NodeForce and SP step by step at a node

[Lei Lei]

Dear STKOteam,

I try to make a two-step simulation.
In the first step, I add vertical node force at two nodes and make the load constant after analysis.
Then, I add vertical displacement at the same two nodes with sp.
However, the second analysis seems to not work. When I change sp with node force, it works.
What's the reason for it?

Looking forward to your reply!

[kesavapraba]

In FEA, you can't apply both force and displacement to the same spot (node) at the same time. Think about it like a tug-of-war: you can pull one rope (apply force) or hold onto it (specify displacement), but not both at once. This limitation comes from the basic idea behind FEA. The software solves for either the force needed to achieve a certain displacement, or the displacement caused by a specific force. It can't handle both unknowns at the same location.

[Lei Lei]

Thanks for your reply!

Then, If I want to apply force and displacement at one point both in different stages, can I use an auxiliary point to achieve it?
I mean using equalDOF to connect them and then applying displacement on the auxiliary point.

Looking forward to your reply!

[STKO Team]

Then, If I want to apply force and displacement at one point both in different stages, can I use an auxiliary point to achieve it?
I mean using equalDOF to connect them and then applying displacement on the auxiliary point.

The idea is correct, but not using equalDOF (because of the fact you are doing it in 2 stages)

When you apply the sp in a second stage, it will reach the imposed value in an incremental fashion starting from 0!
But you are applying it in a second stage where the initial displacement is not 0!.

One solution is to duplicate a node an connect it with a stiff spring (this second node and spring should be added in a second stage using the modelStage command).
This is because the zeroLength element account for the initial displacement