Internally Wired Bar Loading

[jhughes01]

In our performance hall, the front of house lighting points comprise Internally Wired Bars arranged in an approximately 10m square. The IWBs are roughly a "C" section, with the space filled by a rubber strip to retain the cables. I believe them to be made from Aluminium, and to be manufactured by Furse.

The whole structure is suspended from the ceiling using brackets mounted to the ceiling and then onto the bar by clamps. The sections are joined with tubular collars. This is illustrated in this photograph.

We have previously mounted MAC 250s on the bar as shown in this picture. However, we are planning to use MAC500s for an upcoming show, and are concerned at the increased loading. The 500s weigh 150% of the 250s at 33kg rather than 22kg.

 

Does anyone have any maximum design loading figures for this type of IWB, as one concern is that the C section will collapse in on itself.

Also, I have seen the bar bend on past occasions and am therefore worried that the MACs will cause distortion (or worse) of the IWB. Is it better to "straddle" the MAC's mounting brackets around a roof support bracket, or is there another way of mounting them. One suggestion has been to use a length of scaffolding pole to hang it with multiple clamps along IWB thus bridging roof supports to spread the load. Has anyone had experience of this kind of IWB/problem?

 

Thanks in advance.

[Tomo]

Nobody here can help you unfortunately.

Your question requires a lot of information which we do not have access to!

 

You should have figures for the maximum distributed Safe Working Load (SWL) and sometimes they also give the maximum point load provided in the venue's documentation.

 

Usually there is a tag at one or both ends of the bar stamped with the distributed SWL, as that is a little harder to lose than a file of papers.

I'm not sure if this is a legal requirement on fixed rigging as opposed to movable bars which are considered lifting equipment.

 

This should give you some idea, but if you don't have figures for max point load then you can't tell whether you will locally overload the structure without asking a competent structural engineer.

[jhughes01]

Thanks for the reply Tomo.

I'll have a scout about for SWL or other details on the bar. I expect that if any paperwork was provided on installation it will either be lost, or filed in the records department (as good as lost ), as it has certainly been there for over a decade.

I feared that a Structural Engineer was going to come in at some stage, but wanted to check here first in case "industry people" had any more knowledge of the IWB itself.

 

Thanks again Tomo.

[mike]

Tomo

 

As Jhughes01 says, you are pretty much on your own here - however if the event is not too far away then maybe contact other users of the facilities - if it is a public hall - or local companies - I believe Hi-lights are local to you - to see if they have researched into it, or used the venue.

 

mike.

[damian666]

you need to see if there is any original documentation on the installation first.

If there isn't, then you need to encourage the management that what is in the building is not sufficiently known about, and as you have seen the system show signs of weakness in the past, then it is up to you and the management to decide a course of action. If this means getting an external assessment done then so be it. You are quite right to be cautious, and question the potential for greater loads, or in fact, if there is no documentation at all, any loads whatsoever! If it is indeed of the age it seems, I doubt whether it was ever intended to take the loads which you are hoping to apply. Further research needed.....

[Chris Higgs]

Those bars may be made/supplied by Furse, although other companies did use them.

Try Johnny Mac at Hawthorn Theatrical - he would probably know or know where to find out, as would Richard Atkins (I'm sure someone will help out with his location.) Also try David Harwood at Central Theatre Supplies in Birmingham.

As you appear to have done from your photos, perhaps only hang at the support points (what rating are they?)

My gut instinct is that the bars were installed for the anticipated load of the likely 650/1k profiles and fresnels of the day; 10 kgs per circuit outlet max, I should think. I certainly would not add secondary tubes to them.

HTH

[jhughes01]

My gut instinct is that the bars were installed for the anticipated load of the likely 650/1k profiles and fresnels of the day; 10 kgs per circuit outlet max

 

10 kgs would tie in to the majority of "stock" equipment of similar age to the bars (other than the CCT silhouettes, which are nearly as heavy as the MAC250 - hence why we were not as worried about mounting the 250s).

 

I'll bear this information in mind, and do some socket counting.

Thanks.

[lxkev]

Hello,

 

Im dealing with a similar prob. A grid was installed 6 years ago, I took over 2 years ago and had some concerns. My whole entire grid shakes when riggin on it, I rang the company who installed it, there response is we dont know how much it can hold its not down to us.

 

Well I rang the council (my venue is part of a school) and they said the company was right and they dint have to supply a safe working load as it a install (which idiot decided on that needs shooting). However they sent out some one to give an inspection, ive now been waiting 8 months for a report, detailing what testing needs doing and they will carry it out.

 

So if its part of a school or a council run venue, give them a ring.

[Chris Higgs]

"I rang the company who installed it, there response is we dont know how much it can hold its not down to us."

 

I think they'd find it most certainly is down to them if it was intended as a flexible system that allows lighting to be changed around (which would seem reasonable to expect?).

They must have had a design brief to work from, and at some time someone must have decided what the load should be limited to. This may have been by the 'full' lighting rig anticipated - fine, but it is still a load that was used.

As with my other post - I wouldn't exceed at each outlet what the weight of the appropriate vintage lantern would have been at the time.

Meanwhile, it must be inspected from time to time, especially as part of a state school.

 

The other point to bear in mind is - if there's no SWL marked you shouldn't hang anything on it.

Maybe that's what the contrcator meant? Only hang the intended load (or less).

HTH

[Jivemaster]

Looking at the pics the suspension for the IWB is also interesting! If there isn't a rated SWL then there should be, if you are going to change from the original install.