Snow load on cable trays

[dozer]

We're doing a job in Canada and are applying the flat roof snow load to cable trays and getting a load of 6 kPa (125 psf). I've heard different values for snow density but that could mean the snow is 1.5m to 2m (5' to 6'-6") deep! I don't see how it could pile up that high unless you put several trays side by side and they were the solid type.

I live in southern US so I haven't seen this first hand. Does anybody know if there is any guidance on this? What about first hand observation? Anybody seen snow this deep on cable trays?

If you use the rung type with no cover does that relieve you from designing for such a high snow load?

 

[vato]

Draw it up with a reasonable but somewhat vertical side slope and see what it weighs as wet snow. I'm in Colorado and snow will span small openings pretty quickly, and build up on it like it was on something solid. If it's not a building or life safety issue if it fails, then perhaps reducing the snow load is justified.
Not sure if this will help as I have no idea what a cable tray is, but what about ice? I recently designed a large steel entrance canopy in Texas where ice was a significant load and I almost missed it. FYI It just snowed an inch here in the last two hours.

 

[MiketheEngineer]

Cable trays "fill up" over the years as different trades add and abandon cables (but don't remove them..

I have seen cable trays that were basically "full" and I would definitely use full snow load. And don't forget drifting loads are possible!!

 

[peal724]

I've never heard of cable trays being exterior.

 

[MiketheEngineer]

Well they call them cable trays. Some might have pipe. But I have seen many with power conduit. Go to a power plant sometime. That's how they transfer all those electrons around!!

 

[msquared48]

I agree that if these cable trays are suspended from a concrete wall, then, if open to the weather, significant drifting of snow and ice can occur. I have designed structures with snow loads of 400 psf. I also stand under them when it is snowing!

You might also consider ice buildup below the cable trays if there is any heat in the conduits.

Mike McCann
MMC Engineering

 

[racookpe1978]

These are "outside" cable trays - photographs are from the cable and instrument runs in the "back end" (steam generator) of a combined cycle power plant in upstate NY.

Notice that a couple of the trays are held up by a 8 foot long Unistrut bar: The bar is held at one end by a single Unistrut clip, and on the other end a welded stringer to the structural steel.

For your application, you may consider a steeply sloped "tent" or ridged cover over the cable trays to shed snow and ice. Even closely spaced parallel electrical conduits will trap ice and snow.

 

[Brad805]

Here are a couple of other exterior cable trays from a small compressor station in Northern Canada. Nice to see others are stuck working on these silly things too.

Brad

 

[Brad805]

Only one attachment at a time I see. Here is an overloaded tray. After analyzing the capacity of the 4" aluminum tray I learned that our electrical code stipulates a max. 40% fill requirement if the cables are power cables. I was pleased that the electrical engineer did not tell me that.

Brad

 

[dik]

On the last couple of projects, I've designed cable trays for 150 psf, including cables, snow and ice.

Dik

 

[Duwe6]

As for heating, if the tray contains any power [vs. Instrumentation] cables, those cables will be warmed by the current flow. Melts the stuff off in 'normal' winters, but our Canadian brothers do *not* have normal winters.

I would start using a snow [icicle] load at about 35-40° latitude. At 45° latitude, give the tray the "full" load for that region. Keep in mind when designing cable trays, somebody will get into them to pull and dress down the cables. A 300 lb 'live' load is a *very* good idea.

 

[civeng5150]

I suggest looking at the Canadian Building Code supplement (where you'll find all the infomation regarding snow, ice and rain added to sknow). Up here, 6feet of snow is nothing surprising. also I wouldn't count on the current flow melting anything, we also have that nice sub zero weather to go with all our snow! Paradise on earth! LOL.

 

[Duwe6]

Your [damn] cold winters are actually better for the cable trays. The snow will stay as snow, and not melt and form icicles. I have seen icicles in Indiana 2-ft in diameter, but this monster was supported by a sturdy platform, not one of our trays. At our latitude, the current flow keeps the snow and icicles in the trays pretty minimal.

 

[RyUIUC]

Most of the time I have designed pipe racks that supported cable trays, the live load on the trays were governed by the capacity of the tray itself. These trays are typically load rated based on depth/width of available cable space. It is up to the electrical engineers to size the trays on not only what they are going to route in there but the snow/ice loads as well. If in your case the loads are significant they just tell the Structurals to provide supports more frequently since maximum span will be reduced. In either case just use the capacity.