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mooreaaryan

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Electrician
Is there a resource that i can look into for sizing all thread aka ready rod? I am hanging cable tray and a few separate conduits I ask the project engineer and he told me to "go ahead and size what you think will get the job done" I have no issuing learning but need direction on where to start. Thank you all
 
Into this spiral of information I now have more questions then answers. Is there a min. supporting distance for cable tray install overhead in trapeze? where can i find this at. Legrand (manufacture) says ask engineer i figure code committee would have found a min. and put it in the code book for us "build and design" fellows
 
The manufacturer must have the support spacing. It is totally up to them.
392.30 Securing and Supporting.
(A) Cable Trays.
Cable trays shall be supported at intervals in accordance with the installation instructions.
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A little snug is one thing, tight is another; we torque to stretch the bolt a certain amount (against it's tensile strength), pulling on it with a weight does the same thing. If you have 100 pounds hanging from some all-thread then tighten a jam-nut, the force from tightening is added to that 100 pounds on that segment of the rod.
 
zbang said:
Except that had nothing to do with using all-thread and everything to do with the support beams not being constructed and installed to spec.
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The original design called for a continuous length of all thread from top to bottom, and the contractor did not want to have to thread nuts all the way on from the far end and so changed the design.
 
GoldDigger said:
The original design called for a continuous length of all thread from top to bottom, and the contractor did not want to have to thread nuts all the way on from the far end and so changed the design.
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In addition, it was supposed to be a special high-strength rod, and the contractor basically did the equivalent of running down to Home Depot. The engineering firm slit their own throats on the design, because they reviewed and stamped the new configuration as approved.
 
zbang said:
Right, but from what I can see the rods didn't break, the beams did; that's what I mean- the use, or non-use, of all-thread was not the final failure.
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Quite true. Just pointing it out in case the design in question involves two or more vertically separated cable trays supported from common anchor points. :)
How you use something is often just as important as whether you use it.
 
zbang said:
Right, but from what I can see the rods didn't break, the beams did; that's what I mean- the use, or non-use, of all-thread was not the final failure.
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The force on the beam from the lower nut was doubled because it was supporting two walkways instead of one, causing the beam to fail. The failure bent the lower flanges of the two channels upward so that the flanges dropped down below the nut, and then presumably the upper flanges were also bent outward by the nut allowing the beam to free fall. It seems that it would've been wise to have a thick plate above the nut to distribute the load out to the vertical webs of the channels, instead of applying the force closer to the inner ends of the flanges which caused them to bend and open up.
Also, I'm guessing the channels may only have been tack welded together instead of a full weld based on the picture.

So I think there were probably multiple flaws or weaknesses that contributed to the tragedy, although not using the single threaded rod from the original design was the main one.
 
synchro said:
The force on the beam from the lower nut was doubled because it was supporting two walkways instead of one, causing the beam to fail. The failure bent the lower flanges of the two channels upward so that the flanges dropped down below the nut, and then presumably the upper flanges were also bent outward by the nut allowing the beam to free fall. It seems that it would've been wise to have a thick plate above the nut to distribute the load out to the vertical webs of the channels, instead of applying the force closer to the inner ends of the flanges which caused them to bend and open up.
Also, I'm guessing the channels may only have been tack welded together instead of a full weld based on the picture.
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The article notes that the design called for the weld between the two C channels to be on the vertical sides rather than at top and bottom. The actual incorrect construction seems to have been the result of simply ignoring that part of the spec rather than a deliberate design change. I suspect that a full butt weld could still have led to the same weakness. But it may well have been a structurally inadequate weld too.
 
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