Cable try Ice loads

[penpe]

Outdoor cable tray is subject to the typical elements: Wind, Snow, Ice, Seismic. Our current project is located where the design ice thickness per ASCE7-16 is 1.5". Doing calcs according to ASCE the ice cross-sect area on each rail Ai = (pi) x ice thick x (channel diagonal measure + ice thickness). Each rail 7.4" tall has area of ice 40 in^2 per the formula. Per foot of length that's 480 in^3 per foot per rail. Ice density is 56 pcf, so two rails' ice load comes to 31 lbs/ft. If the tray is full of low voltage cable, and ice covers the surface it might be analyzed as a cover plate where Vi = (pi) x ice thick x plate area. Where cable tray is 36" wide Vi = 1936 in^3 per foot. 56 pcf density result is 63 pounds per foot. Ice total weight is 94 pounds per foot. Using max span of column-supported cable tray the total load shouldn't exceed 103 plf, including cable, snow and ice. The snow load in this area comes to about 45 pounds per foot. We've got a problem! (sidenote: A contradiction here is that if there's no cable in the tray the ice can't accumulate on the surface).

The cable tray manufacturer Eaton's B-Line design guide prescribes a method for ice loads: ice in pounds per foot = (cable tray width x max ice thick)/144 x ice density of 57pcf. For 36" tray this comes to only 21.4 pounds per foot! That's less than 25% of the load I've computed per ASCE.

For CYA reasons we like to include on our drawings what is max cable load, but these ice loads are so far apart it's hard to arrive at a compromise, or even feel comfortable leaning toward either.

Of course we don't want to reduce the span which would add twenty columns and foundations because of this crazy ASCE ice number.

Any suggestions?

 

[penpe]

Thread title should be "Cable TRAY ice loads". Damned spellcheck! (kidding)

 

[GC_Hopi]

" ASCE 7 does not have specific provisions for snow and ice on non building structures and components. Process Industry Practices (PI) and American Petroleum Institute (API) recommendations do not give comprehensive design procedures or guidelines for calculating snow and ice loads on non-building structures and components. Engineering judgment is used for adapting the snow and ice provisions for buildings to non-building structures"

The above quote is from this paper published on ASCE. I dont think it has changed.

 

[azcats]

TIA-222 has some pretty thorough ice load calculation information.

 

[dik]

We typically use covers and design for 1" of ice all around, not just on top. On some mining projects, I've encountered over 1' of heavy dust 'debris' sitting on cable trays.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dauwerda]

Personally, I would go with the manufacturers published method and not look back.

 

[penpe]

Thanks for the responses. I did find that ASCE 7-16 says it is acceptable to multiply the ice volume for horizontal plates by 0.6, so that brings the ice weight down to 69 plf per ASCE. Maybe the average of the manufacturer's method and ASCE would be a reasonable compromise at 45 plf (using engineering judgement). My department manager asserts that the total max allowable 103plf should be calculated by using the worst case ASD load factors which will be D + .7 ice + .7 wind on ice (= 0 vertical) + 1 snow. With 45 plf snow and 45 plf ice, 26 plf is left for cable dead load. (Using the .7 factor actually reduces ice to about 31 plf). Electrical says about 0.25 plf for low voltage cable, so we have room for 100 cables. Maybe that's good enough for the initial installation, but they will probably add more over time.

I looked up TIA 222 (telecommunications industry) which apparently most directly applies to antenna supporting structures. It might be worth a look, but didn't appear to be readily accessible, and doesn't seem to actually apply to cable tray. Thanks, though, for the suggestion.

1" all around would come to 34 plf, so that seems to be in a reasonable range, but ignores the 1.5" per ASCE according to location.

 

[dik]

I wasn't aware of the ASCE's requirement...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[penpe]

I'll admit that ASCE doesn't specifically address ice for non building structures, but Chapter 10 starts with this: "Atmospheric ice loads caused by freezing rain, snow, and in-cloud icing shall be considered in the design of ice-sensitive structures."

And cable tray manufacturer Eaton states that "Cable trays can be subjected to static loads like cable weight and dynamic loads such as snow, ice and wind". "The total load = cable load + ice load + snow load + wind load (+ any concentrated loads + other special condition loads i.e. future cabling)".

 

[TLHS]

GC_Hopi, I actually think a bunch of the recommendations from that paper are in ASCE-7 now, if it's the one I'm thinking of that has things like maximum snow pile heights on thin items and snow bridging effects on cable trays.

I don't recall it having much in the way of icing, but I haven't looked at it in quite some time.

 

[dik]

In addition to them I have a 'debris' load to account for lack of housekeeping... often encountered in mining operations.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[oldrunner]

It depends where in the country the cable tray is. Does your contract provide a thickness? I've worked with ice from 1/2" to 5". Had one project in North Dakota where the BR said 1/2" and the local building department said 3". I used the 3" and was chastised by the BR plan checker. But I had to sign the drawings. One substation in Wisconsin had 5" of ice and many things failed. (Not my project but was providing the loading for another project for a 50 year life structure)

 

[penpe]

oldrunner, no, our contract doesn't provide a thickness. The ASCE7-16 map gives nominal ice thickness of 1.5" for this part of Indiana.

 

[PersonalProfile]

I cannot directly assist, though perhaps consider if impact loads etc must be considered:

https://www.reddit.com/r/WTF/comments/sovjgb/snowball/