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Snow loading on and below bar grating

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PSUEng11

Structural
Aug 30, 2012
10
Good afternoon all,

I was having a debate in the office this week regarding snow loading on bar grating. I have looked far and wide and I am unable to find a specific resource for anything conclusive.

This is a large equipment platform with bar grating on top of a roof that is supported off of the building columns. It is generally our understanding that we have to specify the bar grating and design the beams that support the grating for the full snow load. We are also designing the roof framing for a full snow load.

The debate however is whether or not it is required to consider the full snow load on both the bar grating and the roof below at the same time when designing items such as the columns, footings, or determining total roof loads used for story drifts.

Any thoughts on this? Thank you in advance.



 
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Check the depth between T/O Grating and T/O Roof.
Determine the depth of the basic snow load, which is a function of the specified pressure and snow density.
Then build your rational analysis on that.

I would design it for the load on the roof. Find the depth of snow. If the grating is below this depth, I would probably add the maximum depth that can accumulate from the roof with the basic snow load happening on the grating. You can still get build up on top of grating that can insulate below, which prevents that snow from disappearing. So, yes, take it into account somehow.
 

Snow loads shall be considered on all open-frame structures with flooring that can retain snow. Grating will also retain snow with cornicing effect. Please look ASCE 7-16 commentary C7.13 SNOW ON OPEN-FRAME EQUIPMENT STRUCTURES..
 
Columns and footings should be designed for full snow load, regardless of whether it is on the roof or the grating.

BA
 
I think that there is some misunderstanding here, I probably did not phrase my question properly, so for that, I apologise.

For example, say I have a 30 psf balanced snow load. I am designing the bar grating and it's supporting framing for the full 30 psf. I am also designing the roof framing for a full 30 psf. However when I am sizing the columns, footings, and determining overall total roof load, would I be basing it off of a total 30 psf acting over the roof or would I be taking a full snow load on both the grating and the roof for a total of 60 psf essentially over the roof?

Thanks again

 
Originally I wanted to suggest two loading schemes, but I later thought that the more critical case would be snow on all exposed surfaces only, since the grating is likely affecting certain roof members locally (local critical), and without putting any load under the grating, it amplify unbalanced load effect on the roof.
 
Columns and footings should be designed for full snow load, in your case 30 psf. It can be on the grating or the roof or perhaps partial snow load on each but it can't be on both simultaneously.

BA
 
Again, snow load on all exposed surfaces. See sketch below. (30 psf every where).

snow_webdus.png
 
Thank you Baretired, msquared, and retired13. That is what we were discussing. We figured that it could be on either, but not a full load on both at the same time. Even though snow could pass through the grating for a period of time, it would likely begin to accumulate and collect on the grating. It didn't seem reasonable to count on both being fully loaded at the same time.

Just wasn't sure if there were any provisions or studies for this type of situation (like if wind driven snow could drive snow under the platform or if snow could be on the roof and be shaded preventing it from melting before the next snow event.) Probably drawing at straws there. I was looking to see if there was anything that O'Rourke had published or anything else out there, but I wasn't having much luck.
At any rate, I appreciate your responses

 

If the sides open, assuming the snowfall direction 45 degrees, snow loading shall be considered at levels below the top Level. ASCE 7-16 item 7.13.2 states 'Snow at Levels below the Top Level. At all levels with
flooring (grating, checkered plate, etc.) located below a level with flooring, the flat roof snow load shall be applied over a portion of that flooring level near any open edge in accordance with Fig. 7.13-1. The flat roof snow load shall extend from the upwind edge of the flooring a horizontal distance equal to the vertical difference in elevation between the level in question and the next floor above.'.
 
Some snow will be blew off the grating - high roof/low roof effect!?
 
I'd need to see a sketch of your situation, but we typically assume grating does not hinder snow accumulating on the surface below. In some cases this results in 'double' the snow load being applied where snow is assumed to fall through the grating / be blown in from the side and accumulate under the grated surface.
 
I don't think retired13 is correct. Reference Figure 7.13-1 in ASCE 7-16 as suggested by HTURKAK. You'll need to add some flat roof snow load under your platforming.
 
I should also add, I would design the beams and deck under the platform for the full snow load. The columns, footings and anything else supporting both platform and roof framing in that area would be the worst case between flat snow on platform plus the loading per ASCE7 Figure 7.13-1 or flat roof snow on the roof.
 
The code specified amount of snow is perceived as the maximum one can expected in his geographic area. As emphasized by BA, the first step, and the most important, is to account all for your beams and columns to carry, then you should consider local accumulation as described by code (around vertical barriers, and hi-low roofs..). Beyond that covered by code, IMO, all are scientific fictions to everybody's guess. Note that for your case, the roof is partially shielded by the grating, I think it is rather conservative to assume no snow passing to amplify the unbalanced load condition.
 
ASCE has addressed this issue in 7-16. They even produced a handy figure for various conditions.
image_yytxob.png


Robert Hale, PE
 
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