Cantilevered lighting bar off an I-beam

[back_ache]

I've just noticed something unusual about one of our about lighting bars, it is clamped underneath an I-beam, but rather than the load (the lighting bar and lights) being directly under the I-beam it has instead (by the use of short lengths of scaff bar and right-angled couplers) been raised to be in parallel with the I-beam to give it more height.

 

My worry is that instead of the load being an even downward force on both clamps, it is instead a rotational force pushing down on one clamp and up on the other.

 

This all said, it has been like this for donkey's years and apparently "brave" fellows have done chin-ups on the bar but I would still like to know if this is good practice (the bar not the chin-ups) and if not, what should replace it.

 

I have done a little ASCII art to illustrate

 

          ---  x--x--
I-beam==>   |   |  O     <= lighting bar (sideview)
         (---) |  
         ------x 

[Seano]

Nice ascii drawing..

Your poll doesn't have an option for "it depends", so no vote from me. ;)

 

What is it clamped to the beam with? Gravlocks?

 

Things that would make it dodgy:

• Any of the components of your thingamabob being loaded beyond their specification. (Unlikely)
  
• The torsional load on the beam being a bit too much in a twisty kind of a way. (Also unlikely)
  
• Scaff clamps working loose over time. (Less unlikely - needs care.)
  

 

So, while it doesn't sound like the tidiest thing ever, its not necessarily dodgy. Assuming the beam is ok for the load, the obvious upgrade would be to replace the short scaff (and maybe the gravlocks as well) with something fabricated for the job. Its the kind of custom bracketry that doesn't come particularly cheaply, but shouldn't cost a fortune either.

 

Your bar doesn't fall under LOLER, but even so it would generally be considered good practice for it to have a marked (or otherwise written down somewhere) SWL.

[back_ache]

Sounds like it's time to get the ladder and torch out again and check what the exact beam clamp is.

 

Assuming it is a gravlock, I guess as a gravlock is rated for 600kg, normally with a straight-down load with a pair of them the swl would be 1200 (ignoring the other components for now) so given its a rotational force, how much should I de-rate them by, 50% ? (i.e. rate the pair of them based on the max load of one of them)

 

Thanks for the comment on the ascii, if I drew it on a wall, would I get paid as much as banksy? (please say yes) :-)

[Andrew C]

        	---------- 	
        	(---)	|
I-beam==> 	  | 	O 	<= lighting bar (sideview)
  	    	---

Would this work? Fewer bits, but might not be possible to pass the top horizontal between RSJ & ceiling. Same sort of turning moments, but a reduced risk of component failure.

 

Urgh... Try again! Stuff it; why didn't I use a pen & paper and a scanner?

[Seano]

I guess as a gravlock is rated for 600kg

Er.. not so sure about that..

 

how much should I de-rate them by, 50% ?

You probably shouldn't - not meaning to be blunt, but giving the bar a calculated SWL is probably a job for someone who wouldn't be asking these kind of questions.

 

but might not be possible to pass the top horizontal between RSJ & ceiling

Quite likely I should think, otherwise something like what you suggest would've been a much more obvious way to go about it.

[ramdram]

Apropos the turning moment on the beam owing to the load being off-centre, would it be possible to balance the load either side of the beam...move "half" the lanterns over or add a counterweight?

 

Presumably somewhere in your paperwork there are specifications of loading along the bar and that sort of thing.

 

I suppose you might do the belt and braces thing by chaining the bar to the beam, assuming space allows?

 

But before you do any mods you should check out the specs and if they are unfathomable to yourself find a man with the appropriate letters after his name to give the drawings the once over.

 

You could not be accused of being overcautious if you went this route.

[back_ache]

Hi, to answer you questions,

 

* There isn't access to the top to the beam as it's also hold the ceiling/flatroof up

* I think they should get a relevant company in to do it, at the moment though, I am just trying to gather evidence to potentially replace something that people will see as working fine just as it is.

* I am always triply cautious when it comes to putting heavy things above squishy heads

* I got the gravlock's SWL from doughty's site, it does seem a huge amount though, maybe they mean 600kg when mounted as a pair.

 

 

 

http://www.doughty-engineering.co.uk/cgi-bin/trolleyed_public.cgi?action=showprod_T29497

[ianl]

gravlocks should always be used in pairs. 600kg for a gravlock is quite reasonable, however the beam its attached to or the scaff bar may (will) reduce the SWL of the system. relevant info would be the size of the I beam and the style and orientation of the 90 degree clamps