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Reaction Force (Again)

Question asked by Mike Armstrong on Sep 8, 2014
Latest reply on Sep 23, 2014 by Jared Conway

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Hi all,

I have modelled an offshore support frame and applied an external force and gravity to include the self weight of the frame. (Please note the default mass properties have been overridden to account for missed components). However, when I assess the reaction forces underneath the 15mm doubler plates, the reaction in the Y-axis is greater than the applied load + self weight of the frame.

Below is an image of the doubler plate and reaction forces. These doubler plates will be secured onto the deck of a barge by means of a fillet perimeter weld, hence the fixed restraint in all directions in the static simulation.

Does anyone have an explanation why these forces are greater than the applied loads?

Thanks.

Mike

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Offshore Support Frame .SLDPRT.zip727.9 KB

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Gregory Halleck Sep 8, 2014 11:45 AM
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HI Mike. I believe you are seeing a mechanical advantage due to an offset force from your anchor (or pivot) point. If you decouple your moment at the anchor point you would see higher forces than the applied. You can double check your analysis by creating a simple FBD (free body diagram) on paper and comparing your results with the simulation.

..or I could be completely wrong and need another cup of coffee :-)
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- Gregory Halleck @ Gregory Halleck on Sep 8, 2014 1:52 PM
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  Mike - after looking at the FEA results a little more, I found an odd discrepancy between the FB force and Reaction force. The FB force appears relatively symmetric..while the reaction force is not. It could be the frame supports are producing an asymmetric loading condition. Just something to consider.
  
  -Greg
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  - Jim Riddell @ Gregory Halleck on Sep 8, 2014 2:02 PM
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    There has to be a shear force on the surface in the Y(?) direction to counter-act the moment generated by the overhanging mass.
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  - Mike Armstrong @ Gregory Halleck on Sep 8, 2014 2:17 PM
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    Gregory Halleck wrote:
    
    Mike - after looking at the FEA results a little more, I found an odd discrepancy between the FB force and Reaction force. The FB force appears relatively symmetric..while the reaction force is not. It could be the frame supports are producing an asymmetric loading condition. Just something to consider.
    
    -Greg
    
    Greg, the restraints are not symmetrical so I would have expected the reaction forces in the plates to be different. However, I still would have thought that the reations would have equalled the applied load, but as you said above, it may be a result of mechanical advantage.
    
    Mike
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- Mike Armstrong @ Gregory Halleck on Sep 8, 2014 2:12 PM
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  Gregory Halleck wrote:
  
  HI Mike. I believe you are seeing a mechanical advantage due to an offset force from your anchor (or pivot) point. If you decouple your moment at the anchor point you would see higher forces than the applied. You can double check your analysis by creating a simple FBD (free body diagram) on paper and comparing your results with the simulation.
  
  ..or I could be completely wrong and need another cup of coffee :-)
  
  Cheers Gregory, I will have a look in the morning.
  
  I am sure you are correct.
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Mark Ankrom Sep 8, 2014 1:03 PM
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Mike,

The summation of reaction forces has to equal the summation of applied loads. Common problems are
1. Solver is not seeing the same mass as the design model - check the .out file.
2. Solver is seeing loads that are not apparent in the user interface - again look at .out
3. There is a reaction that you are not accounting for.
4. A badly conditioned FEM model or a solver error.

I tried to look at you model. I downloaded the zip file and was able to look at the design model but there was no study included in the file. What version of SW are you using ? I only have SW2014. I will take a look at your model if I can open the study.

Mark
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- Mike Armstrong @ Mark Ankrom on Sep 8, 2014 2:22 PM
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  Mark Ankrom wrote:
  
  Mike,
  
  The summation of reaction forces has to equal the summation of applied loads. Common problems are
  
  1. Solver is not seeing the same mass as the design model - check the .out file.
  
  2. Solver is seeing loads that are not apparent in the user interface - again look at .out
  
  3. There is a reaction that you are not accounting for.
  
  4. A badly conditioned FEM model or a solver error.
  
  I tried to look at you model. I downloaded the zip file and was able to look at the design model but there was no study included in the file. What version of SW are you using ? I only have SW2014. I will take a look at your model if I can open the study.
  
  Mark
  
  Hi Mark,
  
  Thanks for the pointers. I did not upload the simulation results due to size. I will upload tomorrow morning when back in work. I take it the .out file is outputted when a simulation has been undertaken.
  
  I am also using Solidworks 2014.
  
  Mike
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- Mike Armstrong @ Mark Ankrom on Sep 9, 2014 2:14 AM
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  Hi Mark,
  
  I cannot attached the .CWE file as it is too large,.
  
  Mike
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Jim Riddell Sep 8, 2014 1:34 PM
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Mike,

First, if your only connection of the plate is a perimeter weld then you should not be restraining the complete under surface. If the weld bead is mostly the thickness of the plate then you could use those edge faces. Otherwise, I'd suggest just using the edges. As Gregory & Mark say, look at your FBD. I'd expect some difference but I've had good luck with using the resultant forces to get a sanity check on the solution numbers.

