Convergence
Convergence
(OP)
Hi,
I am conducting a displacement controlled analysis.
In a first attempt, I ran 30 000 steps for a 0.1 displacement total ( that make 0.1 / 30 000 displacement per steps).
Then it failed at around 28800.
I said fine, I'll just increase the number of step, to, say 35 000. There I thought I had it.
Well, now it fails at... 15 000 steps.
Have you ever had this problem? I changed absolutly nothing else in the model!
I thought decreasing the displacement increment per steps would help stabilize the system... never imagined it could ruins it? Any clue on what could be happening? Thanks!
I am conducting a displacement controlled analysis.
In a first attempt, I ran 30 000 steps for a 0.1 displacement total ( that make 0.1 / 30 000 displacement per steps).
Then it failed at around 28800.
I said fine, I'll just increase the number of step, to, say 35 000. There I thought I had it.
Well, now it fails at... 15 000 steps.
Have you ever had this problem? I changed absolutly nothing else in the model!
I thought decreasing the displacement increment per steps would help stabilize the system... never imagined it could ruins it? Any clue on what could be happening? Thanks!





RE: Convergence
Although I too find it odd that decreasing step size has that effect on how far the analysis converges.
What software are you using and what kind of nonlinearities are in the model?
RE: Convergence
When I take the same model with a little larger meshing, it's fine (but the answer is a little too high).
When I refine, with same steps number, I crash close to solution. When I increase the number of steps (which should help), I crash sooner... this I don't get.
RE: Convergence
RE: Convergence
I know it gotta have something to do with element distortion because when the mesh is large, I do not get this problems.
RE: Convergence
Best regards,
Sze Kwan (Jason) Cheah
RE: Convergence
I suspect buckling is happening. I've seen cases where refining the time steps caused convergence failure while coarser ones didn't. That doesn't mean the coarse step results were right. The fact that refining the mesh makes it worse is another sign in support of buckling since a finer mesh tends to be less stiff in bending. Try a linear buckling analysis for a rough idea to see if you're close to the critical load. It might not find it, but it might.
RE: Convergence
RE: Convergence
I am trying to obtain the ultimate bearing capacity of a foundation.
I am wondering if it could be some problem with the interpolation function of the element. Since am using quad-element with a stabilized single-point integration (just one gauss point at the element center) I thought maybe it had problem adapt to imposed deformation, hence the problem with finer mesh? I am trying with 9-noded element to see if it will fixe the problems.. any thought on this?
thanks for the help!
RE: Convergence
Best regards,
Sze Kwan (Jason) Cheah
RE: Convergence
I was wondering if decreasing the size of the element could somehow solve this problems (using SSPquad element (SSP --> Stabilized Single Point, see here for "details" : http://opensees.berkeley.edu/wiki/index.php/SSPqua... ).
RE: Convergence
Best regards,
Sze Kwan (Jason) Cheah