Hello.

I got a case where my result force, from a simple simulation, didn't match the result the person asking for was expecting.

So I was asked to explain why, and unfortunately I couldn't. It is probably very easy, but I ask for your help.

In picture below there are 3 plates.

In each outer plate there is a cavity and in the middle one there are 2, one on each side.

Left and right side of the outer plates are fixed so all result forces will end up there.

In example lets say that the pressure in the blue cavities gives F=10 000N and the red F=4 000N.

What is the result force in my fixtures?

Looking at the left side, the only force should be the 10 000 N

Locking at the right side, the force should be the same. Z-> = 10 000-4 000+4 000 = 10 000 N

This works fine and matches the result expected. But what happens if the cavities are off centre?

The expected answer was that the result forces should be the same.

Measuring result forces on left and right fixture shows about 10 500 N for the case where the red cavity is half and half overlapped with the blue.

Moving the red cavity down even further so it is completely off centre (edge to edge) gave 12 000 N result force

What is causing this additional result force?

Thank you in advance for your time.

How are the blocks connected?

Is this a 3-dimensional solid analysis?

Are the left and right sides fully constrained? If so then a free body diagram is not valid, as this is not a free body. Reaction forces at the fixtures will include the forces necessary to keep the stack from growing, in all directions.