Cable Tray - Steel Support Frame

[kburk2014]

Hello there.

I am designing a 3D frame inside of a building to be used to support a cable tray running across the length of the building. The cable tray runs the entire length of the 3D frame I am designing at the same elevation off of the ground. The cable tray is 3' wide and 4" deep and weighs 3.24plf. The frame I am designing is 20'-6" long, 10' high and 8' wide.

What is the best way to assign the loading of this cable tray to the frame I am designing?

Thank you.

 

[Brad805]

Most cable tray suppliers provide a rated capacity for their trays.

 

[paddingtongreen]

You need to know the maximum weight of the cables, not just the tray. You will probably use rods or unistrut to hang the tray. Design for the loads where they attach.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[dhengr]

Kburk2014:
I’d go with 14.3984789kips/ft. of tray length..... Alternatively, you will have to investigate what is going to be on that cable tray, in terms of the total weight. Then, add some percentage (how much?, I’m not sure) per foot because the contractor will likely do that, in any case. But, you say a 3' wide tray, and an 8' wide support structure, so they can potentially add 5 more feet of cable trays or pipe lines to your support structure and they likely will. Then you locate those loads on the cross members to cause the greatest beam bending moments, and then, again, near the ends of the beams for max. shear and column loading, unless you can really control there locations. Then, they’ll probably hang some off the sides too. That’s some of the stuff of real engineering, not a textbook problem where all of the needed info. to solve a narrowly defined problem is given. You should discuss these kinds of questions with your boss/mentors, at your company or within your industry, there must be some basic standards as a place to start. This is the best way for you to learn, not by asking a group of people who can’t see what you are dealing with.

 

[Brad805]

I have attached an example cut sheet from a Cooper Cable tray. I don't suggest trying to figure out the exact number of cables they plan to use. I have done that for an existing undersized tray, and it is tedious. I assume this is an industrial plant. These tend to go thru many equipment changes, so the cabling changes with time. I suggest you design for the rated capacity.

 

[kburk2014]

Thank you. All of these answers are very helpful.

In particular, @dhengr thank you for the detailed response. I have now accounted for additional cable trays being added to vacant space onto the frame because of your advice. After discussing with my lead, this is typical within the process industry. It seems typical to be rather conservative as a structural engineer in this line of work.

 

[racookpe1978]

And then plan for two 200+ pound electricians standing in the cable pulling cable ... Maybe not the first night shift, but sooner or later.

 

[kburk2014]

@racookpe1978 - Just two?.....

 

[rb1957]

without counting each and every cable they could use, estimate a reasonably conservative load ... 10 lbs/ft, 100 lbs/ft ... that also allows them to get on with their job. check with your lead. then if you want CYA with a placard "load not to exceed x lbs/ft". also account for abuse loading (separate loadcase) like the 2 200+ lbs workers standing on it ...

another day in paradise, or is paradise one day closer ?