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Stiffness matrix

Submitted by SivaSrinivasKolukula on
education
FEM

Hi all...........

       I have a doubt in FEM for calculating Stiffness matrix. I am using a mesh where the mesh has combination of two different elements. 4 noded isoparametric quadrilateral elements and 8 noded isoparametric quadrilateral elements. The stiffness matrix order for Q8 elements is 8X8 and for Q4 elements is 4x4. Now my doubt is how to assemble these different order matrices? 8x8 and 4X4 matrix? Is that we need to turn 4X4 matrix to 8X8 matrix by taking zeros at evry alternate positions............? I am not sure of this. I am confused........if not is any other method there? I am new to FEM. 

 Sreenivas

unsrohith

Hi sreenivas,

Assembly of stifness matrix has to be done using the degree of freedom vector. Give numbering to each DOF of the node and using these find the global stifness matrix which actually contains all DOF then automatically the contributions from each element comes from the numbering you followed..

Regards,

Rohith

Mon, 02/22/2010 - 17:56 Permalink
SivaSrinivasKolukula

Hey all

     We know that stiffness matrix evaluated in FEM should be symmetric. I am doing FEM alalysis of fluid using  MATLAB. I have used Q8 elements 96 in number for fluid. After eveluating 96X96 ordered stiffness matrix, I wanted to check whether the matrix is symmetric or not? I checked it, but i got it as nonsymmetric. I checked all my M files and function files again and again. But it dint work.  I checked the elements where they are assymetric. When i was checking elements they were equal. Later I got to know that in format long option the matrix is nonsymmetric and format short option the matrix is symmetric. There is a difference in 16th digit. Now my worry is:

Taking the stiffness matrix symmetric in format short which is not symmetric in format long not going to effect the final result?????????

Why we should neglect this?Is there any solution?

Hope question is not silly...........

Thanks in advance

With regards 

Inspiration and genius--one and the same.
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http://sites.google.com/site/kolukulasivasrinivas/
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Siva Srinivas Kolukula
Junior Research Fellow

Wed, 02/24/2010 - 09:26 Permalink