Jim
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- Mike Armstrong @ Jim Riddell on Sep 8, 2014 2:24 PM
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  James Riddell wrote:
  
  Mike,
  
  First, if your only connection of the plate is a perimeter weld then you should not be restraining the complete under surface. If the weld bead is mostly the thickness of the plate then you could use those edge faces. Otherwise, I'd suggest just using the edges. As Gregory & Mark say, look at your FBD. I'd expect some difference but I've had good luck with using the resultant forces to get a sanity check on the solution numbers.
  
  Jim
  
  Hi JIm,
  
  I have applied the restraints to the edge faces of the doublers plates. Although not entirely correct, it will be a good approximation for this analysis.
  
  Mike
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Jared Conway Sep 9, 2014 1:25 AM
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I think mark's got it right here

let's look at the whole structure's input force + weight and compare it to the reaction force total (all fixed locations selected)

check the .out file to make sure the weight is what you expect it to be

and then do some sanity checks for yourself to show that the software is working as expected. build a really simplified model and check that force in = force out.

something I did want to mention is that if you are grabbing restraints individually and adding them up to try and equal out to the total input load you may run into issues if there are any shared nodes. check the solidworks kb for some good examples.
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- Mike Armstrong @ Jared Conway on Sep 9, 2014 2:44 AM
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  Jared Conway wrote:I think mark's got it right here
  
  let's look at the whole structure's input force + weight and compare it to the reaction force total (all fixed locations selected)
  
  Thanks for the response Jared,
  
  Which faces are you referring to on the restrained plates?
  
  check the .out file to make sure the weight is what you expect it to be
  
  How do I access this?
  
  and then do some sanity checks for yourself to show that the software is working as expected. build a really simplified model and check that force in = force out.
  
  I have done this and it seems the software is producing correct results, so as thought, the issue probably lies with the operator
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  - Jared Conway @ Mike Armstrong on Sep 9, 2014 3:19 AM
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    restrained faces, I haven't looked at your model, i'm talking about fixed faces, that is where reaction forces can be measured
    
    .out file, check the swx kb, enable temp file saving, run the analysis, the .out file will be available for opening in notepad/wordpad
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    - Mike Armstrong @ Jared Conway on Sep 9, 2014 4:43 AM
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      I have attached the .out file and it seems the mass used in the simulation is different than the manually overridden value.
      
      Mass used in simulation - 904kg
      
      Overidden mass - 1690kg - to account for excluded items.
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      Offshore Support Frame Analysis-Operational Analysis.OUT.txt.zip2.7 KB
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      - Jared Conway @ Mike Armstrong on Sep 10, 2014 12:14 AM
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        "manually overrriden value" > do you mean you've done that in solidworks? none of that is transferred over to solidworks. solidworks just uses volume * density + distributed masses + remote masses
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        Mike Armstrong @ Jared Conway on Sep 10, 2014 3:11 PM
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        Jared Conway wrote:
        
        "manually overrriden value" > do you mean you've done that in solidworks? none of that is transferred over to solidworks. solidworks just uses volume * density + distributed masses + remote masses
        
        Yes I didn't realise that the overridden mass properties did not copy from Solidworks into Simulation. I did find a way to account for the additional item weights: by creating a custom material and modifying the density.
        
        However, I do not think this would have made the reaction so high as mass used was less than expected.
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        Jared Conway @ Mike Armstrong on Sep 11, 2014 7:44 PM
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        "However, I do not think this would have made the reaction so high as mass used was less than expected."
        
        you'll need to elaborate here for me.
        
        did the updated model fix the reaction issue?
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        Mike Armstrong @ Jared Conway on Sep 20, 2014 5:49 AM
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        Jared Conway wrote:
        
        "However, I do not think this would have made the reaction so high as mass used was less than expected."
        
        you'll need to elaborate here for me.
        
        did the updated model fix the reaction issue?
        
        Right, I will try and explain without the exact numbers as I away from my work machine.
        
        The initial reactions in my original post were larger than the self weight (remember this is the incorrect weight - not the overridden mass weight used in the Simulation) + applied load, which I thought was not possible. Therefore, I don't think the overridden mass issue is causing the reaction discrepancy, as the frame weight used in the Simulation is less than the actual weight of the frame.
        
        I can only imagine that it has something to do with mechanical advantage as Gregory stated.
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        Jim Riddell @ Jared Conway on Sep 19, 2014 10:41 AM
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        That's odd, I've applied over-ridden mass values to models and they have transferred correctly into SW Sim Premium. Works just fine.
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        Jared Conway @ Jim Riddell on Sep 19, 2014 12:04 PM
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        can you post an example?
        how did you validate?
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        Jim Riddell @ Jared Conway on Sep 19, 2014 1:01 PM
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        *egg-on-face*
        
        Just ran a quick test and you are absolutely correct sir. Fortunately, (for me) the time I had done this; it doesn't change the outcome since I lumped two steel parts anyway.
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        Mike Armstrong @ Jim Riddell on Sep 20, 2014 5:41 AM
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        James Riddell wrote:
        
        *egg-on-face*
        
        Just ran a quick test and you are absolutely correct sir. Fortunately, (for me) the time I had done this; it doesn't change the outcome since I lumped two steel parts anyway.
        
        The only way around this is to manually adjust the density of the material with Simulation. Not the most elegant solution, but the only one I know of.
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        Jared Conway @ Mike Armstrong on Sep 23, 2014 7:49 PM
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        or use remote masses
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Reaction Force (Again)

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