall seeing a drift leaning on a structure. (NE Ohio Lake effect)
I always though (hypothesized) that as the snow fell a little wind would swirl at the wall and hold the drift back somehow.

 

[BAretired]

This link gives an ASCE reference at the end of the thread:

http://www.eng-tips.com/viewthread.cfm?qid=225399&page=6

BA

 

[JAE]

The attached jpg is a snap shot of a section of the ASCE 7-05 commentary on the snow chapter. This is really the only reference to "lateral" loading from snow - here specifically for a sawtooth roof system.

I think this would be a case where a "V" shaped sawtooth roof would create a wedge of snow (sloping roof driving snow into a vertical clerestory window) that would develop some kind of lateral load.

But on a vertical parapet, with no wedge-shaped snow load from an adjacent sloped roof, there just wouldn't be any lateral force developed in my opinion.