Nowadays, equipment that has been operated for more than 20 years is still available in many companies. Such equipment includes bridge cranes and the railways on which they move. After so many years, it is necessary to determine whether their operation is still safe. The operator of such equipment has two options. The first option is the equipment replacement with a new one at the end of period for which it was designated. The second option allows the operator to test the existing equipment and its mechanisms based on the available methods and decide on its further use according to the results obtained. If the equipment complies with the safety and operating regulations, it is kept in operation. However, in that case, the number of preventive maintenance operations for such structures and machinery will increase.
We can distinguish two types of crane failures. The first type is associated with the control system of individual crane mechanisms, which can be eliminated by a relatively simple replacement of a defective part. The second is mechanical damage to the steel structure associated with the formation of cracks, which in most cases is directly related to fatigue damage of the material. Nowadays, due to the variety of influencing factors caused by the complexity of loading, fatigue analysis and lifespan assessment is still a very challenging problem. Systematic and detailed research has been carried out for many years in the field of theory and testing of different crane constructions. In addition, various fatigue life assessment methods have been summarized. Overall, they can be divided into three fatigue life assessment approaches based on S-N curve and fracture mechanics [1
], probability and statistics [3
] and information technology [5
]. Kadirgama et al. [5
] developed the tool life prediction model for P20 tool steel with the aid of the statistical method, using a coated-carbide cutting tool under various cutting conditions. This prediction model was then compared with the results obtained experimentally. In general, the results obtained from the mathematical model were in good agreement with those obtained from the experiment’s data. According to the relationship between fatigue life and evaluation parameters, fatigue life assessment approaches based on S-N curve can be divided into nominal stress method [6
], structural hot spot stress method [7
], local stress and strain method [8
] and equilibrium-equivalent structural stress method [9
]. The nominal stress method is often used to assess the fatigue life of the bridge crane structure in operation and the accidental load is treated by the Rainflow Counting method. The cumulative fatigue damage of the crane metal structure is calculated by the Palmgren–Miner (or Corten–Dolan, Heibach, etc.) model to obtain the fatigue life prediction results. The complex influence of multiple factors that include geometrical characteristics, loading conditions, operating characteristics and failure mechanism will affect the safety of the bridge crane structure. The evaluation of bridge crane structure safety by real-time fatigue life prediction method based on a digital twin is described in [10
]. The computational model is enhanced by the analytical model and information transfer, and subsequently, the main factors affecting the fatigue life of the bridge crane structure are determined. Ávila et al. [11
] point out that the most frequent failures in these structures are related to fatigue in the welded joints. They conclude that the S-N method combined with finite element analysis proves to be an effective technique for determining the most critical damage at the joints of a structure. Fatigue damage studies of crane girders have led to significant improvements in design, eliminating premature fatigue failures. Some focus on crack formation [12
], others on determining remaining life as mentioned in [14
]. Tong et al. [13
] performed finite element analyses to investigate the stress concentrations at the girder ends of the crane runway (two types). The analytical results agree well with the data measured. Kuľka et al. [14
] describe a procedure for determining the residual life of the casting crane girders, which determines whether it is worthwhile to make any structural modifications to improve the crane’s operational characteristics. Frankovský et al. [16
] deal with the repair of an already damaged beam. The longitudinal fillet welds between the top flange and girder web on the rail side of the 200 t trolley were damaged due to specific stresses after its long-term operation. Consequently, the girder’s cross-section opened, substantially reducing its strength and stiffness. Bomas et al. [17
], Darisuren et al. [18
], and Trebuňa et al. [19
] discuss how fatigue damage can affect structural elements and nodes of load-bearing structures due to inappropriate design in manufacturing or inappropriate retrofitting, e.g., major overhaul (design factors), infringement of technological discipline in the manufacture or repair of equipment (manufacturing technology factors) and intentional or unintentional use of equipment in a manner inconsistent with its intended use. The influence of these factors then makes the actual nature of the stresses much more unfavorable than expected in the structure design, often resulting in a change of the direction, gradient, magnitude or redistribution of stress. In such affected areas, structural elements and nodes, there is a higher risk of fatigue damage (failure) [20
]. Liu et al. [22
] identified the dangerous locations of crane girders using the finite element method. They performed fatigue life tests on a simplified model considering welding details. By the fatigue test results of the specimen, they quantitatively described the actual fatigue life of the welded structures of the bridge crane girder. By transferring the data into the equation, the S-N curves of the bridge crane girder included the specific materials, stress ratio, welding process and structure.
Most of the scientific articles published in the field of experimental-numerical analyses of bridge cranes are devoted to analyses performed immediately after the equipment reaches a state of failure. The present paper deals with the problem of reducing the fatigue resistance of the girder cross-section associated with periodic maintenance of the equipment. The bridge crane’s remaining service life was assessed at the request of its operator, who registered the limiting values of the height difference deviations of the rails’ top edge. Since the analyzed crane has been in operation for more than 50 years, it was necessary to use an analytical-experimental assessment of its safe operation, considering actual operating conditions. The performed analysis has shown that improper (non-professional) intervention in the crane structure can significantly deteriorate the technical state of an already significantly stressed structure, which will only accelerate its degradation, as confirmed and described in the experimental-analytical parts of this paper.