Effects of the loading history on the local buckling behaviour and failure mode of welded beam-to-column joints in moment-resisting steel frames

The cyclic behavior of welded beam-to-column joints for moment-resisting steel frames was assessed by constant amplitude cyclic quasi-static tests. Reanalysis of the results showed that the failure mode of the joints strongly depends on cycle amplitudes. A premature brittle failure of the welds may occur if the cycle amplitude is not large enough to cause local buckling of beam flanges. Influence of both flange and web slenderness ratios on local buckling behavior, investigated through an experimental parametric study, is discussed. Four tests, carried out adopting different variable amplitude displacement histories, confirmed that isolated large amplitude cycles have benefical effects on the joint response, extending its life: on the contrary, many large cycles clustered together endanger the seismic performance of beam-to-column joints. Numerical analyses allowed interpretation of the experimental data in terms of local stresses and strains. For "large amplitude" cycles numerical results indicate a local state of strain causing a progressive collapse for low-cycle fatigue while, in case of "small amplitude" cycles, brittle failure mode is due to the ratcheting of material.