Stiffness Matrix of Frictionless Plate Support: Puzzling Differences

In summary: There are three BCs (frictionless supports) in the photo. One is at the center, one is on the upper right, and one is on the lower left. The stiffness matrix (defined at the node at the center) is not equivalent between the three BCs. The Kxx and Kyy values are different but not far apart. The BCs on the upper right and lower left are not the same. What do you mean by the stiffness matrix at the node at the center? The Stiffness matrix applies to the entire structure (or a substructure), and doesn't describe a single point.
  • #1
Mohamed_Wael
23
0
ImageUploadedByPhysics Forums1469943346.394802.jpg


Hi all,
In the attached photo, you can find a plate supported along the edge by "frictionless" support and I am trying to obtain the stiffness matrix using the node at the center. I wonder why the Kxx and Kyy (highlighted) are not equivalent or even near to each other, any suggestions?
 
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  • #2
Im not familiar with ANSYS, but are those arrows your BCs (the supports)? It looks like you have "BCs" at the centerpoints between the edge nodes...Is that the case?

Also, are the BCs on the upper right hand side of the upper right half the same as the BCs on the lower left (asking because the arrows are different)?

Also, can you explain what you mean by the stiffness matrix at the node at the center? The Stiffness matrix applies to the entire structure (or a substructure), and doesn't describe a single point.
 
  • #3
First thanks for your care,
1-actually I wanted to divide the plate in such homogeneous way trying to get equal Kxx and Kyy, but this lead to triangular base and not circular edge, but when u define the boundary conditions you define it for the circular edge geometry, that's why they look like flying supports.
2-I defined a frictionless support allover the edge they should be the same, I don't know why Ansys mark them in this way
3-when I extract the stiffness matrix I should define the DOF by selecting the nodes and the required DOF, that's what I mean
4-I have just tried changing the type of support to fixed support and I got equal Kxx and Kyy! i will attach the image.
thanks again
 

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  • #4
Good to hear.

The "support type" (constraints or BCs) is very critical in FEA. When I was learning, my constraints were the cause of most of my mistakes. It helps to think of things in terms of nodes instead of physical shapes (that is, as a series of triangles instead of a circle).
 
  • #5
What kind of elements are you using for this model? Have you considered using elements with mid-side nodes to allow for a curved outer edge?
 

1. What is a stiffness matrix?

A stiffness matrix is a mathematical representation of the stiffness or rigidity of a structure. It is used in engineering and physics to model the behavior of structures under different loads and conditions.

2. What is a frictionless plate support?

A frictionless plate support is a type of support used in structural engineering that allows for movement in all directions without any resistance or friction. It is commonly used in structures that require a high level of flexibility and allow for thermal expansion or contraction.

3. What are the puzzling differences in the stiffness matrix of a frictionless plate support?

The puzzling differences in the stiffness matrix of a frictionless plate support refer to the discrepancies that have been observed in the calculated stiffness values from different analytical methods. These differences can be attributed to the assumptions and simplifications made in each method, as well as the complexity of the structure being analyzed.

4. How do the puzzling differences in the stiffness matrix affect the analysis of structures?

The puzzling differences in the stiffness matrix can affect the analysis of structures by leading to inaccurate predictions of the behavior of the structure under different loads and conditions. This can result in potential safety hazards and costly errors in the design and construction of structures.

5. What are some ways to address the puzzling differences in the stiffness matrix of a frictionless plate support?

Some ways to address the puzzling differences in the stiffness matrix include using more advanced analytical methods, conducting experimental tests, and refining the assumptions and simplifications made in the analysis. Collaboration and comparison of results between different methods can also help to identify and resolve any discrepancies.

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