Who is competent, exactly?

[revbobuk]

Got a performance coming up in our Church building. Need, if we can, to not have tripod lighting stands - they mean the loss of seats, they get in the way, and they look awful. So I'm investigating a length of truss spanning the width of the builing - there is a concrete 'shelf' at the top of the side wall.

 

The issue is this. Who exactly would be the right person to give a professional evaluation of what would be required? What's the job title? Where would I find such a person?

[paulears]

If you mean you are going to have just end supports, then unless you understand the truss specs and can calculate the deflection - and then see if it's ok, then a local firm of structural engineers could do the maths for you, given the loading details and the manufacturers spec. They could also tell you the load the concrete shelf can support. Equally, the truss manufacturer could comment on the loading of the truss based on end supports only.

 

In examples like yours, how will you stop the truss falling off the shelf?

 

Where are you exactly, maybe somebody local can give you the name of somebody suitable.

[revbobuk]

I've deliberately avoided detail - because I do know that a) I'm not competent to do the maths, and b) I'm not asking an internet forum to give me the answer. We're just outside Leicester; and we'd be hiring the truss for a two day performance. We're talking light loads - 4 small fresnels a side, 9m span - here's one side of the interior, and the truss would span right across to the other side.

 

Here's the image...

 

So a 'firm of local structural engineers' is the kind of answer I was looking for - and I'd be delighted to hear from a BR member who can point me in the right direction...

[gordontech]

For that kind of lighting system you would be spending more money on hiring the truss then the rest of the lights. I know its not necessarily a relevant point but its worth considering if you have to convince higher ups etc.

[revbobuk]

What else can I do? I appreciate the truss may cost a bit, but I can't currently think of any simple alternative. A skyhook?

[paulears]

I know what I'd do. Those columns have a smaller section at the very top. It would be quite simple for a local engineering firm to make up two u shaped brackets that would be bolted together and circle the smaller section, resting on the edge. I'd then use some of the Doughty gizmos, to allow a length of scaf to fit between the front and rear column. It's short enough to be rigid, and you could mount your Fresnels there. A 9m span of truss, supported at the ends will sag rather a lot with just it's own weight to support if it's 300mm type sizes with spigot coupling. To be rigid enough it will need to be BIG.

 

If you need to put kit in the middle of the space it's a snag, but in between those columns could be much simpler.

[revbobuk]

I like that idea a lot. Re-usable, too. Any suggestion as to which Doughty gizmo you had in mind? I know a couple of local engineering firms who can fabricate the brackets, so this shouldn't be too hard.

[Pete McCrea]

9m of litec Qx30 truss will give 57mm if deflection with a 430kg centre point load (Google Litec Qx30s truss loading table for details). With a relatively light load of approximately 100kg, located along the length of the truss, I'd expect minimal deflection on it.

 

I'd expect that to come in under £100 ex VAT for a weeks hire. If you pushed a supplier possibly under £100 inc VAT.

 

Assuming that the ledges can support the total loading your proposing that is...

[ImagineerTom]

Just to bring the truss issue to a close....

 

1) a structural engineers would charge you more for doing the sums than your entire budget

2) for a 9m unsupported span you'd need some of the medium duty truss (ie not the small, thin, cheap stuff) which is going to cost a chunk

3) in that venue your biggest problem wouldn't be the truss span but rather securing the truss at either end to prevent forwards/backwards movement so that it couldn't twist and fall off the shelf

 

None of these issues are insurmountable but every one adds a bit more cost and specialist knowledge.

 

Paul's suggestion of a custom engineered clamp to fit around the columns is a much better solution in every way/

[adamharman]

I like that idea a lot. Re-usable, too. Any suggestion as to which Doughty gizmo you had in mind? I know a couple of local engineering firms who can fabricate the brackets, so this shouldn't be too hard.

 

I think Paul means something like this http://www.doughty-engineering.co.uk/cgi-bin/trolleyed_public.cgi?action=showprod_T13100 which is a convenient way of fixing the tube to the brackets.

[revbobuk]

I'm curious as to what forces are going to be causing this backwards / forwards / twisting movement, as the loads are static and straight down (i.e. gravity) and the truss itself is going to weigh quite a lot more than what we are hanging from it? Surely the weight of the truss and associated fixtures will prevent movement?

[adamharman]

I'm curious as to what forces are going to be causing this backwards / forwards / twisting movement, as the loads are static and straight down (i.e. gravity) and the truss itself is going to weigh quite a lot more than what we are hanging from it? Surely the weight of the truss and associated fixtures will prevent movement?

 

People focussing the lights? Difficult to tell from that photo but doesn't look like the truss would be sat on very much. If you could clamp it down it might be ok, but that's not necessarily as simple as it sounds.

[Roderick]

I do like Paul's idea just concerned about the height, it looks fairly low under the 'shelf'.

Any chance you could clamp a pipe along the side of the shelf?

Then over-rig the fresnels for a nicer angle?

 

The other thing to consider with the truss: how are you going to get it up there?

[kerry davies]

You could form a threaded bar around the tops of pillars, bolt on two half couplers and use a short ali scaff bar to mount the lanterns directly onto the pillars. Back in the dim and distant days I have even used heavy duty loadstraps to do the same on rounded church pillars with similar "ledges".

[Shez]

In terms of suppliers, Hawthorns (Melton Mowbray) can certainly sort you out with the truss. I'm hiring a 10m span of X30V from them later this week which would do the job for you and comes in well under your budget. They may also be able to suggest ideas for fixing the ends if you can give them sufficient details of the job.