The camber of the main beam gradually decreases, and the structural elasticity gradually declines
Deflection of main beam
≤27 mm
10 mm
12 mm
The 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 index
Standard requirements
Measured value in December 2016
Measured value in March 2021
Change significance
Dynamic stiffness of the whole machine
＞2 Hz
2.17 Hz
2.06 Hz
Decreased 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 index
Measured value in December 2016
Measured value in March 2021
Change significance
Maximum stress in midspan
85.1 MPa
86.6 MPa
The strength of the main beam is macro stable
Maximum shear stress at the end
55.1 MPa
71.2 MPa
The 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 part
Detected in December 2016
Detected in March 2021
Change significance
Sequence 1: Variable section arc weld of bridge end beam
There are 3 cracks in the total 4 common areas, with the length of 300 mm, 400 mm and 400 mm respectively
Failure in the middle to late fatigue life
Sequence 2: lower base metal of wheel seat supporting main beam
There 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 respectively
Late fatigue failure
Prediction of sequence 3: weld at variable section of girder end
Maximum shear stress at the end 55.1 MPa
Maximum stress at end 71.2 MPa
Prediction of the next common failure area, late failure of fatigue life
