Nonlinear Stress Amplification Mechanism of Shell-to-Bottom Fillet Welds in Large Oil Storage Tanks Induced by Local Bottom Plate Detachment Under Harmonic Settlement

Although previous studies have investigated global deformation responses of large storage tanks subjected to uneven foundation settlement, the stress redistribution mechanism of shell-to-bottom fillet welds caused by local bottom plate detachment remains unclear. In this study, a fully coupled bottom plate–shell–fillet weld model was established to quantify the nonlinear transition from continuous contact to local detachment and its influence on weld stress amplification. Uneven settlement was simulated using a Fourier series calibrated with field data. The model achieved high accuracy with a maximum error below 10%. The results indicate that under a 40 mm settlement amplitude and 19.76 m liquid level, the bottom plate reaches a critical local detachment state during the fourth harmonic (n = 4). Compared to the full-contact state at n = 3, the local detachment at n = 5 increases the peak stress in the large fillet weld from 500 MPa to 615 MPa, yielding a stress amplification factor of 1.23. These findings clarify the mechanical interactions between the tank bottom and its foundation, providing crucial theoretical support for settlement risk management and structural design.