In this paper the effect of local damage on structural behaviour of steel components is studied and simple mathematical models for stress analysis of damaged cross sections are created. The bars of the Urfahr Railway Bridge are used to determine the geometry. They consist of four L-profiles and comply with usual cross sections of historic truss bridges made of steel. The first part of the master thesis deals with the stress determination of cross sections, where the individual profiles are considered as 'points' (without moment of inertia). A new and simple mathematical model for calculations of damaged point cross sections is created. As a result it is possible to determine stresses without using cross section values. In addition to this, the transition to stress analysis of damages real cross sections is performed. The commonalities and differences to the studied point cross sections are shown. The collected knowledge is used to create and evaluate two mathematical models for determining stresses of damaged real cross sections with a simple calculation process: - model of a stiff plate - model of a stiff plate with damage-modifications As a result of using these models, the complex transformation of coordinates and internal forces caused through damage, as it is usual with the classic stress determination, is no longer necessary. Considering these facts, they represent a practical way to verify damaged steel components.