A cyclic yield-line plastic hinge model is developed to estimate the rotation capacity of fully-restrained steel moment connections designed for moment-frame structures in the US under seismic load conditions. The geometry of the beam after local buckling occurs in the plastic hinge is modeled using the yield-line approach. A simplified force-displacement relation for the plastic hinge is proposed based on large deformations and plastic moment at the yield lines using the principle of virtual work. Two limit states are considered using the cyclic yield-line plastic hinge model. First, post-peak connection strength degradation due to local and lateral-torsional buckling is used to establish a connection rotation limit similar to the limit used in FEMA-350. Second, low-cycle fatigue crack initiation model based on a cumulative local strain concept at the critical yield line is used to predict crack initiation at the creases of the local buckles in the plastic hinge and, thus, limit plastic hinge rotation. A comparison of connection rotation capacities recorded in recent SAC Joint Venture tests is presented to validate the proposed cyclic yield-line plastic hinge model. The proposed cyclic yield-line plastic hinge model is a calibrated analytical model for estimating connection rotation capacity. This model is intended for use by designers to develop new connections before the required proof-tests.
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