# Prescribed displacements questions

In Karamba it is possible to prescribe displacements at supports. Would it also be possible to prescribe forces instead of displacements? I am simulating a doubly curved concrete shell where I want to tie the supports (like a tied arch). Now I want to know how much pretension of the tie-members this system can take - in order to ideally reduce tension stresses in the shell. Yes I can prescribe displacements here, but the resulting pretension force would be dependent on the cross section of shell and not the value I am looking for…

Any ideas?

Hello @Rudi,
apply a point-load instead of a displacement boundary condition at the node and degree of freedom in question.
– Clemens

I am always trying to break down my questions as much as possible. What I am actually trying to do is to apply pretension to a concrete shell. Therefore I opened up some DOFs and applied a corresponding force there. Used a list with desired Fx & Fy pretension values and divided it by the number of support points to get the correct total force respectively (see sketch below):

The support with DOF in x & y dircetion gets a pretension force in x & y. This results in vector addition as can be seen with the 45° arrows in the corner.

Question is: Is this now modelled correctly? Would you do sth different?

Thanks a lot and all the best,
Rudi

Hello @rudolf.neumerkel,
your approach seems correct. The disadvantage is, that you get a non-symmetric displacement pattern. Alternatively you could try this: fix the mid-point horizontally and rotate the supports at the corners such, that one direction points towards the center. Then support them vertically and one point in addition in such a way that the hole structure can not rotate about the vertical axis.
– Clemens

Very good point, makes a lot of sense!

Like so? All white arrows indicate fixed support directions, the red ones indicate pretension forces - now on all corners.

Regarding the support direction perpendicular to the center line, we have to assume that the resulting deformation of the shell is in exactly that direction. Any deviation from that direction would introduce (tiny amount?) stresses, I guess?

Hello @rudolf.neumerkel,

the supports indicated in white look good to me.

Regarding the support direction perpendicular to the center line: without this the shell would be free to rotate about the vertical axis. So any deviation in direction would lead to a slight rotation of the system but to no additional stresses.

– Clemens

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