Why and how to input Internal forces into a detail?

If we have a detail that is trimmed out of a global model of the structure, it is crucial to input both the external forces (load impulses) and internal forces correctly. This is because the trimmed part does not know about the geometry and the loading of the rest of the structure, and through internal forces in a defined section we are able to tell the application, how the whole model behaves like. The internal forces must be taken from the global analysis of the structure.