There are two important research fields, which called specific attention from research and standard development point of view in the last years: (i) FEM-based design of steel structures and (ii) assessment of existing steel structures. The current paper presents an investigation which combines these two topics and presents the correct application of advanced FEM-based design (direct resistance check) on a specific structural detail of an existing structure, which needed a detailed static check, during reconstruction and strengthening. The paper presents a detailed investigation on an existing 30-year-old highway Danube bridge in Hungary, which has an extremely large truck traffic. During the recent reconstruction the bridge was strengthened at the end cross-girders after changing its bearings. The new bridge bearings had significantly different geometry from the old ones and induced different stress distribution within the diaphragm of the cross-girders. The effect of local load introduction and stress concentration within the numerical model is always a question, which is investigated in a detailed manner on this bridge. An extensive static check is executed by the BME Department of Structural Engineering to investigate the stress concentration within the diaphragms by using the old and new bridge bearings. The stress distribution and the resistance of the diaphragms is studied and determined by using an advanced FE model and direct resistance check. After installation of the new bearings, loading test and detailed measurement of the whole bridge with a focus on the cross-girders are executed to validate the numerical model and investigate the real effect of the local load introduction. The paper introduces the advanced numerical models and presents the details of the direct resistance check method on the existing steel bridge validated by on-site measurements and loading test.