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Large generator step-up (GSU) transformer units are particularly important components of the power generation network. In order to ensure a reliable functioning of the power network, GSU transformers have to be able to withstand a wide variety of stresses, of which Lorentz forces generated during an external short-circuit can be potentially dangerous for the transformer. In this case, a reference mock-up unit designed to withstand similar mechanical stress underwent the actual short circuit testing. Upon the demonstration of short-circuit withstand capability of the mock-up unit, a destructive test was undertaken in order to demonstrate the safety margin of the design. This was achieved by gradually increasing the current until mechanical deformations occurred. This paper describes the process of model generation, calculation, numerical behaviour and the results of a 3D transient magneto-mechanical FEM simulation of the winding assembly of the mock-up transformer unit. This unit underwent short circuit testing with the standard short circuit test currents. This type of simulation allows for a deeper insight into the actual generation and distribution of the magnetic forces and the resulting mechanical stresses during a short circuit which cannot be achieved by presently used simulation and calculation methods used in the calculation of the short circuit withstand capability of power transformer. [ABSTRACT FROM AUTHOR]