hanging lights outside span

[numberwrong]

If you had a straight truss (lets say 9m) that was rigged from 2 dead hung points so that the span between points was 7m.

This would leave 1m either side of the truss span from where the truss was slung.

 

Is it acceptable to rig lights on the ends - out side the span?

 

I assume this would not count towards the load between points and actually reduce the load between points due to leverage - all be it very slightly in this case.

[LeeStoddart]

You can certainly add load outside the pickup points. We do this all the time with our lighting bars - there is always a "pipe end" lantern at each end. Unless we had the pickup points literally at the end of the bar there will always be something added outside.

 

These end loads do not reduce the overall load on the bar/truss but you are right in that the weight at the end of the bar will have the effect of "unloading" the centre of the bar.

[brainwave-generator]

What he said.

 

The weight is still being carried by the points / motors, so you can't discount it from the overall load, however, it does become a more EVENLY DISTRIBUTED LOAD (EDL) and thus will reduce the POINT LOAD at the centre of the truss.

[numberwrong]

I thought load tables for truss (in general) were between points?

 

I asume that the week point in a system like the one I've described would be in the middle of the span and that loads outside the span would be calculated separately

 

ie. a 9m truss with pickup points 1m in from each end will refer to the loading for a 7m span for loads between points.

 

or does the crushing force for the slings on the truss come in to play?

 

EDIT : Sorry - I'm talking about capacity of the truss I am assuming the actual points and hoists are well within limits

 

Thanks

[Shez]

I think Cantilever is the word to be looking up in the manufacturer's guidelines.

[numberwrong]

I think Cantilever is the word to be looking up in the manufacturer's guidelines.

 

It would be Slick Lite beam II - only very basic information avalable

[Hilary Watts]

The weight is still being carried by the points / motors, so you can't discount it from the overall load, however, it does become a more EVENLY DISTRIBUTED LOAD (EDL) and thus will reduce the POINT LOAD at the centre of the truss.

 

Surely not? The point load - ie the downward force of the weight being carried at the centre of the truss will remain the same. What might happen however is that (depending on the relative weights involved and the flexibility of the truss) the weight on the pipe end will counteract and possibly reduce the downward deflection (sag) at the centre of the truss. This does not mean however that the load on the centre of the truss is reduced, merely that an opposing force has been added, placing more stress on the truss. It is unlikely that such a situation can be catered for in standard loading tables, given the many variables involved, so that specialist knowledge and experience might well be required to confirm that a cantilevered hang is safe (please note that I'm not suggesting that a pipe-end hang is wrong in itself, just that it complicates the loading calculations and certainly does not increase the capacity of the bar or truss, as seems to be suggested in some of the earlier posts).

[Easter5053]

I would say its fine,

I dont have enough knowledge of physics to really back up this but I do thoroughly agree that it would be a more EDL

and if there was any cantilevering, which I dont believe there would be it should theoretically cause down force on the center as the 3.5meters to center would be the fulcrum to the dead hangs pivot.

 

again this is what I believe based on very cloudy gcse physics memory, feel free to correct me.

[son of lx dad]

 

and if there was any cantilevering, which I dont believe there would be it should theoretically cause down force on the center as the 3.5meters to center would be the fulcrum to the dead hangs pivot.

 

 

It is known as a Cantilever - a rigging project manager of the country's biggest lighting and rigging company just told you so.....

[Seano]

Is it acceptable to rig lights on the ends - out side the span?

Generally, yes - though as with all rigging it needs to be done with care by a competent person.

As a rule of thumb its usually considered acceptable to cantilever up to 1/6 the length of the truss.

 

Surely not? The point load - ie the downward force of the weight being carried at the centre of the truss will remain the same. What might happen however is that (depending on the relative weights involved and the flexibility of the truss) the weight on the pipe end will counteract and possibly reduce the downward deflection (sag) at the centre of the truss.

Exactly right.

 

It is unlikely that such a situation can be catered for in standard loading tables, given the many variables involved, so that specialist knowledge and experience might well be required to confirm that a cantilevered hang is safe

Again - exactly right. It's not that difficult and you don't need a *lot* of 'specialist knowledge and experience', but there is a clear legal duty for the person hanging the truss to be a "competent person" within the meaning of LOLER. Probably the most important aspect of that competence is to be aware of one's own limitations and know when to seek advice.

 

Edit to add:

A rather good free download from Prolyte: 4.6Mb pdf

[trussmonkey]

Different types of truss will handle cantilevers in different way. Normal load tables dont cater for cantilevers so you would have to speak to the manufacturers. I know in the office we have load calcs from Thomas for their Supertruss range specifically dealing with cantilevers etc.

 

TM

[brainwave-generator]

The weight is still being carried by the points / motors, so you can't discount it from the overall load, however, it does become a more EVENLY DISTRIBUTED LOAD (EDL) and thus will reduce the POINT LOAD at the centre of the truss.

 

Surely not? The point load - ie the downward force of the weight being carried at the centre of the truss will remain the same. What might happen however is that (depending on the relative weights involved and the flexibility of the truss) the weight on the pipe end will counteract and possibly reduce the downward deflection (sag) at the centre of the truss. This does not mean however that the load on the centre of the truss is reduced, merely that an opposing force has been added, placing more stress on the truss. It is unlikely that such a situation can be catered for in standard loading tables, given the many variables involved, so that specialist knowledge and experience might well be required to confirm that a cantilevered hang is safe (please note that I'm not suggesting that a pipe-end hang is wrong in itself, just that it complicates the loading calculations and certainly does not increase the capacity of the bar or truss, as seems to be suggested in some of the earlier posts).

 

Sorry the point load isn't reduced, the strain is.

 

The truss's point load rating is usually based on what it can do before it buckles and bends; where the EDL is based on what it can take before the whole lot falls apart.

 

So the actual point load at that point will be the same, but because the overall load is more evenly distributed across the truss, there is less strain on the centre.

 

That said, truss only flexes a little bit.

 

 

As for 'Cantilever' - that is indeed the correct term. You'd be surprised what can be taken on a cantilever. I know that Prolyte V40 stuff can take 400kg on a 1m cantilever. That is really quite a lot!

[Easter5053]

and if there was any cantilevering, which I dont believe there would be it should theoretically cause down force on the center as the 3.5meters to center would be the fulcrum to the dead hangs pivot.

 

 

It is known as a Cantilever - a rigging project manager of the country's biggest lighting and rigging company just told you so.....

 

 

Cheers.......honestly not sarcastically I appreciate the correction

[Brian]

...EVENLY DISTRIBUTED LOAD (EDL) ...

I've always known this, and seen it referred to in all the classic texts, as UDL (as in Uniformly Distributed Load). Is it the same thing?

[LeeStoddart]

...EVENLY DISTRIBUTED LOAD (EDL) ...

seen it referred to in all the classic texts, as UDL (as in Uniformly Distributed Load). Is it the same thing?

 

When (many moons ago) I did my Civil Engineering Degree course we did indeed talk about Uniformly Distributed Load when designing structures like bridges. So maybe your "classic texts" are about as old as me :).