Truss Circle

[Edlight]

I will be hanging a ladder truss circle 5.5m above a stage, this is to be rigged from 5 1/2 ton loadstar motors.

The truss will have a tab track hanging from it at a weight of 80kg evenly distributed over the truss. There are 8 cloths each 5mx5m which will also hang on the track, these at a weight of approx 25kg each (200kg total)

Obviously I need to take the weight of the truss and all rigging components into consideration before I get a total loading.

 

My question is regarding safetys.

 

The truss has to be set in a interval (it forms part of the set for a mixed bill of dance) The loadstars will be lowered in to the truss attached then flown out, (a relitvely quick procedure)

 

Would I need to then bring out the scope to attach a safety to each point of the points the motors connect to the truss or beacuse if one point were to fail the remaining four points are still able to take the weight are safetys required in these circumtances?

 

With kind regards.

[timmath]

Three options:

 

1) Use load arrest (inertia) reel's, with these you have a retractable safety that will come down with the motor.

 

2) Unusual Rigging have a policy in Earls Court by where if you are using less than 25% of a motors SWL it doesn't need a safety.

 

3) 'Normal' steel safeties that you go up and fit.

 

Which you chose it down you your risk assessment, if it were my choice I'd go with option 1.

 

Regards

 

Tim

[Neil Frazer]

A further option would be to use double braked motors.

 

AFAIK you don't legally need a safety for these in the UK but don't quote me.

We use them on trusses that fly during the show.

[Tomo]

You don't legally need a safety at all - what you need to show is that you took all reasonable steps to prevent an accident.

In many situations a safety bond is the most practical way to prevent a failure from causing an accident.

 

Not always though!

If you are using sufficiently overrated hoists in a suitable arrangement to prevent the thing dropping, you've covered yourself.

You need to prove this of course!

 

I'd guess that five motors on a ring would be enough if any four motors can hold the entire load without exceeding the SWL or causing it to swing 'too much', but nobody here can tell you whether that is the case, or indeed how far 'too much' swinging would be.

Only someone who looks at your rig design and venue can decide that.

 

Finally, according to an article in the latest ABTT mag, double braked motors don't really add anything. They are a requirement in Germany for certain things, but not anywhere else.

[timmath]

A further option would be to use double braked motors.

 

Using double braked motors will only protect against motor brake / clutch failure, what if a top point or truss fixing fails?

 

Regards

 

Tim

[Neil Frazer]

Using double braked motors will only protect against motor brake / clutch failure, what if a top point or truss fixing fails?

 

 

The discussions I've had with riggers in the past vary but most are along the lines that the safety is there to protect against break failure within the motor. Recently we have been using cluch chains to safety from the truss to the chain just above the motor on static pieces.

 

Truss points and top fixings are far less likely to fail than a motor break. (if inspected, maintained, installed and rated properly).

 

However it does all come down to your risk assesment at the end of the day.

[Jivemaster]

You have to do the easy calcs for all the loads applied, and the harder calcs for failure conditions Probability of slippage or failure of one or more parts and the survivability of the remainder. Then Risk Assess the lot, minimise the hazard, minimise the risk, then insure the rest.

 

Consider how you will ensure that the hoists distribute the load evenly as the assembly rises. Unless all the hoists lift together evenly the load will be wildly uneven.

[Chris Higgs]

What's the span between each pick up, what's the 'cantilever' (such as it is) and what's the load per point?

Sounds like you've got it covered to me; the various posters have got it right, I believe.

Just make sure the motors have been thoroughly examined and are as well maintained as they can be...

[damian666]

a principal we use in many exhibition venues and the like is:

top points are construted of equipment of a suitable safety factor to remove the need for safeties, eg a 2T SWL (at 7:1 to max break) roundsling in a basket over a beam is effectively saying that it will fail with an induced load of 28T! providing that roundsling is sufficiently away from sources of high heat (eg a parcan) then that leave the top point OK on it's own. or use a steel wrap if in a more vicious environment.

Similar applies to the truss pickup.

