**7. Conclusions**

During their service life, bridges, like other structures, undergo functional and material degradation. In many cases, tests show that despite non-strict conformity with actual design standards, they are able to carry actual service loadings. According to [1], calculations of existing steel bridges resistance should be carried out more precisely and in a more readable manner following a three-phase assessment: a preliminary evaluation, a detailed investigation, and an expert investigation. Some other studies have recommended using five assessment levels: a bridge rating, a preliminary evaluation, a detailed investigation, an expert investigation, and advance testing [16, 35–37]. Application of the assessment procedure led to improvements in knowledge and a calibration of confidence factors.

At the same time, the project "Sustainable bridges – assessment for future traffic demands and longer lives" with nine packages, deals with the increasing capacity and service life of existing railway bridges [35]. New methods for the resistance assessment of existing bridges were developed in standards and guidelines using calibrated partial safety coefficients.

The Steel Structure Chair at the Technical University in Szczecin (now WPUT) contributed to these issues by carrying out a multiyear program for the quality-level assessment of welded butt splices in existing railway bridges in service in Poland. Radiological tests on butt splices in the steel girders of around 200 railway bridges were carried out. Nearly, 200 bridges were tested using X-ray examination over a 37 year period. The number of joints tested was over 15,000; cracks were discovered in 400 welded joints in 34 bridges tested. Repeated tests on welded joints with cracks were undertaken as well as laboratory investigations for their complex assessment. Partial results of the laboratory fatigue testing are given in the paper.

Numerical analysis of tensile stresses in welded splices was undertaken to support and confirm the results of laboratory fatigue tests. The results of the FEM analysis of tensile stresses in welded joints with rhomboid cover plates are given. The FEM method allowed us to determine tensile stresses at each service stage. The analysis allowed for:

