Snow load on grating

[TLHS]

Does anyone have any references or opinions for how to account for snow load on grating?

For the most part, snow passes relatively easily through any large grating, however I usually include snow load in my calculations knowing it won't govern because the National Building Code of Canada includes a clause that says that pedestrian only areas do not need to apply occupancy and snow at the same time.

However, in this case, I'm involved in a project with very controlled access with a small occupancy load in a situation where it will be quite expensive to build in extra capacity for unrealistic snow loads. Parts of the structure are to be in the water and will be supported via buoyancy and the snow load would be incredibly significant. Technically, the NBCC (or, rather, the provincial building code derived from it) won't apply to this structure, but I'm following the general thrust of it anyway. Snow load only needs to be accounted for on surfaces that snow can accumulate on and while snow can't really accumulate on grating significantly, there is generally still a little bit that sticks around that should be accounted for, even in the best case.

I'm also slightly concerned that with icing the holes of the grating could provide surface area for snow buildup, but it hasn't been observed in a similar installation.

I'm actually surprised that I haven't been able to find a grating manufacturer that's done a relatively quick study they can use as a selling point.

What have other people done with regards to grating and snow? I'm fairly sure that a significant amount of snow can't accumulate, I'm just arguing with myself over whether I can properly justify that opinion if I ever need to back it up.

 

[csd72]

TLHS,

for outdoor signs in New York city we ignored the snow load though I do concede that you have a valid point about icing.

To allow for this look up what the angle of recline is for the snow mound from the edge and this will give you a much greatly reduced snow than the full normal load.

Hope this helps.

 

[dik]

Snow will bridge across the grating... design for full snow loading.

Dik

 

[paddingtongreen]

In general, the grating never sees it's full design load, that is especially true on top of snow and ice, during and after storms. The first job, if the floor is needed, is to clear the stuff to make it safe to work on. Even walking on it forces the snow through the voids.

The only exception that I can remember was during a re-fit. A lot of pipe was laid out on boards on the floor, enough to provide a base for the snow. However, that was a special condition, not a requirement of the original design, we checked on it as part of the refit design.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[TLHS]

Thanks for the thought csd.

I had actually considered working out a load reduction based on that concept. Unfortunately, when I started working through it I realized that I've seen fresh snow pile up with perfectly vertical edges for well over a foot of height on my very own patio table. Snow has a fair amount of cohesion and, while I'm sure it will rearrange itself to a stable angle if it grows high enough or is disturbed (avalanche anyone?) I'm not sure if it's really a valid assumption for reduction of loads?

dik, this is something I'm concerned about. I know that if we somehow transported a two foot covering of snow onto the grating it would manage to bridge the gaps. I just can't really work out a way that it would get to that stage on its own. The bars are so thin in comparison to the grating holes. How can enough snow gather to bridge the holes before it loses stability and falls through the grating?

 

[BAretired]

It depends on the size of the openings, but a thin plate can ice up and become much thicker than its original size thus blocking the openings as well as adding to its weight.

Designing for full snow load would be my approach if the openings are less than about 2" in either direction. Could consider a reduction for larger openings.

BA

 

[paddingtongreen]

To those who say design for full snow load, with or without simultaneous LL?

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dik]

I would design for full snow loading or live loading, whichever was the greater of the two. It won't be part of a 'dance floor' if there is 2' of snow on it...

Dik

 

[BAretired]

I agree with dik.

BA

 

[zstructural]

I agree with the people who say design for full snow. I do not know of a reference to confirm that's required. But, I don't think it would take much to get enough ice and/or snow pack as a "base" on the grating, and the snow would accumulate on top of it.

I would not combine it with other Live Loads. If this is something that needs to get used during the winter, then they would probably shovel the snow away for access.

 

[TLHS]

Thanks for the input everyone. I'm still of the opinion that the loads will likely be less than on a solid floor, as I can't see the entire area icing up and having snow bridging occur at the same time. However, since I also can't assume no snow load and I wouldn't be able to rationalize any specific reduction factor I applied beyond "I wanted it to be smaller," I'm just going to suck it up and apply the whole load.

 

[dik]

Once it ices up, the snow doesn't bridge.... <G>

Dik

 

[msquared48]

I would look at a total of four conditions:

1. Full Live
2. Full snow
3. Full live and .5 snow
4. Full snow and .5 live

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

If you check for 3. and 4. is it possible that 1. or 2. would be critical?

BA

 

[dik]

Only if the live load is uplift <G>...

Dik

 

[msquared48]

And... with snow, there is no LLR, but there is with live load.

Is the water muddy enough yet?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[IFRs]

See attached, written 23 years ago to demonstrate that bar grating was "self cleaning". Probably not much help. Snow did accumulate on the bar grating much less than the perforated metal tread.

 

[TLHS]

Thanks IFR. It doesn't get quite deep enough into things for me to apply it in this situation, but it's more testing than I was able to find and was an interesting read.