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IWB fixing distances


sunray

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I've just been to survey a job to move yet another IWB.

The bar has 24 sockets and currently has only 6 fixings, I estimate it's 12metres long so well over 2m between fixings.

Is this correct?

I've always aimed for about half of this but I haven't done much lighting bar work [only 2] since a change of job in 2002, suddenly I have 4 and possibly 5 this year, so a reality check is due.

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The issue is not the IWB itself as the thick wall tube vs typical loading weight make the suspension distance for the bar very rarely problematic in terms of deflection, the suspension interval is more a factor of the supports above. Studding has amazing weight carrying strength and I seem to remember when somebody (possibly Brian) looked into this a few years back, the results were surprising. The points above tend to set the spacing, especially if beams run at 90 degrees to the IWBs.

 

A span of 2m doesn't seem a problem unless you are hanging very heavy items?

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In terms of lowest strength / most likely to fail the studding is the least of your concerns.

 

M12 stud bar in perfect tension will take several tonnes before it fails. Standard scaffold tube (horizontally) can take 100kg point load in the middle of a 3m span and still be in acceptable limits.

 

The weakspot will be the brackets used to connect the stud to the scaffold and the stud to the roof joists.

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I've seen some very lightweight IWBs out there so I'm not sure you can take it as read that the bar itself is suitably strong for any particular span. Surely you fix it appropriately and label it with an appropriate max loading based on deflection within those spans?
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Theoretically the shape is what gives it the strength, not the wall thickness (lots of caveats here about material quality, cut-outs, damage all reducing the strength more than wall thickness) and obviously you should make a proper assessment of your unique installation and not blindly trust a stranger on the internet. My key point remains though - the weakest link in the rig is often the brackets/connectors between the stud/tube/joist and more often than not you will find that is what defines the weight loading capabilities. A 2m span doesn't concern me in and of itself.
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Its all source 4's so fairly middle of the road weight wise and I predict it's standard Andolite spec of scaff bar.The new setup has joists every 750 to 800mm so 1.6m max looks good.Place I used to work for spec'd 1250mm max and that's stuck in my head, 1200mm max works well with 400mm joists and/or 600mm tiles.Thanks for the input.
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1200mm? 4 foot spacing seems very strange to me - I've got far more than that unsupported on the ends of my stage bars - which are single tube, and 3 wire supported without bridles - and get inspected every year and were put in in 1959 onwards. So mine have 3m between points, and move!
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<br />1200mm? 4 foot spacing seems very strange to me - I've got far more than that unsupported on the ends of my stage bars - which are single tube, and 3 wire supported without bridles - and get inspected every year and were put in in 1959 onwards. So mine have 3m between points, and move!<br />
<br /><br /><br />

This is useful info, it could simply be a case of 'set the spacing at 1250 max [~4ft] so we don't need to calculate every time'. The 'habit' is ingrained in me and until I saw this set-up I had never needed to query it.

The last installation I did had metal brackets 15ft FOH welded to the I beam at 4ft intervals to match the ceiling panels which they poked through, these were originally designed for individual fittings and it felt so right to me to hang a 6m bar from them.

I have emailed Andolite asking for this info but not heard from them so far and not found anything on their website.

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Andolite have always quoted me that their bars have an SWL of 200kg at 1.5m spacing and 150Kg at 2m spacing.

 

I know that doesn't directly answer your question but is hopefully some useful information to help make an informed decision. If you've already attempted to contact Andolite I'm sure you'll get a definitive answer, I've always found them to be very easy to work with.

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<br />Andolite have always quoted me that their bars have an SWL of 200kg at 1.5m spacing and 150Kg at 2m spacing.<br /><br />I know that doesn't directly answer your question but is hopefully some useful information to help make an informed decision. If you've already attempted to contact Andolite I'm sure you'll get a definitive answer, I've always found them to be very easy to work with.<br />

Brilliant thanks. Those figures will work well for this job at 1.5 to 1.6m.

My only involvement with them in the past has been the purchase procedure, which they are very good at [urgent phone call at 4pm and a selection of 12 bars and hardware arrived by 10am, in that time some had been adapted too].

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Just phoned Andolite and they sent a sheet: @1m point load = 115Kg or 230Kg/m UDL @2m = 55Kg or 55Kg/m @3m = 35Kg or 25Kg/m @4m = 25Kg or 10Kg/m @5m = 20Kg or 5Kg/m @6m = 15Kg or 5Kg/m On those figures my previous 'guide' of 1.2m is well OTT.
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