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Calculation Error - Staticly indeterminate beams


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There is a error in braceworks calculation when calculating statically indeterminate beams. The hoists acts as they are fixed in position and the truss is supported by this fixed support.

While in real life there is slack(unless you actually use a fixed connection somehow) via the round sling connecting the hoist to the truss. While braceworks in a situation with one beam with 3 supports(S1, S2 and S3) and only one heavy load in the middle of S1 and S2, the deflection results in a uplift on S3, and here Braceworks immediately applies the weight of the hoist to the beam, and not resulting in a error before the sum of loads on the hoist is 0 or below.

 

Braceworks should error out due to slack or instability when the deflection results in point loads equal to hoist weight or below.

 

Example from VW:
image.thumb.png.f7bfdf43f0bffd1d88ca762a2bb482d1.png

 

Example from RSTAB (it was hard to quickly illustrate, but there is "cable" between beam and supports):

image.thumb.png.defd3f948f853a649637ef1423c37478.png

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  • Vectorworks, Inc Employee

Braceworks, like most structural analysis programs has to assume that the hoists and their connections are rigid or it is almost impossible to calculate the system. This, from my understanding, is in line the assumptions a structural engineer has to make to be able to do the same calculation.
 

Yes as you've highlighted the S3 hoist will have an upwards force excerted on it, but this will not cause the calculation to fail or to be unsafe to hang. Technically its still within the capacity of the truss and hoists (or atleast it looks to be from the images supplied), that is why Braceworks is not throwing up an error message.

To look at this from a real world point of view how would an onsite rigging team deal with this issue?
Mostly likely they would bump the S3 hoist up a couple of clicks to take the slack out of the sling or steelflex, if the hoist is not fully under load like this.

You can do the same in Spotlight using the chain shorten field in the hoist properties, when you add a the equivalent of the length of a couple of chain links to the chain shorten field and then rerun the calcualtion the hoist will now have a positive load acting on it.

Would adding a warning to better communicate this be useful?
 

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Structural analysis programs often have the possibility to add hinges within member connections, and the supports can be rigid, but hinged supports are greatly used, if not the most. While this is the case, making a real-life scenario into a statical calculation, is indeed difficult. If you add a truss as it is rigged, with all supports being in a cable (chain, unable to support any other forces than Tension), the structure does not calculate due to instability (in RSTAB, this is the case).

 

I'm not concerned about how to handle this in real life, this is a totally different matter. My concerns are that the loads on S2 is wrong, it is potentially whatever the motor weight is on S3 wrong. So, if I have a 1kN hoist, the potential false load applied to S2 is 1kN. This is not neglectable load. In a static analysis software, I would add the "chain" or "sling" to make sure the structure behaves like in the real world, and you do add the sling in Vectorworks, but it has no effect to the calculation. But, indeed, this is only a false load until the deflection is so great that the hoist itself is lifted, but this crossing point is impossible to evaluate due to unknown variables. What slings are used and does the hoist land on the truss or "fall" off to one side or in the middle if the truss is big enough? So, in my opinion the calculation should fail if there is negative load below the hoist. Since this is a calculation software for amateurs (in the statical analysis world).

 

Why does Braceworks fail if the support result is 0 or lower or in other words, slack chain? The truss and the hoist can indeed handle this. The hoist is like this in the flight case all the time, and most trusses can handle 1kN on a cantilever. A static analysis software would give you the option to "fail if negative" or a custom value. But these are not options in Braceworks. Thus, I assume good choices are made for me in beforehand.

 

Real life scenario could be a mother grid, where you do not see what is going on up there. Later, there could be added loads between S2&3 where twisted/rotated rigging indeed would matter. The end results are the same, but the Load scenarios is very different.

 

I came across this error when I was pushing the limits of a venue. Where this false load was making the show not possible.

 

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  • Vectorworks, Inc Employee

@Stefan B.
I've been discussing this further with our dev team to get more info and they raised a couple of points/questions

  • Tolerances, Braceworks has a number of tolerances built into the calculation process, such as the geometric precision and ax deflection in the settings, how do these compare to RSTAB? (I have no experience of using RSTAB so I can't answer this myself)
  • The hoist is designed to allow a certain amount of upwards pressure to account for the flexible nature of the pickup, from the images posted we can't tell if this is working correctly in this case.
  • There is a braceworks error that should trigger when Braceworks calculates that the cahin has to much slack and isn't supporting the system, which I think is what is happening here but the warning is not being triggered

    Message: The hoist is slack; system is not supported.

    The chain is under too much pressure and will not participate in calculations (in the Braceworks Preferences, Second order analysis is selected).


From my discusion yesturday its possible that this is a bug or that the threshold to trigger the warning needs to be looked at. We will investigate

Can you provide the file you used for this test?

 

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Great to hear this is being discussed.

  1. Tolerances: There are tolerances to adjust in RSTAB, but i have never adjusted these setting. There are "limits" to where the software warns you that the maximum displacement is reached. This value is standard to 1,5%.
  2. Indeed, when looking at this i see some strange results. And digging into this a bit deeper I find even greater concerns that I need to investigate further. But applying a safety factor to "Rigging" acts favourable to the self weight in this topic. While the SF should be applied to the Top hook load, it is applied at hoist level, making the applied load to s2 even greater. If i have a safety factor of 2 on the rigging, it makes the slack chain warning pop up first at 2x the load. While in real life this is dangerous long before. So, this contradicts with guidelines on how favourable and unfavourable factors/loads are applied. Enhancing my initial meaning, the slack chain warning must be applied if load below the hook is 0 or less, before factors.
  3. Yes, but this warning should appear before as indicated in section above.

 

File attached. Play around with safety factors on rigging and see how it behaves. The bottom truss has a negative 82,98kg load on low hook, meaning the hoist is essentially lifted by the truss long ago, since this is with chain weight where some chain is above the hoist. 2 trusses above are too prove principles.

rstab wrong calcuation statically undeterminat beam.vwx

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  • Vectorworks, Inc Employee

@Stefan B.
I've done an initial look and as far as I can tell there are 2 main issues
All 4 example lines are correctly failing, based on the point loads and rigging configuration.
The calculation results look broadly correct to me in the calculation overview, except for how the 2 hoists with negative loads are behaving and being reported.
Issues

  1. First looks to be the error message itself on hoist M6, I would expect to see the message
    'The hoist is slack; system is not supported.
    The chain is under too much pressure and will not participate in calculations (in the Braceworks Preferences, Second order analysis is selected).'


    Instead the message 'the chain is under pressure and has failed' displays.
     
  2. Hoist M3 hasn't triggered the error message, despite the truss deflection causing it to be pushed upwards by a similar amount to hoist m6. This should trigger the warning, which suggests the internal threshold for the error message needs to be looked at.
     
  3. The status of the Hoists in the Calculation overview, Hoist workload tab. I think fixing the first 2 issues will likely solve this, since M3 will change status when the warning message correctly triggers. 

I'll get the Jira's opened


 

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