Abstract: Micromechanics-based fracture criteria of structural steels can be used to predict ductile fracture initiation with large scale yielding and no initial flaw. High-fidelity finite element analyses were carried out on ten welded connection specimens between the steel tube column and beam flange under monotonic tensile loading. The calibrated micromechanics-based fracture criteria, including Stress Modified Critical Strain (SMCS) model and Void Growth Model (VGM), were used to predict fracture initiation for each specimen. Good agreements were found between the predicted results and the test results. Therefore, it is applicable to use the micromechanics-based fracture criteria to predict ductile fracture initiation of connections under monotonic loading. Subsequently, the VUMAT subroutine was developed in ABAQUS software by the authors. By taking the SMCS and VGM as fracture criteria, the post-fracture load-displacement curves of two welded connection specimens between the steel tube column and beam flange that fractured at different locations were traced by deleting the failure elements one by one. The predicted results agree well with the test results, which indicates that these micromechanics-based fracture criteria and associated simulating techniques can be used to carry out collapse analysis of steel structures under extreme events.