# Braceworks Calculation Asymmetry

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I am working on a multi layered truss structure and I noticed an interesting Braceworks behavior that I would love to understand a bit better.

The load calculations shown are on completely empty truss and all truss and symbols are inserted based on a symmetrical centerline. I notice a couple of interesting things.

Looking at just the B section - Based on symmetry it seems to me that the loads on either side of the centerline should be equal. 1B-5 should be equal to 1B-1. They are close, but not exactly equal. Looking at 4B-1 and 4B-2, the stage left #2 hoist is almost twice the calculated load of the stage right #1 hoist.

I then took the structure and rotated it 180° and created the A section. The load calculations end up being exactly the same, kind of. The loads have switched to the other side of the stage. Comparing 4A-1 & 4A-2, the stage right #1 hoist is now the one that is almost twice the stage left #2 hoist. I got the same results whether or not compensate drops is active or turned off.

I am assuming this has something to do with how the FEA engine approaches a drawing and it seems to be affected by the coordinates of the objects. Is there a way to control this? Are there settings to improve the accuracy of the calculations with more complex layered truss systems? When the differences are rather large - how do we know which one is most accurate?

• Vectorworks, Inc Employee

Are you able to post a file for testing? The info in the screenshots is small and the data limited. I'd want to see the x,y,z values of the truss and hoists before being able to offer guesses.

Hoist 2B-9 is not connected (or other failure), so the system is not symmetrical.

@Scott C. Parker Thanks for the reply and maybe you are still willing to give it a guess.

It seems the question is more about my custom truss and hoist symbols than it is about Braceworks and and FEA process. I have customized many of the properties and look of the stock truss and hoist symbols. A total rebuild of my truss and hoist symbols is on my schedule, so understanding better exactly where I screwed up would be very helpful.

I rebuilt the truss system using stock symbols and I get the behavior I expect to see. The calculations are symmetrical.

But when I use symbols I've customized, I get this.

There are many reasons for the various customizations I have made. I have always tried to be as thorough as possible when I make any changes, knowing that I don't want to be screwing with the Braceworks process, but obviously, I missed that mark!

I have attached the (2) files.

@Scott C. Parker I had a little time this weekend to investigate the behavior I was seeing. As near as I can tell it is not the customization of the hoist or truss symbols, but rather an issue of having Compensate Drops turned on or off.

Honestly, I have always had a weak understanding of exactly what compensating drops is actually doing. With the attached drawing of the simplified truss system I originally referenced, when I have Compensate Drops turned on, the load calcs get very asymmetric, especially at the hoists I have labeled A1 & A2. When the option is off, the loading is symmetrical as expected.

Compensate Drops OFF

Compensate Drops ON

File attached.

If you have any insight to help explain this behavior, that would be awesome.

The Hoists A1 and A2 (Hoist 1/2 Ton CM ProStar) seems to be corrupt. When calculating i get this failure message:

Change the two hoists and the system is correct:

Even after I replace the hoist. I get the [BrxFemHandle not set] error if I run the calculations and have "Compensate drops" turned ON.

If I run the calculations with "Compensate drops" turned OFF I get the symmetrical loads of 204.33 lbf that you show in your screenshot.

I've always been a bit confused about what compensate drops does to a system and when is it appropriate to use. I am a bit surprised though that it can generate such drastically different load calculations on two points that in reality are going to be close to identical. In reference to the cleaner version of the sample file "CompensateDropsAsymmetry.vwv" posted above - With compensate drops turned on, point A1 comes in at 427 lbf. and A2 at 195 lbf.

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