Simulate Early, Simulate Often... In Rhino
Solid versus hollow test with Rhino design
My latest project contains a retractable roof (www.villo.nl).
Now it is an interesting question what the z-displacement is just before it is supported by the mast. The roof will be made of an aluminum frame covered with aluminum sheeting.
First I did a Scan and Solve test as if it is a solid piece of aluminum and specified a scalar force for the gravity. Then the question raised; what's the difference with a hollow structure made of frames, stiffeners and sheeting? So I set up a simple test to find out.
To keep it simple I modeled a simplified geometry both hollow and solid. One constraint of SNS is however that I can only select a single solid. When modeling the structure, several solids are created which are not connected. I tackled this issue by adding a small rectangular pipe connecting the outer shell with inner cavities. Thus creating an open cell structure.
Furthermore I didn't use the gravity option but simply added a scalar load representing gravity.
The results for the z-displacement were:
- Solid: -9.5 mm
- Hollow structure: -19 mm
This seems a big difference, but wall thicknesses are small and I only used one internal stiffener.
Attached is the test file.
As I am not a specialist in FEM, I am interested in your comments.
Please find a full report about Villo roof analysis over here
Gerard Petersen
Now it is an interesting question what the z-displacement is just before it is supported by the mast. The roof will be made of an aluminum frame covered with aluminum sheeting.
First I did a Scan and Solve test as if it is a solid piece of aluminum and specified a scalar force for the gravity. Then the question raised; what's the difference with a hollow structure made of frames, stiffeners and sheeting? So I set up a simple test to find out.
To keep it simple I modeled a simplified geometry both hollow and solid. One constraint of SNS is however that I can only select a single solid. When modeling the structure, several solids are created which are not connected. I tackled this issue by adding a small rectangular pipe connecting the outer shell with inner cavities. Thus creating an open cell structure.
Furthermore I didn't use the gravity option but simply added a scalar load representing gravity.
The results for the z-displacement were:
- Solid: -9.5 mm
- Hollow structure: -19 mm
This seems a big difference, but wall thicknesses are small and I only used one internal stiffener.
Attached is the test file.
As I am not a specialist in FEM, I am interested in your comments.
Please find a full report about Villo roof analysis over here
Gerard Petersen
Views: 1682
- Attachments:
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SNS-test-hollow-solid.3dm, 5.5 MB
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Replies to This Discussion
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Permalink Reply by Vadim Shapiro on
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This looks great and appear to make sense. Thank you for posting this example.
However, you should be cautious: this is just one numerical computation of an idealized model, and you may need to do a lot more before making important engineering decisions. For example, you should always run your models at different resolutions. The picture below shows that at higher resolution (60,000+) your max displacement is significantly higher, and high stresses will show up a few more places. Also, you connected your hollow structure with very "small rectangular pipe" that may need to be analyzed separately, e.g. for buckling, or simply if you want to get more accuracy/resolution. Etc. ... you should definitely talk to a structural engineer before making such design decisions.
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Permalink Reply by Gerard Petersen on
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Thanks Vadim.
Yes I can understand that it is important to run at different resolutions. In reality I will not have the 'rectangular connection pipe' and by placing them near the neutral line I hoped it doesn't affect the stress simulation. If any hot spot occurs near the 'rectangular connection pipe' edge I don't mind or do I make fundamental mistakes then?
For me the challenge is to be able to analyze a hollow structure build up from frames, stiffeners and sheeting instead of a non realistic solid.
No decisions will be made solely based on this analysis but it certainly helps me in having an idea of the behavior of the roof in this specific situation.
I already have the idea to make the beam more triangular shaped in top view in order to minimize the deflection at the other end.
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I cannot tell you if a single solid is a reasonable approximation of a complex structure. The way individual structural members are manufactured and joined may be also important (or not). All I can say is that you are using Scan&Solve as it was intended: you can compare different configurations and idea quickly, use it to check your intuition, and to improve your design. But Scan&Solve can be also used to perform detailed analysis, to determine significant locations and magnitudes of stresses, and make critical design decisions. This requires a lot more skill, knowledge, and experience that many of us lack.
But I would encourage other members to comment and share their opinions. This is the whole point of having this network!
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Permalink Reply by Richard.L on
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The retractable roof looks cool.
In my opinion, jut only to consider the worst situation, that is the roof has reached the support(the mast),but the the support 'Gone'(No constraints there).
If the deflection is acceptable, you may lucky.
First of all, I think aluminum-alloy frame is needed.
The previous three pictures appears that the roof may be to 3000mm x 2000mm at least, only three constraints；the wight of the whole assembly roof may make deflection in trouble.
So the shape&position of the frames are very elegant design by calculations ,even the numbers is important, too.
More frames or big shapes will reinforce the stiffness,but the wight raise up ，too. The deflection is uncertainty.
You can compare several proposals according to calculation to find the acceptable proposal.
So the S&S is helpful， fast to perform detailed analysis.
As above ,I also think your test sample by S&S make sense
The test model :Solid may the better than the hollow，because of the structure is very different. Try some ribs with the hollow one.
To your compared samples the same conditions is made the results reasonable，only if each displacement converged.
Actually, the constrain on the beam may be not mached the real situation，I don't know,I need more ditail of that area,if the both side constrains are support only ，the Z direction fixed is enough.
Thanks to your sample，I wonder whether the S&S support the mutil-constrains，
may Vadim give us the answer？
I 'll test it lately.
BTW，
How to out two post results together,can it be more? Pls tell me，i'm not good at Rhino.
Thanks.
Like as fellows，
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Thanks Richard,
Although I do not understand all of your reply. I'll try to respond.
>In my opinion, jut only to consider the worst situation, that is the roof has reached the support(the mast),but the the support 'Gone'(No constraints there).
>If the deflection is acceptable, you may lucky.
>> The purpose of the calcualtion is to find out the deflection just before the roof hits the mast. Due to the sloped topside of the mast, a little deflection is accepted. The mast will be permanent so I do not fear the stresses in the roof without mast support.
>First of all, I think aluminum-alloy frame is needed.
>The previous three pictures appears that the roof may be to 3000mm x 2000mm at least, only three constraints；the wight of the whole assembly roof may make deflection in trouble.
>So the shape&position of the frames are very elegant design by calculations ,even the numbers is important, too.
>More frames or big shapes will reinforce the stiffness,but the wight raise up ，too. The deflection is uncertainty.
>You can compare several proposals according to calculation to find the acceptable proposal.
>So the S&S is helpful， fast to perform detailed analysis.
>> Yes, you are right that the challenge is to have a stable roof with a frame containing a minimum amount of small stiffeners. It is Weight versus deflection.
>As above ,I also think your test sample by S&S make sense
>The test model :Solid may the better than the hollow，because of the structure is very different. Try some ribs with the hollow one.
>>Over here you can find a more informative validation of this case
>To your compared samples the same conditions is made the results reasonable，only if each displacement converged.
>Actually, the constrain on the beam may be not mached the real situation，I don't know,I need more ditail of that area,if the both side constrains are support only ，the Z direction fixed is enough.
>Thanks to your sample，I wonder whether the S&S support the mutil-constrains，
>> What do you mean with: 'mutil-constrains'?
>may Vadim give us the answer？
>I 'll test it lately.
>BTW，
How to out two post results together,can it be more? Pls tell me，i'm not good at Rhino.
Thanks.
>> What do you mean with: 'How to out two post results together'?
Thank you,
Gerard Petersen
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