© 2021Widely used design guidelines and specifications provide methods to calculate plastic resistance of end-plates considering yield line mechanisms. Experiments reported to date showed that the ultimate resistance of end-plates can be considerably larger than its plastic resistance due to strain hardening. Using the ultimate strength in design can be a viable and cost-effective option; however, limited studies exist on quantifying the ultimate strength. In this paper, a database of four-bolt extended end-plate moment connections was developed. The capacities of these connections were calculated using AISC Design Guidelines and EN1993-1-8. The results showed that the plastic resistances of connections are on average 29% greater than the capacities calculated using the AISC Design Guidelines. On the other hand, the test-to-predicted ratios were found to have an average of 1.08 for EN1993-1-8. The ratios of the ultimate resistance to the calculated plastic resistance are 2.07 and 1.73 for AISC and European approaches, respectively. A numerical study was undertaken to propose modified design expressions for the plastic resistance which are compatible with AISC Guidelines. Furthermore, the ultimate resistance of end-plates was quantified. Stiffened and unstiffened T-stub models were analyzed using the finite element method. Expressions based on yield line mechanisms were developed to calculate the plastic and ultimate resistance. The evaluations showed that the averages of the test-to-predicted ratios are 1.06 and 1.17 for the plastic and ultimate resistance respectively when the proposed expressions are used.