Springs in shells 2

[moebius]

Hello from Spain:
I have calculated a water tank but i´m not sure that the results were good and i think that the problem are the springs that I put in the shells because the strange resuts are at the joint between the wall and the floor. So I have three questions:
1st) What simulate better the soil reaction, springs or Nlinks
2nd) Where can i find an example of a slab (I think that is the name of the floor)
3rd) What are better for shells springs in joins or in shells and what´s the difference.
Thanks in advance.

 

[stressed]

If you have a problem with the connectivity between the tank wall and the floor, graphically select the wall and floor and Assign menu>Areas>generate edge constraints if you have version 8. The edge/line constraint automatically connects mismatched joints at shell intersections..which saves you the hassle of having to change the mesh.

Define an area/shell with the properties of your slab, 3 foot thick concrete, for example. Typically, you would give the area a "shell" type which gives stiffness in-plane and out of plane. If you have version 8, you can use the Edit menu to extrude a shell into solid elements and then assign springs to the solid element if you want. In older versions of SAP, you have to import autocad or edit with the text file in order to model solid elements

For soil, a spring restrains joints in both tension and compression. I hate to keep bringing up version 8 if you don't have it, but version 8 lets you make an area assignment for springs which automatically calculates nodal spring constants for you. This saves you the trouble of manually figuring out joint spring constants based on element size and tributary area. If you have a subgrade modulus of .3 kip-in, you would simply assign>area>area spring and assign an area spring value of .3 kip-in to your mat foundation and the program automatically calculates joint spring constants for you.

If you want to consider the possibility that the foundation may uplift and redistribute load, then you will need to assign links - gap type links, at joints instead of springs. Unfortunately, there is no area assignment for links as there is for springs. You will need to calculate the soil spring constant based on size of element and tributary area

One other bit of advice. You should consider putting soil springs in all three translational directions, not just vertical, to prevent numerical instabilities.

 

[moebius]

thank you for your replay, but i have another doubt about springs. If i put springs in a plate and i don´t know why but without loads it has a deformation.
I put springs only in the face number 6 and the value in in the direction 1 and 2 is 10000 KN/m3 and 20000 in the direction 3.
What´s the problem? without loads the deformation and the stresses should must be zero.

 

[IYIeI2cY]

moebius,
Did you ignored the own wieght of the mat?

 

[moebius]

Yes I did it. that´s why I don´t understand this.