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Non Uniform Load on Multi Hang Truss


HorusLP
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Hi All,

 

So recently I've decided its high time I got my head around weight loadings rather than getting other people to do it for me.

I've gotten my head around basic point loads on a truss between two motor hangs and calculating the weight on each hang depending on the loads position using this formula - Hang 1 = point load(kg)*distance to hang 2(m)/span between two hangs(m). Obviously that doesn't include the weight of the motor/truss etc, but that's simple enough to work out.

 

Built myself a little calculator template in Excel to prove it with/experiment with different loads at different points on the truss etc.

 

My question is how do you do the same calculation for multiple hangs?

 

I've found loads of examples online but they are all based on a uniform load, but in the real world you aren't always going to have a uniform load...you might have a few extra moving head profiles in the middle of your 6 motor straight run of truss and less on the outside, that's going to have a big impact on the weight on each hang, especially based on the examples I've seen online for UDL where the central most points take more load than the outer ones.

 

Anyone with a bit more brains than me able to advise?

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Welcome to the forum!

 

As you've discovered for a two point hang it's fairly easy to work out using moments without knowing anything about the truss itself. Unfortunately as soon as you go to a multi-point hang things get a little more complex. If you try and set up a system of equations in the same way that you have you won't be able to get it to solve neatly - for example, for a three point hang, a single point load (and ignoring the mass of the truss) you have three unknowns (the load on each motor) but only two knowns (the load weight and the position fo the load on the truss) - can't be done. The key thing you're missing is how much the truss bends when the load is imposed, which obviously depends on the truss. In structural engineering terms, a two point hang is a 'statically determinate' structure, and a three point hang is a 'statically indeterminate' structure. There are various rules of thumb you can use which it sounds like you've found - often in a theatre/events context even if the load isn't exactly uniform it's close enough that these rules of thumb are useful, and also that there's often a large margin between the SWL of each individual hang/motor and the actual load.

 

It goes without saying that you should consult a suitable qualified structural engineer if you are at all unusure about what you are doing or are doing anything complex. For proper calculations from scratch, the material properties of the truss are used with some calculus to work out the 'bending moment' at each position along the truss as well as the deflections - I'm not a structural engineer myself so I'm afraid this isn't a very satisfying answer, but sadly it's not as easy as it looks!

Edited by Jevans
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Hi Jevans, thanks for the welcome and the info!

 

Interesting stuff. I would be quite interested in figuring out the calculations required to actually know the load on each motor in a multi motor scenario.

We typically use all Milos truss, I have found some info on Deflection under load etc on their website...time to do some more digging!

 

 

 

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Interesting stuff. I would be quite interested in figuring out the calculations required to actually know the load on each motor in a multi motor scenario.

We typically use all Milos truss, I have found some info on Deflection under load etc on their website...time to do some more digging!

Again welcome to BR Lee but do fill in your profile and location because it helps when people respond to know a bit about you and your background.

 

Having said that you may already have guessed that I am just another random old pedant on the interwebby thingy so take this as intended, advice and nothing more. I am not exactly brain dead but as soon as one moves past simplistic loads I hand it over to a man who has spent five years studying the academic side and ten more working out how to apply it to the events business. It isn't just something you do a bit of digging on, for instance, a cousin of mine went to Uni in the 60's and proudly showed off his paper on "The Moment of Inertia of a Cornflake". That being an unloaded cornflake with one suspension point so I understood it. Multi-hang entertainment trussing, invented since the 60's, when loaded with some of those loads being dynamic, is a whole other galaxy to a simple cornflake.

 

I daresay that there are plenty of BR members who could do the job but few enough who could teach others and even fewer who could do it via this forum. The NRC itself specifically does NOT assess either;

Specification or design of rigging equipment or Structural design or engineering

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On a multipoint hang you'll never truly know what any point has without a load cell, as it depends on the tension of the various points. You can calculate a worst-case and check that's within SWL, but without a load cell you can only look at the point/shake the chain and guess if that point is actually taking weight or not.
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Thanks for the replies all!

 

Yes I will get round to sorting my profile soon, and yes I accept this isn't something I'll master simply by doing some digging, but it is something that both interests me, and having a little more knowledge of will benefit me greatly in day to day work.

It's always nice to have enough of a basic understanding about something you rely on daily, although I certainly leave it to those qualified when it comes to designing and signing off a system : )

 

Jon - I'd be very interested to know more about calculating a worst-case to check that it's within SWL if you have any info you could point me at?

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The Prolyte BlackBook provides some useful background if you are interested in trusses and the implications of various applications. Whilst obviously geared towards their products to an extent, there is a wealth of broad information and useful ‘rules of thumb’.

 

https://www.prolyte.com/getattachment/Support/BlackBook/BlackBook2020.pdf.aspx?lang=en-US

 

I cannot stress enough the importance of calculation when deviating from manufacturers load tables and this is also stressed at various points in the BlackBook too I believe.

 

As previously mentioned, when working with statically indeterminate systems, load cells are rather essential.

 

E2A please do not rely on this as a single resource. I mainly linked to it for the general structural principals, there are elements of the BlackBook that do need updating to reflect recent changes to standards in other areas.

Edited by operalx
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The simple answer is that there isn’t “a sum” you can do. I dabble in this area and we make the calculations using specialist software that costs thousands and which requires a fair bit of engineering / physics knowledge to use. Then once a job is out being used there’s load cells everywhere to check it is working as expected and records/feedback is generated so that we can improve the design in future.

This sort of question is answered with mountains of data and equipment rather than a few sentences and a reference table. If you’re trying to design a load get someone who can do this stuff to do the sums for you.

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As a thought experiment, consider a truss suspended on three points. Bump one motor just 1cm up or down. That significantly changes the loads on all of the points whilst being virtually indistinguishable by eye. Whatever you calculate in advance, the real world implementation can be subject to quite large variances.
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Bearing in mind all the issues with full calculation and the practical reality of statically indeterminate systems mentioned above, the method I use for estimating fixture loads on a multi-point truss is to treat every span as a two point and total each point up.
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As a thought experiment, consider a truss suspended on three points. Bump one motor just 1cm up or down. That significantly changes the loads on all of the points whilst being virtually indistinguishable by eye. Whatever you calculate in advance, the real world implementation can be subject to quite large variances.

 

I did this as a presentation to a lot of our touring engineers when I worked at a hire company. We put 4 motors on a 15m length of A-Type and loaded it with a fairly typical 'festival package' of large moving heads (Vipers I think), strobes, molefays and then all the associated cabling. Unlike the touring systems, I put loadcells on it.

 

I sent the plot around a few days prior and asked everyone to bring their loading calculations.

 

Then we trimmed it to everyone's satisfaction in the warehouse. I asked everyone what the point loads on each motor were. Everyone was fairly confident they knew pretty much what was taking what load.

Then I put the Loadcell screen up on the projector. The estimates were mostly ballpark correct, but there were deviations up to about 250kg from what people had written down.

Then clicked each motor up or down an inch (indistinguishable to the eye) and we all watched together as the loads changed hugely.

Then I also ran motors up and down until the 'theoretical' load distribution was showing on the cells... the truss was rather "wonky".

People were very surprised how far away estimations were from reality.

It was interesting because when we'd previously talked about touring loadcells, it was unanimously voted down, due to the extra time to rig, loss of trim height, and cable faff.

After doing the demonstration, it become much more normal for people to come to me and ask if they could have loadcells for certain aspects of their shows - particularly video walls.

I think (and, to be fair, hope) that the more people come aware of this, the more common loadcells may start to become in entertainment applications.

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