Rigging advice needed please

[mattcoomber1]

Hi all, I have some new speakers which I would like to rig up on one of our FOH bars. The bar has been rated at a SWL of 250KG evenly distributed. I have 2 moving heads on the bar at the moment that weigh 32KG each and are rigged over one of the uprights going into the roof which I believe are 12mm threaded bar going into an RSJ in the roof. I would like to rig the speakers which weigh 17 KG each at the end of the bar next to the movers however there are no upright supports going up to the roof on the end of the bar. I am worried about how much weight is going to be at the end of the bar unsupported. Could the bar break? Would it be safe to rig the speakers there? Unfortunately I cannot move the movers further along the bar as we are also a cinema and it will get in the way of the throw of the projector. I have attached a diagram of what I am trying to say. Its not to scale, just for reference.

 

http://i300.photobucket.com/albums/nn12/mattcoomber1/Rig_zpsbwyv7wov.png

Any advice greatly appreciated.

 

Thanks

 

Matt

[J Pearce]

What is the length of the bar? The SWL UDL divided by length would give you a UDL per metre, which might indicate whether this arrangement is suitable or not.

 

 

My concern would be the bar bowing upwards in the middle, placing a force up on the centre studding, and down on the outer studding.

[mattcoomber1]

Thanks very much for the reply, the bar is 6m long.

 

Cheers

 

Matt

[Seano]

If it's a continuous scaff tube (either steel or ali) with no joiners anywhere and the cantilever at the end is less than 1m, I'd say yep, that's ok. Just about.

 

You can't really take my word for that unfortunately, I'm just some random anonymous bloke on t'internet and my advice is only worth what you're paying me for it. ;)

 

JP is probably correct, the bar will most likely be a little bit sad-face, putting the centre studding under compression and slightly increasing the load on the outer two.

[mattcoomber1]

Thanks Seano, yes it is one continuous length of ali with no joins and the end drop to end of bar is about 0.8M. The droopy ends was my main concern (isnt it always ;) ) and if that weight would be enough to cause too much strain on the bar.

 

Are there any good companies that anyone can recommend who could survey it?

 

Thanks

 

Matt

[kerry davies]

As with Seano I am an old fart on the interweb but the only real reservation would be the type of tube. Ali scaff tube varies in wall thickness from 4 to 5 mm where obviously the 5mm suits you best. However it may not be scaffold grade tube and be even thinner or very old and be thicker.

 

If it is 4mm wall minimum I can't see it "breaking" with 20Kg at roughly 600mm turning moment and it should bend very little if the fixings are secure and there isn't anything amiss with the tube. Let us know what happens when you hang the test apprentice off it.

[Don Allen]

Can you rig a diagonal brace from the top of the 12mm threaded rod to the end of the bar?

[mattcoomber1]

Thanks all for your replies, I will check the wall thickness when I get to work today. Good idea about additional bracing Don, I will have a look at that whilst I'm up there. I will keep you posted.

 

Thanks again

 

Matt

[ImagineerTom]

Threaded bar is great in tension but in compression it's only slightly better than spaghetti. It's also usually attached to RSJ's using little beam clamp gadgets which are also no-where near as effective when dealing with a backwards force. With all that in mind I'd suggest that if rigged as suggested (and assuming the pole itself is structurally correct) then the outside 2 threaded bars would be acting as a fulcrum and the centre 2 threaded bars would effectively not have any load on them at all (because they are being forced upwards slightly) which effectively reduces the load-baring capabilities of the bar as a whole as the 2 centre bars would effectively not exist. The items you propose to hang would still be within this reduced weight loading capacity but you would now be using the bar at the top end of its safety margins.

 

Personally I'd be wanting to hang /something/ in the middle of the bar - it could be rigged (using an appropriate clamp) on the top side of the bar so as to eliminate the projector problem you mention. Doing some quick fag-packet calculations a 20kg weight lantern/speaker rigged in the centre of the bar would effectively cancel out the cantilever effect of the 2 speakers sufficiently thus spreading the load across all 4 threaded bars again and keeping your SWL at 250kg with plenty of headroom and in-built safety.

 

As others have said, I'm just a bloke on the internet; undertake your own safety checks and calculations.

[pgpro2]

Be careful around assessing the local strength of the bar itself with regard to bar wall thickness etc. The SWL on the bar (evenly distributed) is 250kg. This means that each threaded rod may take 62.5kg load.

If you perform a basic calc based around evenly placed supports, then the outer 2 uprights would be seeing in the region of 76Kg (tension), and the inner 2 supports around 7.3Kg (compression)

 

Some assumptions on equipment placement have been made, but based on the details presented, I would not be loading in this manner as you are exceeding the SWL by around 20%

 

I agree that hanging something centrally (20kgish) would even the load across the four supports, but I don't suppose this is good practise.

[mattcoomber1]

Thanks everyone for all of your replies. I have decided that I am going to take the movers down and just fly the speakers. I would not want to take any risks. If the future allows us to add more supports to the bar then maybe I can look at it again but for the amount the movers actually get used, it is more important to have the speakers rigged. I may be able to swap the movers for some lighter ones but I will have to see if its worth it.

 

Cheers

Matt

[empyfree]

Just move the movers near the central threaded bars? Would help with the weight distribution and you can still use the lights, just need to update any programming that is already done.

[adamharman]

Just move the movers near the central threaded bars? Would help with the weight distribution and you can still use the lights, just need to update any programming that is already done.

 

But....

 

Unfortunately I cannot move the movers further along the bar as we are also a cinema and it will get in the way of the throw of the projector.

[Seano]

If the future allows us to add more supports to the bar then maybe I can look at it again ...

 

No need to add additional supports or otherwise modify the bar, you just need someone to give it to you in writing that the existing bar is ok for what you want to do. (Which it is.*)

Get back to the original installer for clarification would be the most obvious thing to do if poss.

 

It's been suggested above that your bar having been given a 250kg UDL implies that each support therefore has a SWL of 62.5kg. Doesn't really work like that, you're not going to get useful information by looking to 'reverse engineer' the rating that's been given to the bar. What it's been given is at least a little bit arbitrary, so the figures you end up with that way aren't likely to be very useful.

 

62.5kg would be very low for a 12mm threaded rod in tension, and also for the type of clamp generally used to attach a threaded rod that size to a beam.

(Lindapter type FL flange clamp would be the most likely candidate - you can find the specs online.)

 

If you were going to modify your bar at all, if possible it sounds like the best thing you could do is lift it a bit higher to clear the top of the screen and make the middle of the bar usable.

 

 

 

* - I'm making some assumptions here, obviously. ;)

[paulears]

He can't do that because there is a projector beam going through the space.

 

I know you mentioned the beam masking by the movers but how close is it? Even moving the movers 300mm towards centre would make it work. The 17Kg of the cantilevered load isn't massive, and is not as bad as hanging a big Fresnel on the end of a normal fly bar.