As regards the motor, a lot of venues use the 25% rule, ie if the single braked hoist is only holding 25% of it's max rated capacity then a 4:1 safety factor is sufficient. If a single braked hoist is carrying more than it's 25%, then as said either apply a bypass from the truss to the chain using an appropriate sized clutch chain, therefore allowing for failure of the brake/motor assembly ( as the chain is at least a 5:1 in itself) or you run an independant safety form point to truss.

Or you utilise double braked motors. The usual thought being that if one of the highest levels of safety requirement (ie moving loads over audience) is doable in germany with suitably certified double braked hoists, then we can use that specification in the UK in similar situations.

It sounds to me that if you can acheive 25% usage and/or redundancy then you are happy to go with single braked motors and no independant safeties.

What you do need to do though is be able to record your reasoning for going with whichever system, why you do it that way, and the steps you are taking to ensure the safety. As Tomo pointed out, there is no law to say you must put an independant safety on every hanging point, PROVIDED that the reasons for not doing it are sound.

That said, every council and licensing department has their 0.02p to put in, so definitely worth sounding them out too.

If you would like consultation or equipment hire, PM mate.

[Nick Evans]

Recently we have been using cluch chains to safety from the truss to the chain just above the motor on static pieces.

I bet there is no manual for a motor which advocates this method as an acceptable practice, neither is the purpose for which chain clutches were designed.

[Edlight]

What's the span between each pick up, what's the 'cantilever' (such as it is) and what's the load per point?

Sounds like you've got it covered to me; the various posters have got it right, I believe.

Just make sure the motors have been thoroughly examined and are as well maintained as they can be...

 

The diameter of the circle is 6.5m, 5 motors over a circumference of approx 21meters. 4 Meters between each pick up.

 

Weight Per Point with equally distributed load works out as follows:

200kg Cloths

80kg Tab Tracks

Waiting for confirmation of truss weight but truss spec can be found here: http://nobleox.com/?tab=System&chapter=FD32

Other weights that I need to take ito consideration which sometimes get over looked I think is the weight of the Motor itself (1/2 ton loadstar with 20m chain)and all rigging used on the truss.

 

I hope this helps.

[Chris Higgs]

Looks like it will be fine, to me. Do your sums, it seems you know what you are taking on.

 

The point about the secondaries you plan is interesting - the two main players in the hoist market distance themselves from the practice.

A couple of the main players in the rigging field are conducting research on the method.

The clutch is unlikely to be matched to the hoist chain and isn't necessarily going to do the chain any good or remain captive. Much depends on the type and size. Better to do a completely separate bypass; in this instance, though, will you actually need any secondaries?

 

I have two views on this.

1) If it is really the case that you have, say, twice the capacity per motor, per point and per span that you are using, you might rely on redundancy unless there is some other factor that influences the decision.

2) 'Safety it' and sleep well. Keep them taut!

[Chas at Halls]

I'm sure you have also considered using a D8+ rated hoist, which would not require a safety at all.

 

I understand there are such hoists available in the UK now - designed to get around this exact problem....

 

Chas at Halls (Home of the Plasa Innovation Award 2005 - for the ASM D8++ chain hoist...:)))))

[sam.henderson]

Can you expand on why it doesn't require safety's, Chas at Halls??

 

Ta,

 

Sam

[Peter Hind]

I assume that the circle is horizontal.

It would appear that the ladder circle is required to span about 4 metres between points on a curve.

Clearly, the strength of a ladder in a curve is very different from a ladder in a straight line.

You probably need to consider a dynamic magnification factor of 1.25 as the circle will be lifted with the loads attached.

Someone has determined the allowable load that can be applied to the ladder circle.

One for the manufacturer I would think.

Obviously you need to sure about the capcity of the motors and the supporting structure above because I would very much doubt that a ladder circle would go 8 metres between support points.

The discussion about the safeties and allowable loads on chain hoists would appear to be secondary until you know that the truss will work.

Perhaps the risk assessment needs to include comment about allowable and actual loads on the ladder truss?