## General Mechanical

#### Force seems to be applied in opposite direction

• wlane
Subscriber

I was getting some odd results trying to run a basic structural simulation and while I've been able to sort it out, I can't figure out why it's an issue. Any insight would be very appreciated!

The only boundary conditions applied to the system are gravity [0,0,-9.81] m/s^2 and fixed supports at the top. I added a force to the bottom face [0,0,-802] N. I ran the simulation and checked the reaction forces of the fixed supports and to my surprise, the reaction force is less than the combination of the gravity/mass and the added force. Turns out, it's exactly double the force. I flipped the force direction [0,0,+802] N (but kept gravity the same) and re-ran the simulation. With the reversed loading, the reaction forces balanced as expected. But the force vector that ANSYS displays literally shows it opposing the gravity vector, so I'm very confused about what's going on. I've attached some pictures to show the loading conditions and results.

• Erik Kostson
Ansys Employee

Can you please try to do a simple steel block, with a force (say +100 in +Z direction) on one face/side and that we get the equal and opposite reaction on the opposite fixed face/side?

Thank you

Erik
• wlane
Subscriber
Erik Thanks for the response! That was actually the first thing I did when I couldn't find any other errors. With a simple beam the displacement, stress, reaction forces, etc. were all consistent with Euler-Bernoulli theory. Setup: 0.1m x 0.1m x 1m, Fixed @ 0m, F=[0,0,-1]kN @ 1m, E=200GPa yields a reaction force of [0,0,1] kN and max displacement of 0.2 mm.

• Erik Kostson
Ansys Employee

That is good.

Can we now take away all the loads, gravity and supports. Then add a fixed support (please show where it is scoped), and then just add a force of 100 N (show the details of the load please).
So we should only have a fixed support and a force (do not call it Other please).

Run that and see that we get equal and opposite reaction in the fixed support.

All the best

Erik
• wlane
Subscriber
Erik Thanks again for the support! After simplifying down to only the supports and running a combination of different loading conditions I have finally identified the issue. Instead of using the built-in Standard Earth Gravity conditions, I used the generic Acceleration condition with components [0,0,-9.81] m/s^2, thinking this would be equivalent, but that's not the case. You need to accelerate the bodies upward to feel the opposing downward force. Once I figured this out I searched and found the following answer: https://forum.ansys.com/discussion/11409/difference-between-standard-earth-gravity-and-acceleration

• Erik Kostson
Ansys Employee
Hi

That is good - was thinking of that - all the best

Erik