The topic of the article are digital models of bridge structures used in dynamic analysis. FEM numerical models used in modal analysis for the purposes of diagnostics of real objects should be consistent with the real structure, in particular in terms of mass distribution and element stiffness, but also in terms of nodal connection stiffness. Experimental research and calibration of computational models performed on the basis of real measurements are very important. A well-adjusted digital dynamic model makes it possible to determine the boundary values, i.e. the critical values of selected parameters, only such the real-fit and validated model can be the basis for building monitoring systems. Damage to structure changes its eigenfrequencies and eigenforms. The article describes the experimental studies and the way of fitting FEM models of damaged and undamaged steel frames. The difficulties of modeling real damages in modal analysis, even in the case of simple structures, are shown. The last stage of research work was the calibration of the FEM model of a real footbridge located in Poland over the A4 motorway. The basic dynamic characteristics were obtained as a result of field tests on a real footbridge, the FEM model was calibrated and finally a very good correspondence between the dynamic characteristics of the FEM model and the real bridge object was obtained.