safe truss spans

[gingertech_2000]

Hi

 

I have been through a lot of topics and the search function but cannot find a direct answer to my question

 

I need to know whether a 10m span of truss from 3m uprights is safe to do. Is there a recommended safe span allowed before too much sagging appears in the middle or it becomes dangerous

 

Thanks in adavnce for your help :)

[dunk_1984]

What's the truss? The manufacturers information will tell you load ratings and deflection over different spans.

[gareth]

Two things about your question suggest, unfortunately, that you really oughtn't be doing this yourself and should be getting someone experienced to help you or do it for you.

 

One - it depends on what type of truss. 20.5" A-type, for example, can handle much more load over much larger spans than, say, Trilite.

Two - it's not all about the span. It's more about the load you're applying and how it's distributed on the span.

[smalljoshua]

Find out what make of truss you have, go to the manufacturer's website, get the loading charts.

 

This will tell you how much weight a 10m span can take both evenly distributed and point load in the middle of the span. It'll also tell you how much the truss will deflect.

 

Josh

[AndyJones]

To be fair, the Loading tables won't tell you how many vertical supports you need. or how much you can load each vertical. The best thing to do is to find out the makers of the truss, and then give them a call. Most manufacturers will be fairly open and easy to talk to about different set ups.

 

Chat soon

 

Andy Jones

[Joe Betts]

However, that does not matter, as a 10M truss hung on 3 points, as actually 2 x 5M spans from a loading point of view.

 

Ie., If you load is evenly distributed, the Points 1 & 3 will take 19% of the load, while Point 2 (centre point) will

take 62% of the load.

 

I suggest you get the manufacturers loading tables (usualy available for download from manufacturers website)

And check that the load you want to apply is within their specifications, for the distance between supports

(I.e 5M if your centre point is actually centre, and your outer points are at the end node)

 

Assuming this is ok, you should then work out the total load that will be applied to each point.

(I.eIf your total UDL is 1000Kg, then P1 and P2 will be carrying 190Kg, and P3 will be carrying 620Kg)

And check that the points are suitably rated.

 

Deflection is to be expected, the more you load the truss, the more it will deflect.

The manufacturers loading charts will tell you how much defection different loads and spans

will incurr, but as long as its within the limits set out on the chart, it is allowable.

 

Remember - if your not sure about anything, consult a rigger. Most production companies will employ riggers,

or will point you in the right direction, there are also pleanty of free lance riggers around.

[Seano]

To be fair, the Loading tables won't tell you <snip> how much you can load each vertical.

Unless the vertical is a tower truss (as it really should be) - in which case, yes they will. (Because the truss is specifically designed to be used as a ground-support leg, and the manufacturer's data reflects that.)

 

Ie., If you load is evenly distributed, the Points 1 & 3 will take 19% of the load, while Point 2 (centre point) will take 62% of the load.

 

This is correct, but its based on some assumptions that won't usually be true. Firstly it assumes that the pickups on the ends of the truss are right at the very end - but its actually quite desirable to have a little bit of a cantilever there from both a loading and a practical point of view. Also it assumes that the pickups are all at exactly the same height - in the real world its quite likely that this won't actually be the case. If you're talking about a flown truss on motors, for example, a small 'bump' on one or other of the motors can make quite a bit of difference to the way the load is distributed.

 

The problem with looking at a relatively simplistic calculation and getting a figure like (say) 19% is that its tempting to think you then know exactly what's going on.

If you do a calculation and the answer comes out to 18.6675, its better imo to tell yourself the answer is "about 20 ish" - it helps you to not forget there are underlying assumptions that mean your result isn't going to be all that accurate.

 

There's already been some comment on the span/loading of the horizontal truss, and a brief mention of the compression loading of the verticals. Really, they're pretty easy to check out. What's more at issue is the stability of the legs (and therefore of the structure as a whole) - that's a much more likely 'danger zone' for mistakes to lead to the structure collapsing, and as its much less straightforward to assess whether a proposed design is going to be adequate that's an area that really does require a certain amount of knowledge/experience.

[Joe Betts]

Quick apology needed here - I have mis read the OP, and gone off on a bit of a tangent.

 

Im in agreement with Seano, the manufactures loading charts do tae account of use as a vertival tower,

and in many cases this is actually stated on the sheet.

 

The only other thing to bear in mind is bases - are you building a box, or free standing goal post?

If the latter you will need suilable bases.

 

These questions are really for the manufacturer to answer, as what you can and can't do depends entirely on the truss.

[Jivemaster]

The first place to start is to identify the truss, by make and model. There are lots of sorts and sizes of truss, the smaller ones will not support their own weight at ten metres. The larger trussing will have a large SWL(WLL) and larger safety factors at ten metres.

 

Remenber that while all trilite has three tubes, not all three tube truss is trilite. Unless you have seen the purchase information you need to see the inspection documents for each piece of truss.

 

Once you have the maker's name, they or their (UK) importer will likely be pleased to look over your rigging plan and tell you if you are within the design parameters.