What is the load on this cantilevered channel? 2

[bdn2004]

Based on my sketch at the top of the attached page and the data given on the Vendor's chart....What is the MAXIMUM weight that can be supported by the channel? What would you do make this installation stronger?

I'm not a structural engineer or designer.

 

[PicoStruc]

Hire a structural engineer !

 

[jayrod12]

Let me ask you a question. I want to put an air conditioner into my house. Can my panel handle it? I'll give you a cut sheet of the air conditioner I'm looking at but no other information. I have zero electrical knowledge.

 

[TehMightyEngineer]

What is the MAXIMUM weight that can be supported by the channel?

It can support one engineering code of ethics pamphlet, however you probably want to include a safety factor. Never use the maximum load.

What would you do make this installation stronger?

Simple, hire a structural engineer.

Maine EIT, Civil/Structural.

 

[rb1957]

well, maximum we'd say is maximum considering code safety factors.

how'd you make it stronger ? it's impossible to answer sensibly without knowing the constraints. another fastener into the 10" I-beam might help (if this attachment is critical), another supporting brkt from the cable trays, a support on the LH side of the cable trays would be great, change on material on the channels, ... depends on what is the critical loadpath.

also, it's much better to ask can this support X lbs. the best way to determine "maximum" is to test the final installation, but analysis for X lbs would be very straight forward.

Quando Omni Flunkus Moritati

 

[bdn2004]

Can your panel handle an additional load? Put a power meter on the panel for a month during the summer to see what your panel is now loaded to. Then calculate the VA load on your AC unit and see if when you add the ac unit you are still within the ratings of your panel. It's not that complicated. Would you hire an electrical engineer to do this? If not, are you breaking a code of ethics? What is this forum for?

I thought, actually hoped this would be a straightforward calculation. And one of you structurals could knock out in second. It is obviously basic statics that every engineering student had his 2nd year of college.

More information....there is one of these double trapeze supports every 8 ft. Each level supports a 12" aluminum cable tray that weighs about 65lbs per 20' piece. And each tray has two #4/0 triplexed cables that weigh about 3.5lbs per foot.

So for a 20' section, there would be 2 supports, and the weight would be (2 X 65lbs) + (4 X 3.5lbs/ft X 20ft) = 410lbs/20'

 

[EngineeringEric]

Per your logic... Load that beam with a factor of safety 1.67 and see if it works.

 

[bdn2004]

And one more thing...it would still be helpful to know the maximum load or what percentage of the maximum load this cable is placing on the supports. Cable trays tend to get filled up over the years.

 

[jayrod12]

To answer your question. I would hire someone with the knowledge to tell me if the solution is possible and not attempt to teach myself how to do something I have no idea about. This forum is for people to trade information and discuss topics that they themselves have knowledge in already.

Don't come here with a poorly worded question with a shoddy sketch and expect reasonable answers. Had you worded your original post with the information that you included in your second post you probably wouldn't have gotten as many smart ass remarks.

 

[bdn2004]

Ok the answer seems to be that I do need to hire a structural engineer. Fine.

What worries me a little bit is that these supports are done all over the country every day but not by iron workers but by electricians. And they aren't hiring structural engineers, they are slapping them into place. They usually have a "standard" detail to work from that was probably originally prepared by a structural engineer, but not all real situations fit that "standard" detail. The electricians make the call on where and how to do the support. A little background on how this calculation is done would help give some assurance that what they are doing is right.

Ok I'm done now.

 

[rb1957]

simple question ?

1) how many of these 20' sectiona are there ? is it a one off or a continuous tray ??
2) but the double trapeze are 8' apart ... does that mean that 1 20' cable tray is support by two supports 8' apart ?
3) if so, then if 20' weighs 410 lbs, then each support is carrying 205 lbs.
4) based on your sketch, 75% of the load goes to the LH support, and 25% to the RH one
5) 5/8" threaded rod is good for this load
6) the upper beam (B22A section ??) is taking this load into the I-beam. it sees two loads, 42" apart, and reacts them where it connects with te I-beam (not well defined). the maximum moment in the channel beam is at the LH support. since it's about 6' long, you should include the weight of the channel, about 24 lbs, as a distributed load.
7) how do the attachments to the I-beam feel about the load applied ? probably not a problem, but a question that needs answering.
8) how will the I-beam feel about he loads applied ? again, probably not a problem, but ...
9) this response started when you wee able to define a load requirement, rather than asking what the maximum is.
10) if you don't like our free answers, you're welcome to ask someone else ...

