Prediction of Ultimate Capacity of Welded Steel Moment Connections under Extremely Low Cycle Fatigue and Progressive Collapse

In order to determine the performance level of steel structures or to strengthen them, it is necessary to determine the ultimate capacity of their connections under the influence of extremely low cycle fatigue caused by cyclic load. On the other hand, the ultimate capacity or failure of the connections, under unavoidable monotonic loadings such as explosions or fires, needs to be determined in order to predict the progressive collapse in structures utilizing intermediate and/or special moment frames. In this study, the ultimate capacity of welded connections, subjected to monotonic loading, is estimated using the Stress Modified-Critical Strain model (SMCS). However, the Cyclic Void Growth Model (CVGM) is used to investigate connection capacity under cyclic loading. These models could be used to estimate the ultimate capacity of connections to determine the performance levels of the structures where it is not practical or possible to test laboratory specimens. In this paper, the ultimate capacity of connections are obtained based on the crack initiation criterion in three types of moment connections, namely reduced beam section (RBS), welded unreinforced flange-welded web (WUF-W), and increased beam section (IBS) moment connections; (۱) subjected to monotonic loading in the column removal scenario by using the (SMCS) model, and (۲) under cyclic loading by using the (CVGM). This research shows that by using the ductile fracture models, it is possible to predict the ultimate capacity and fracture mode of the weldedmoment connections with great accuracy. The results of this investigation, using fracture models, indicate that out of the three types of connections, the seismic rotational capacity of the IBS moment connection is the largest (۰.۰۵ radians), next is RBS (۰.۰۴۴ radians), and WUF-W is the last (۰.۰۴ radians) with the least rotational capacity under cyclic loading. The results also reveal that the rotation capacity of the connections under monotonic loading is two to three times thecapacity of the same connection under cyclic loading (IBS:۰.۲۰ radians, RBS: ۰.۱۶ radians and WUF-W :۰.۱۴۷ radians).

Keywords:

Ultimate capacity , ductile fracture , monotonic loading , Cyclic loading , CVGM model , SMCS model