Analysis of a Structural Determinant by FEA 1

[feajob]

Hi Guys,

I would like to know that based on your experience how FEA will predict the reactions of a Structural Determinant?

If I have a square plate subjected to uniform pressure load and this plate is restrained in 3 nodes in order to avoid all 6 rigid body movements then with 6 equilibrium equations, I can calculate all 6 reactions?

Do you think that linear FEA will predict the same thing? To be more specific, do you think that the reactions for a Structural Determinant are independent of stiffness of this plate?

As you know, [K] {U} = {F} where [K] is a global stiffness matrix, {U} is vector of displacement and {F} is load vector (reaction or applied loads). Based on this equation, Displacement field depends on the stiffness matrix and consequently reactions seem to be dependent on the stiffness matrix.

I appreciate to know your feedback.

A.A.Y.

 

[prex]

Don't know the meaning of Structural Determinant.
It is however obvious that, in a statically determined structure, support reactions may be calculated from equilibrium conditions. The simplest example is a beam onto two simple supports. And of course FEA will give the correct reactions for a correct analysis.

prex

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[rb1957]

assuming "structural determinant" means determinant structure, then yes you can calculate the reactions by hand or by FE as there is only one solution that is easily reached (solving the 6 degess of freedom). and yes, the reactions are independent of the structure's stiffness.

 

[COSTASTSAPROUNIS]

Generally
The reactions are idependent of stiffness except the case
where the stiffness matrix is illi conditioned
Specifically for the test problem you said you willhave problem because it is a aproblem where fem fails.
See the site of

http://femci.gsfc.nasa.gov/

to see what is the problem of the specific test problem.
But generally and theoritically if you have convergency and the stiffness matrix is not ill conditioned then you predict the correct reactions indepedently the stiffness.


Dr. Costas J. Tsaprounis

 

[rb1957]

stiffness of the structure only affects the reactions when the structure is statically indeterminate. in a determinate structure there is only one solution to the 6 degrees of freedom, as there are only 6 reactions. for an indeterminate structure there are different ways to react the applied loads so stiffness as an effect.

i went to the link but didn't see a "specific test problem" where FE fails.

 

[feajob]

Thank you for your responses. I am agreeing with you guys. As rb1957 mentioned, "for a determinate structure there is only one solution to the 6 degrees of freedom, as there are only 6 reactions."

I am dealing with a 2-part structure, which is indeterminate (globally), however one part of this structure is determinate.

Structure = part 1 + part 2

Part 2 is determinate and part 1 is indeterminate, so both together are indeterminate too. Now, I am wondering what is the situation for the interface loads between them.

If part 2 is determinate then it means that no matter what is stiffness for part 2 then I have to get the same interface loads between them. Is it correct?

Thanks again for your responses,
A.A.Y.

 

[rb1957]

if part 2 is determinate, then you can remove it and replace it with it's reactions.

the real world rears it's ugly head if you consider contact between the parts (not knowing how the 2 parts fit together). could part 1 deflect in a way to contact part 2 (making it indeterminate) and changing the loadpaths ?