If you are just starting, then, pushover analysis may be a bit beyond you. There are several books that go into this; my old reference books by Maissonette and Save and Baker are 50 years old, but, just as valid today. Pushover and shakedown analysis are the same...
It's a consequence of loading a system into the plastic region and the residual stresses left when unloaded can cause a premature failure of the system with a different loading pattern.
When I mentioned that it may be a bit beyond you, I wasn't trying to discourage you. There are a lot of engineers in this forum that don't use plastic design for steel or concrete. Designing based on collapse mechanisms is not everyone's cuppa... Best you get a grasp on regular structural engineering, first. I've been using plastic design, off and on depending on the project, for over 45 years and I think it's great.
Dik