Fatigue of structural steel is closely related to the accumulation of plastic strain under cyclic loading and eventually leads to the formation of cracks or even failure of a structure. Fatigue is very important in seismic design where for economical reasons localized damage is allowed and structure is subjected to cyclic loading. Usually very rough models are used to access damage accumulation e.g. accumulated plastic strain or accumulated plastic work.. The use of AceGen has enabled assessment of low-cycle fatigue based on damage mechanics by the means of fully implicit, quadratically convergent Newton-Raphson procedure. A three-dimensional finite element for large strain is developed based on the Pedersen-Tvegaard material model for cyclic damage, that combines kinematical and isotropic hardening with damage evolution. Formulation leads to the system of 28 nonlinear evolution equations for each material (integration) point that have to be solved and consistently linearized and is one of the most complex material models ever implemented implicitly. The derived numerical model permits a life prediction of the structural details for arbitrary loading histories.