Detection and late change of structural health index of casting crane bridge

Stiffness indexStandard requirementsMeasured value in December 2016Measured value in March 2021Change significance
Camber of main beam18.9~37.8 mmElectrical beam 24 mm, non electrical beam 25 mmElectrical beam 19 mm, non electrical beam 20 mmThe camber of the main beam gradually decreases, and the structural elasticity gradually declines
Deflection of main beam≤27 mm10 mm12 mmThe static stiffness of the main beam is increased and the rigidity of the main beam is weakened

The 260t-27m casting crane of a steel plant was put into use in 2007 to handle the molten steel ladle, with a daily working load of about 260t. The working frequency from 2007 to 2020 is estimated as 84 working cycles per day, and the working frequency from 2021 is estimated as 100 working cycles per day. The working frequency changes from busy to very busy.

Dynamic stiffness indexStandard requirementsMeasured value in December 2016Measured value in March 2021Change significance
Dynamic stiffness of the whole machine>2 Hz2.17 Hz2.06 HzDecreased dynamic stiffness and structural flexibility

The inspection and test will be carried out in December 2016, and after 4 years and 3 months, the inspection and test will be carried out again in March 2021. The two inspection tests provided direct data for observing the middle and later stages of the fatigue life of the equipment bridge structure and the changes of the fatigue damage positions.

Strength indexMeasured value in December 2016Measured value in March 2021Change significance
Maximum stress in midspan85.1 MPa86.6 MPaThe strength of the main beam is macro stable
Maximum shear stress at the end55.1 MPa71.2 MPaThe end stress increases significantly, indicating the next failure point

The static stiffness change of the main girder of the bridge is shown in Table 1. Table 2 shows the variation of crane dynamic stiffness. The change of bridge strength is shown in Table 3. The expansion changes of the damaged parts are shown in Table 4.

Damaged partDetected in December 2016Detected in March 2021Change significance
Sequence 1: Variable section arc weld of bridge end beamThere are 3 cracks in the total 4 common areas, with the length of 300 mm, 400 mm and 400 mm respectivelyFailure in the middle to late fatigue life
Sequence 2: lower base metal of wheel seat supporting main beamThere are 6 cracks in the total 4 common areas, with the length of 30 mm, 50 mm, 60 mm, 70 mm, 80 mm and 100 mm respectivelyLate fatigue failure
Prediction of sequence 3: weld at variable section of girder endMaximum shear stress at the end 55.1 MPaMaximum stress at end 71.2 MPaPrediction of the next common failure area, late failure of fatigue life
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