Effect of low cycle fatigue on steel moment frames with prequalified rigid connections (bolted unstiffened and stiffened extended end-plate moment connections)

The moment beam-to-column connections include those connections that can transfer the moment between the beam and the column. Bolted unstiffened and stiffened extended end-plate moment connections are from this category. This type of connection, which was investigated after severe damage to connections in the Northridge earthquake, Due to proper behavior under cyclic loading, was introduced as one of the pre-qualified connections of the AISC code and subsequently by the Iranian Code. Since low cyclic fatigue is another cause of failure in connections under the Northridge earthquake, it is necessary to study the fatigue behavior of this type of connection under the effect of low cycle fatigue. So far little studies have been done on the effect of low cycle fatigue on various types of connections, including end plate connections. In the present study, the S-N fatigue chart (Effective stress to the number of load cycles) that was used to estimate the effect of high cycle fatigue was developed to the low fatigue region for the end plate connections. Then, with using the S-N chart, the effect of low cycle fatigue on a 12-story structure under the influence of various earthquake records was studied to investigate the effect of low-cycle fatigue on structures with end plate moment connections. The analysis of steel structures showed that in end plate moment connections at the plastic hinge position, the effect of the cumulative cyclic fatigue before the plasticization of the connection under severe earthquakes is influential. Since the effects of fatigue are cumulative, it will be significant and destructive for end plate connections of buildings that have experienced several medium and large earthquakes.