# Experimental Investigation of the Fatigue Life of a Bridge Crane Girder Using S-N Method
###### Abstract  
Experimental measurement methods used in operational mode provide valuable information about the behavior of mechanical parts of equipment that cannot be determined in advance by analytical calculations or numerical modeling. The strain gauge method, which is often used to investigate the stresses in the load-bearing members of steel structures under operational conditions, was used. The advantage is the fast and accurate acquisition of stress values at critical locations selected based on analytical computations or numerical modeling. In the present paper, the residual operating life of two main girders of a bridge crane was assessed by an analytical-experimental approach. The input parameters for the assessment were obtained from the evaluated stress time records and using the Rainflow Counting method. Experimental measurements identified an almost 50% decrease in the residual life of one of the girders. It was caused by non-compliance with the technological procedures for the regular replacement of the rails, where the rail was welded to the top flange on one of the girders. Considering realistic operating conditions, predicting the effect of welded rail on fatigue damage accumulation, performed by other than experimental procedures, is almost impossible for such complex structures. This paper not only documents the importance of experimental measurements but also highlights the significance of selecting measurement locations with consideration of the current technical state of the structure. crane girder; fatigue life; stress analysis; strain gauge +
Footnote †: journal: Applied Mechanics and Mechanical Engineering  
**Criticality:** [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON] Experimental Investigation of the Fatigue Life of a Bridge Crane Girder Using S-N Method. _Appl. Sci._**2022**, 12, 10319. [[https://doi.org/10.3390/](https://doi.org/10.3390/)]([https://doi.org/10.3390/](https://doi.org/10.3390/))  
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to assess the fatigue life, the operational measurement was continued with the remaining strain gauge transducers. Modes 1 to 3 described in Section 2.2 are documented in Figures 3-5. Figures 3a, 4a and 5a show the time courses of the normal stresses occurring at measurement points A1, A2, B3, B4, B7 and A8 and the shear stresses occurring at point A6. The details of the time courses corresponding to the passing of the crab through the center of girders are depicted in Figures 3b, 4b and 5b. Figure 3 represents the manipulation of the crab without load, i.e., Mode 1 with the cycle repeated twice. Figure 4 shows the manipulation of coils 1, 2 and 3 corresponding to Mode 2. From the series of measurements performed for Mode 3, Figure 5 presents the manipulation with a load of 20,930 kg (coil 6, Sector 2B). +++  
A phenomenon that could not be assessed during measurements was identified for each time record presented for the modes above. It was because of different characteristics of the strains' time records registered at locations A1 and A2 versus B3 and B4 by passing the crab through the center of girders. As the crane was normally used by the operator before application of the strain gauges, the experimental measurement continued. The cause of this phenomenon, i.e., welding of the rail to the top flange, was only discovered by subsequent analysis of the measured data. Therefore, the fatigue life at all measured locations was assessed. Table 4 shows the maximum values of the normal stresses that occurred at locations A1, A2, B3, B4, B7 and A8 and the shear stress that occurred at location A6 obtained when handling a load of 20,930 kg in Mode 3. The mentioned case corresponds to an actual operation, as it considers the crab weight, the load and the dynamic effects from the lift of the load, the crab travel and the bridge crane acting on the weight of the load and the crab.