Quando Omni Flunkus Moritati

 

[mastruc]

bdn2004:

rb1957 gave a good idea of some of the devils in the details. You're right that giving a load capacity for the horizontal "trapezes" is simple if all you're worried about is its plastic bending capacity, if we have the steel yield strength (which isn't on the sketch.) Other concerns are connection details (top trapeze to bottom trapeze, connection to top channel, connection to I-Beam), etc. Connection design isn't quite so back-of-envelope. Neither are local-buckling conditions for that shape.

 

[BAretired]

One possibility is to move the upper member B22A? up so that it rests on the flange of the beam. See the attached sketch. The right end needs to be held down with a bolt through the beam flange.

The cantilever length is reduced to 32", so the moment is 32P"# where P is the hanger load in pounds at the end of the beam.

The section modulus S is 0.5989 in[sup]3[/sup] and the working stress is f = M/S. Assuming a working stress of 20,000 psi, B22A would be capable of taking a moment of M = f*S = 20,000*0.5989 = 11,978"# provided it didn't buckle before that. Recalling that the moment arm is 32", this means that it could carry a force of M/32 = 374# by that calculation.

It would be necessary to consider the buckling capacity of member B22A which I do not have at my fingertips. Maybe the best way to determine that is to load it to failure and see.

BA

 

[bdn2004]

Here is a picture of the actual installation.

 

[BAretired]

Too late for my solution. Didn't realize it was already built. I would have to agree with others that you need to retain a structural engineer to check it out.

BA

 

[Once20036]

BA - Careful, local buckling of the compression flange is going to lower your moment capacity. It's strange to me that the manufacturer wouldn't provide load tables or an allowable moment that can be used to check installations like this.

For what it's worth, Unistrut offers a similar shape with much better design information.
The closest equivalent is a P1001, good for 14360 inlb. This is based on a 42ksi steel member with 2.0 FS.

It's interesting to me that nobody has mentioned deflection, especially when the question was well defined. When asking, "How much can it hold" deflection may be the governing factor. I`m guessing it doesn't matter in this case.

To BDN: If an electrician installed one of these and then overloaded it, I believe the response would be, "eh, he guessed at the wrong size". You asked a group of professional engineers who could be accepting some liability simply by answering a vaguely worded question on the internet.

If I needed to re-wire my house and asked my neighbor to help and my house burned down, it would be my fault.
If I asked a professional electrician to help and my house burned down, I`m not sure I'd accept that same responsibility.

 

[rb1957]

not too late ... easy to reposition the channel above the I beam flange. or you could add a fastener to the channel near the end of the I-beam (minimise the 10" off-set).

but you probably wouldn't do it unless you thought there were going to be problems.

i guess this is typical of "build first, analyze second" projects.

looking quickly at things, i'd suggest testing ... easy enough to do, test to load*safety factor. rough numbers say it's unlikely to have a SF = 2.

Quando Omni Flunkus Moritati

 

[paddingtongreen]

Historically, cable trays were field run from some "typical support" diagrams. The nuclear plants then had gangs of degreed structural engineering assistants field measuring, analyzing and designing fixes for them. If you have seen a cable spreading room you will have an idea of just how much work that was.

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]

I generally agree with Rb1957's and BA’s approach to this problem, and of course buckling should be considered and checked. The OP’er also already said “Cable trays tend to get filled up over the years.” So, what is the final loading on those cantilevered struts? Someone better take a good long look at the 5/8" all-thread and the strut which make up the primary cantilever reaction point. That load will be much higher than the total load on the cantilever. This is almost like (not quite) the Kansas City Hyatt Regency bridge failure detail. A place where a whole bunch of load accumulated, at a really crappy detail, and nobody give it a second thought, until a bunch of people were on the ground and dead.