Snow build-up 5

[jayrod12]

Hey all,

I've got a couple of sizeable rooftop units going on a fairly large building.

By the numbers I need to account for build-up but that causes failure of the entire system basically (deck, joists, beams and columns). If I only need to account for unit weights then the beams and columns are within an acceptable range. This equates to one shit-ton of reinforcing and steel work in an already cramped roof space.

A fellow employee proposed putting these units on braced stilts approximately 3 feet clear of the roof top. His justification for that was if you provide enough space below the units then you won't get the build-up.

I don't necessarily agree, nor do I disagree with his approach. there has to be some height above the roof at which the build-up can be ignored.

What are others opinions of this?

And no it is not possible to build snow sheds or anything around the units to negate the build-up.

 

[TehMightyEngineer]

Actually I would think putting them on stilts would make it worse unless the stilts were unreasonable in proportional height to the roof width.

The reason we get drift loads is the sloped or vertical surfaces act as barriers for blowing snow that would normally get blown off a roof or evenly distributed around a roof. So, say if you have a low sign right at roof elevation (no gap) then it will build up a small drift in front and/or behind the sign. But, if you have a similarly sized sign raised up then the drift might start out small but as snow hit the sign and then fell down and filled in the gap at the bottom the small drift would soon become very big, likely bigger than the original sign. Plus, the higher up the vertical surface the more you might "shade" areas behind it. Thus, even if you don't get a big drift right in front of the raised projection, the snow that may have blown to the other side of the roof will be stopped and blow around on the windward side, possibly causing a higher distributed snow load. You might also get weird wind vortexes that could gather more snow on the roof as well.

Overall I think you'll have to look at your snow load and imagine how that roof will look with a code level snow load. If you're designing for 30 PSF snow load you're probably right that most of the snow will just blow under the RTUs and any captured snow that would have blown over the roof is probably minimal. However, in northern Maine I'm used to designing for 90+ PSF ground snow load and if that's your case as well you better believe that a 3 foot gap will fill in pretty quick. Also, keep in mind that code level snow events are generally two snow storms back to back or at least snow on top of existing snow. Thus, you may get 1-2 feet of balanced balanced on the roof over the course of a winter and then you get hit with a blizzard and your gap does nothing but make your projection effectively taller.

As for code references I know of nothing in ASCE 7 or the IBC that allows for credit of "gaps" below a rooftop projection. O'Rourke's excellent guide to ASCE7-10 snow loads covers RTU design for snow loads but makes no mention of anything with a gap below it. Engineering judgement is about your only reference here I believe.

Maine EIT, Civil/Structural.

 

[jayrod12]

Design snow load of 36 psf. this equates to an average depth of 24".

For the record, I don't believe I have ever seen snow build-up around a large roof sign that has been raised clear of the roof (to be fair they were raised a lot more than 3')

My concern is just that, the gap filling in, and now instead of a 7' tall unit it's 10' or more. I'm less looking for code references and more looking for everyone's general feel for this type of situation. I'm sure I'm not the first nor will I be the last to deal with something like this. Especially in the pre-eng world we live in where there is next to zero additional capacity in the roof.

 

[TehMightyEngineer]

Well, remember that it's wind that drives where snow goes and wind will generally go around and over rather than under (and thus the gap does little to nothing). At the very least you'll get a vortex that forms on the leeward side the RTU which will cause a low pressure area that will "suck" in snow (same reason the back of your car gets covered in snow when you're driving).

If I were you I'd check the design with the raised RTU for the snow load with leeward drifts only assuming a RTU that's got a solid height (no gap) up to the top of your raised RTU. See what that does for you as far as snow loads. If that's too much then you probably have to reinforce the roof no matter what as you'll have some if not all of the leeward drift. If you're that close to the capacity of the roof steel then likely you will have to reinforce the roof.

