Snow drift against parapet on flat roof 1

[Alexander Usdan]

Anyone have any good (retrofit) solutions to prevent/limit snow drifts against a parapet?

(Even though it summer thought I would shake things up with a winter related question

 

[KootK]

1) build another, shielding parapet inboard of the existing one.

2) sheath the kickers to create a ramp at about the same pitch as the expected snow drift such that the resulting snow depth on the ramp is just the flat roof value.

Frankly, I'm not sure that I believe that either of these solutions is theoretically sound based on the aerodynamics involved but they are things that I've seen other engineers do on multiple occasions.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[jayrod12]

A shed roof on top of the kickers to limit the snow accumulation works. Was the build-up missed in the original design, or was a new taller parapet installed without any thought of the repercussions?

 

[jittles]

I think there's good logic in the above replies to sheath the kickers. If you fill in the aerodynamic shade area that the drift would normally fill in, then no drift can form there. Just confirm that the drift can't now form in another new location, and I think you're looking pretty good.

 

[Brad805]

What is the underlying problem? If experiencing leakage, you need to hire a real roofer rather than the hillbilly that used tin with exposed fasteners. You can flash that roof using better materials. If this is a low slope pre-eng using tin with exposed fastners, I think you have an architectural problem and not a structural one.

 

[Alexander Usdan]

Thanks everyone. Yes, I thought that creating a sloped plane on top of kickers may be a solution. And yes I don't think when the parapet was installed/added that anyone thought about the issue (sufficiently).

Is there any concern of the snow accumulating at base/valley of slope?

 

[structSU10]

Look into the snow load guide put out by ASCE. They have some recommendations to mitigate snow drifts. Professor O'rourke (author of snow drift guide)will also answer emails (in my experience) with specific questions you may have. I believe they talk about overbuilds / creating a 'ramp' as solutions.

 

[jittles]

Is there any concern of the snow accumulating at base/valley of slope?

Depending on the shape of the sloped plane, maybe.

I bet you'll find a pretty big difference in the ASCE snow drift triangular shape and the kickers that are there. I'd expect you'd have to take the slope quite a bit farther out to actually fill in the drift area.

 

[Once20036]

Back to Brad805's question, what's the problem that you're trying to solve.
I`ve found that the lateral load from snow drifts on a parapet pales in comparison to the wind forces on these parapets.

Are you dealing with a leakage issue best addressed by a roofer?
A structural force problem of a design you're reviewing?
A structural force problem on an existing structure that has manifested itself as a failure of some sort?

 

[KootK]

I`ve found that the lateral load from snow drifts on a parapet pales in comparison to the wind forces on these parapets.

How does one calculate those lateral load? It's something that I've contemplated from time to time.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[Aesur]

@KootK, just a thought here, but I would think you could calculate lateral loads similar to that of fluid pressure. Snow is just a "solid liquid" with a different density, think of this as saturated soil. Once you calculate the snow density you could theoretically calculate the lateral pressure.

 

[jittles]

Snow drifts are exceptionally solid. I truly doubt there's any appreciable lateral load there.

 

[KootK]

Snow is just a "solid liquid" with a different density, think of this as saturated soil. Once you calculate the snow density you could theoretically calculate the lateral pressure.

Right, but it is the "K" value for this equivalent fluid pressure that is missing from my understanding.

Thinking on this more, a likely outcome is that water gets trapped between the vertical surface and the snow as it might between a retaining wall and the retained soil mass. So maybe the equivalent fluid pressure should just be the actual fluid pressure for water?

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[jayrod12]

If that were true, then it wouldn't pale in comparison to the wind loads. You're be at 120 psf with only 2 feet of snow buildup. That would be quite detrimental to how I've seen the standard parapet designed.

 

[KootK]

You're be at 120 psf with only 2 feet of snow buildup.

Nah. In terms of base moment, which is all that matters, 2' of water is equivalent to 30 psf wind acting at a 1.4 load factor. About on par with parapet level wind. Buy yeah, I'd consider pure fluid pressure to be the upper bound. I suppose that trapped water could freeze but then that would surely press laterally through the adjacent snow drift.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[dold]

Doesnt hydrostatic/LEP from soil assume that groundwater level is constant for an effectively infinite distance beyond the wall? The only place I can imagine "LSP" (lateral snow pressure) happening is at the back/low wall parapet of a lowslope roof with clogged primary and secondary scuppers like you'd design for in ponding. Do you design your parapets for hydro pressure in a ponding event?

 

[Once20036]

How does one calculate those lateral load? It's something that I've contemplated from time to time.

I was tipped off to the procedure from one of the engineers at MetlSpan when I was looking at options for an IMP parapet.
It's in a publication called, "Snow loads - guide to the snow load provisions of ASCE 7-10" on page 137.
Basically, they recommend treating the snow as a cohesionless soil based on the snow density at the mid depth of the drift.
Based on the angle of repose for snow, they recommend an active pressure coefficient of 0.59.

Keep in mind that parapets will have wind load on both faces. The load on the roof side is based on the roof suction in the edge zone which can be substantial.

 

[Ron247]

I rarely work in heavy snow areas but I recall years ago a PEMB having to have additional wall girts because of snow drift accumulation. They in fact called them "snow girts". So at one time, there was some calculation of lateral pressure from snow. I do not know if research has determined it was not needed but I remember at one time it was used. Probably PEMB participants will have some information on this practice.

As I recall, the additional girts were placed in the lower part of the drift. I also remember the calculation contained a triangular pressure similar to soil other than the magnitude. As I recall, the vertical offset of the roofs was about 20' but I am not sure about the height of drift. Both buildings that met to create the drift were fairly wide.

 

[KootK]

@Once: thanks for that. 0.59 is actually quite a bit higher than I'd expected.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[skeletron]

Could you sheath the kickers with grating rather than a solid decking material? The grating may act as a "snow break" and provide some air flow to prevent accumulation. Although there would be a trickle down (excuse the pun) effect on providing drainage and preventing ice build up below the kickers, as opposed to solid sheathing.


...but I can't recall if I have ever solved that problem yet.