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Surface spring type comp. only. From: IVIA



I wanted to report a Midas problem related to surface spring type comp. only. In the model I have defined elastic support on the whole structure + one point spring of the linear type to maintain the stability of the structure. 

According to the given direction, only the compression reaction should appear on the supports. For most loads it is ok, however, the problem appears in the case of soil pressure load.

The deformation of the structure due to pressure looks OK, while the reactions are not.

In the place where there is "uplift" of the structure, soil resistance should occur, but it does not appear, while on external walls, which due to pressure are "squeezed" inwards, there is no soil resistance, so there should be no reaction. In the model, unfortunately, this reaction appears with the opposite sign, despite the definition of a support of type comp. only.

Another load case shows that the same supports defined as comp. only show the correct response so it is not a matter of incorrectly setting the direction of the load (normal +/- or imposing the UCS direction).

The reactions began to appear correctly when the staging was removed. I don't know how this affects the supports, but I can't afford to remove them due to the rheology.

Please find attached model. Could You please help us clarify the problem.



The answer to this issue is actually very simple.  We need to understand that compression-only support stays in compression under a combination of forces. Let us consider a compression-only spring and DL, LL & Earth pressure loads.  Now, kindly refer to the table below.
In case 1, there is no confusion.  All acting forces are causing compression and so the spring would be active.
In case 2, live load is causing tension.  However, the total forces keep support in compression.  So, in this case, what the negative force of live load does, is the relieving effect, i.e., it reduces the total compressive force.
In case 3, the summation of forces is coming down to 0.  Meaning, whether this support is active or inactive, we would get similar results.
In case 4, the summation of reaction is tension.  In this situation, what the software does is that it ignores this particular spring and does redistribution of forces in existing springs in compression.
So, essentially the summation case should be checked, in which the behavior would be exactly as expected.  For individual cases, there could be tension in compression-only spring, and that is not tension but in fact a relieving effect for overall compressive force.  Hope this helps.  Kindly let us know if further assistance would be required.
Creation date: 9/13/2019 3:29 AM (nandeep)      Updated: 9/13/2019 1:00 PM (nandeep)
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