You mentioned no snow sheds were allowed around the RTUs, but what about baffles around them? If you can force air to flow under these RTUs then perhaps this method could be considered reasonable? Also, 3 feet might be small but what about something higher like 6 feet?

I know what my boss would say if he were in charge of this. "Just be conservative and avoid designing outside of the code. It's not worth the liability and, in the end, the client gets a stronger roof."

Who knows, you might have an easy way to reinforce the roof that you haven't thought of yet.

Maine EIT, Civil/Structural.

 

[steellion]

-ASCE 7 does not require snow drift to be considered for roof projections less than 15 feet long.
-We routinely raise the bottom of roof screen walls by about 3 feet (or above the flat-roof snow height) in order to avoid drifting snow against the screen wall. This is the same general concept your colleague is suggesting. I can't point to a code reference that allows it, more of a judgment call. Most of our designs are in a region that only receives moderate snow; I probably would not try it in Buffalo.

 

[TehMightyEngineer]

Good point about the 15 foot limit for drift loads steellion.

I figured probably the best way to see if this happens would be to google image search for road signs along highways and see if they created drifts. Easier said than done, it's really hard to tell if the snow was blowing the right way to cause a drift. I did find this image though:

http://media-cdn.tripadvisor.com/media/photo-s/05/36/d4/15/white-isle-niseko-snowmobile.jpg

As best I can tell the snow was blowing toward the sign which should have caused a noticeable roof drift but no drift is seen. This obviously isn't proof but after 30 minutes of searching I can't find a single image the conclusively shows a drift caused by a raised sign.

Maine EIT, Civil/Structural.

 

[msquared48]

What you missed were the horizontal icicles on the backside of the sign.

Mike McCann, PE, SE (WA)

 

[Canuck67]

There is research published quite some time ago by RWDI giving the results of wind tunnel data that confirms that snow drift accumulation does not occur underneath elevated rooftop units. Within the research was a formula derived for minimum required gap between the roof and the underside of the unit and the example calcs are for northern snowy climates. I believe it is use at your own risk as it is not codified, so depends on your own tolerance for risk. Undoubtedly there will be some additional snow due to aerodynamic shading, however, the research suggests not much. In my opinion, it's always better to reinforce, even if the roof needs to be over framed to accomplish the task, however, sometimes there are situations that become quite difficult due to site conditions.

 

[Once20036]

A coworker shared some slides from a presentation he attented, entitled, "Advanced Snow Loads" by Michael O`Rourke through the Structural Engineers Coalition of CT.
I understand Mr O`Rourke was involved in developing the snow code. The first slide admits that the items discussed are beyond the scope of ASCE 7-05 and merely represent the presenters opinions.
He recommends a 2' gap between the top of the balanced snow load and the bottom of the units/support steel. Any event in which drifts will form involve wind, and this wind can blow snow under the unit as it accumulates, thus, no drift loads.

As Canuck points out, it isn't codified, however, I believe the logic is sound and 've used this method in the past without issue.

 

[ajh1]

Dr. O'Rourke has been recently working on a study regarding snow drifts around solar panels installed on a roof. The 2' separation between balanced snow depth and the lowest edge of the obstruction mentioned above is also the recommendation in this new study.

 

[jayrod12]

Thanks Guys,

Canuck67, any chance you've got some more info on that research. A quick google search of RWDI and build-up returned nothing.

 

[TehMightyEngineer]

Indeed, I too would love to see a copy of this as I run into issues like this all the time being in Maine.

Maine EIT, Civil/Structural.

 

[H57]

My only additional comment would be to make sure and consider connections from the RTU down through the roof (ductwork, piping, etc.). If your elevated roof top unit will have supply and return ductwork that extend down from the unit through the roof this may impact the ability for wind to blow the snow through and you may want to consider more drift loading that if it were only piping. Small localized drifts at the ductwork could lead to larger drifts. Just another consideration.

 

[Canuck67]

I will look for the publication over the weekend and post. The version I have is quite old, however, I wonder if it has been updated.