Rigging Speakers From A Lighting Bar

[Pete Alcock]

Dear Blue Room,

 

Your comments/opinions would be appreciated on this issue...

 

12m long ally barrel rigged permanently in the ceiling of the venue on 4, 1 foot lengths of M12 studding (usual clamps etc), which are very well fixed into the roof structure. Bar is internally wired and has 6, 15A outlets which we'll be using for evenly spaced 1k fresnels and profiles.

 

I would like to rig a central speaker cluster (just a pair of 12 + 1 boxes) with a total weight of about 48kg. This is essentially going to be a point load on the bar, mid-way between the centre two studding supports.

 

Question is, what is the swl of standard 12mm studdingso I can calculate whether the rig is acceptable? I've been told by a major supplier that a point load of 60kg is OK in a situation like this, but I welcome opinions of those with more rigging experience.

 

Many thanks,

 

Pete.

[Brian]

***DISCLAIMER***

 

It is imposible without a lot more information to work out the answer to the whole question.

 

***/DISCLAIMER***

 

the tension breaking load on an M12 stud (not high-tensile) is...

 

tensile strength of material = 40kg/sq mm

minimum cross-section of M12 stud = pi * 4.8mm * 4.8mm = 72 sq mm

 

so it will break at 40 * 72 = 2,895kg

 

we would normally work on 10:1 derating for 'unknown' situations, so the SWL would be around 290kg

 

HOWEVER

 

You may find that the thread strips off the stud/nuts before 2895kg or that fixings into the roof gives way first or that the ali scaff tube bends and fails before these figures.

 

In other words it is impossible, without a lot of information, to work out the answer.

 

BUT

 

48kg is not much weight (unless it falls on you) the best advise is to test it. Using the rigging that will suspend the speaker, place a load of at least 4 times the weight of the speaker and see what happens. And don't forget safety bonds which should be as short as possible to reduce breaking.

 

 

Notes

4.8mm is the minium radius of the stud at the bottom of the thread.

Quick rule of thumb is 25 tons/sq inch for mild steel

[Chas at Halls]

I would definitely agree on the caution required...my kids school had a minor accident in similar circumstances. The staff had been hanging some fresnels off a tube clamp bolted to the wall of the school hall for years. It failed under load - overnight thankfully, so no damage caused - other than to brair fresnel.

 

We would also agree on the maths for the M12 stud and have often hung considerable loads on the stuff.

 

What you don't mention is what the studding goes into. There are various chemical fixings and Fischer bolts that are very good if used in good concrete, but we wouldn't recommend putting anything on it until you'd had a load test carried out by a 'competent person'.

[Pete Alcock]

Chas and Brian,

 

Thanks for your comments on this - caution fully understood. I'm definitely not reckless about things like this, hence the post.

 

The studding (actually 5 suport points not 4, so one being exactly where I plan to hang the speakers), go into the structural steelwork that form the roof trusses of the hall, so it's very solid indeed. No wall anchors or rawlbolts or anything like that.

 

I'm satisfied the safety factor is more than adequate for what we plan to rig, and will be using secondary bonds in any case.

 

Kind regards.

 

Pete.

[Paul J Need]

Very impressive sums ther from Brian...... hoooorah someone who can do real maths!!! <pats brian on the back>

 

We work to a SWL of 250kgs per M12 studding...... but this is reduced depending on the number of studding supports.

[Brian]

Very impressive sums ther from Brian...... hoooorah someone who can do real maths!!! <pats brian on the back>

 

[bLUSH] Gosh, thanks [/bLUSH]

[Paul J Need]

:blink: Sorry Brian

 

<Paul passes you some anti-blushing balm> ;)

[ramdram]

wrong thread, ** laughs out loud **.