Learning Forum

[javat33489]

Hi all. I'm interested in the rollover problem in static analysis.

For example, we have a block standing on a plane and if we push it, it will fall:

I made a test model, there is frictional contact between the block and the surface, gravity, I apply force or displacement to the top edge and push the block.

I just need to understand whether there will be a fall and that’s it.

Static analysis allows one to evaluate the distribution of stresses and strains in a structure under given boundary conditions and loads, but does not allow one to determine whether the structure will be stable or not.

But can I accept that if there is a calculation error that the part falls? Usually this is what happens, if I apply enough force to the top of the block, I get a calculation error and the results show that the calculation stopped when the block tilted.

[Asitang Lahurikar]

Hello,

Static Structural Analysis is performed to assess the system in an equilibrium state when subjected to some load. However, in your case, following the initial displacement (push) provided, the falling of the standing block will be a time-dependent activity under the influence of standard earth gravity. Thus, you may consider using a Transient Structural Analysis system to simulate this model.

Hope this helps!
Thanks,
Asitang 

[javat33489]

I would like to use static analysis. I'm not interested in stress or what will happen next, only whether the part will fall or not.
When completing the calculation with an error after the push, I can accept that it fell. And if the calculation is completed without errors and I see that the part is standing, it means it didn’t fall. Is that acceptable?

[Asitang Lahurikar]

Hello,

Could you please explain why you prefer to use the Static Analysis system over the Transient?

Thanks,
Asitang 

[javat33489]

Because it's fast and easy

[peteroznewman]

“we have a block standing on a plane and if we push it, it will fall”  Are you sure about that?

If the coefficient of friction is small or the force is applied low on the block, it will not tip over, it will slide. But I don’t think you meant that it will fall because it slides off the base plate.

Assume the coefficient of friction is high, or you used a Rough contact condition so the block will not slide. Now you can think about using a force to tip over the block. Assume a coordinate frame where Y is up, X is to the right and the origin is on the bottom, left edge of the block, the one the block will rotate about.

“I apply force or displacement to the top edge and push the block.” These create very different analysis results. Let’s first consider applying a force.

The force that will tip the block over can be calculated by hand.  Let’s say the block has a height h, width w and thickness t.  Assume a horizontal force F is applied at the top of the block in the -X direction to tip it over. The coordinates of the CG of the block is (t/2, h/2, -w/2).

Calculate the static moment equilibrium about the origin for a constant force F at the top and the weight (mg) of the block acting at the CG. This is the force to tip over the block. At forces less than this, there is a reaction force on both sides of the base. At this force, the reaction force on the right goes to zero. At forces greater than this, the block accelerates as it tips over.

Now consider applying a displacement. For a small displacement, the reaction force at the top will be equal to the tipping force calculated above. As the displacement increases, the force reduces until the force reaches zero when the CG is exactly over the origin. Any displacement more than that will cause the block to tip over. Any displacement less than that is a stable equilibrium and if the displacement returns to zero, the block will return to being supported on its base.

[javat33489]

Thank you very much for your detailed answer sir! I need time to understand everything.

 

But I didn’t hear the question about whether it’s possible to use the Workbench static module for capsizing. What do you say about this? When a part slides and does not fall, there is no error - it did not fall. When it starts to tip over, there is an error in calculation, it has fallen.

[peteroznewman]

There is a simple Workbench Static Structural model to decide if a constant applied force will tip a structure over, assuming a no slip condition at the base.  Insert a Revolute Joint to Ground along the tipping edge. Insert a Remote Displacement on the opposite edge and set Y = 0 leaving all others Free. Apply Standard Earth Gravity in the -Y direction. Solve. If the reaction force on the Remote Displacement is up, it will not tip, if the reaction force is down, it will tip.  The downward force prevents the structure from tipping.