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You could try performing a modal analysis on the structure after the time history analysis to capture the updated vertical frequency while keeping the horizontal mass and stiffness intact. This way, you can isolate the vertical frequency without losing the time history results.

That's great! space bar clicker

When it reaches this stage, does it begin to yield? If this is the case, the noncovergence could be connected to the characteristics of the steel02 post-yield transition mechanism. You are free to experiment with the default settings and see what results you obtain.