Convergence of a solution algorithm typically means your model satisfies equilibrium and compatibility for the current applied loads and any nonlinear response. But there are pathological cases where the algorithm converges while the model remains in a state that violates equilibrium or compatibility. A simple case (brought to my attention by Chen Fei) involves a … Continue reading Pathological Convergence
Tag: Convergence
Could Not Invert Element Flexibility
Due to inversion of section stiffness to flexibility, then inversion of element flexibility to stiffness, the force-based frame element implementation in OpenSees is finicky, to say the least. A common error message spawned from the force-based element is "could not invert flexibility..." You will see the same message many times, but, ultimately, you will get … Continue reading Could Not Invert Element Flexibility
Non-Convergence Is Not Structural Collapse
Legend has it that some published research results based on nonlinear dynamic analysis--incremental dynamic analyses, seismic fragility curves, Monte Carlo simulations, etc.--considered a non-convergent OpenSees model to indicate structural collapse or failure. Let's think about this for a minute. Here is the displacement response in two orthogonal directions at the top of a nearly 50 … Continue reading Non-Convergence Is Not Structural Collapse
See the Convergence
Surely you have seen norms fly across the screen when running OpenSees with the print flag of the convergence test set to 1. The screen output slows down your analysis significantly, so you should only use print flag equal to 1 when you are trying to diagnose convergence issues. From a Jupyter Notebook. With OpenSees.exe, … Continue reading See the Convergence
Failed to Get Compatible …
If you've used the force-based beam-column element in OpenSees, you've likely come across this warning involving element forces and deformations: I've encountered this warning many times and so have others. In fact, I lifted the above image from a recent post on the OpenSees Facebook group. I tried to come up with a MWE to … Continue reading Failed to Get Compatible …
Norms and Tolerance
Convergence tests in OpenSees measure how close the algorithm is to finding equilibrium at a time step. Several convergence tests are available, and they all operate on the linearized system of equations that is solved at every equilibrium iteration within a time step $latex {\bf K}_T \Delta {\bf U} = {\bf R}$ where $latex {\bf … Continue reading Norms and Tolerance
A Marathon, Not a Sprint
When tasked with developing an OpenSees model for simulating the nonlinear dynamic response of, let's say, a multi-story reinforced concrete frame, you may be tempted to go straight to force-based frame elements with fiber sections comprised of Concrete23 and Steel08 material models. This sprint to the finish line will undoubtedly lead to an analysis that … Continue reading A Marathon, Not a Sprint
Handle Your Constraints with Care
Manipulating the nodal equilibrium equations is necessary to enforce constraints between degrees of freedom (DOFs) at two or more nodes in a structural model. These multi-point constraints arise from assumptions of axial and flexural rigidity of frame elements, e.g., rigid diaphragms, and also between two nodes at the same location where some of the DOFs … Continue reading Handle Your Constraints with Care
Non-Convergence Does Not Mean OpenSees Crashed
I once had the following conversation with a concerned user (CU) of OpenSees: CU: "I was running a response history analysis, then all of a sudden OpenSees crashed." PD: "Really, it crashed? Did you get the blue screen of death?" CU: "No. The analysis stopped and I saw 'Failed to converge'. It crashed." PD: "Oh, … Continue reading Non-Convergence Does Not Mean OpenSees Crashed