Theoretical and numerical study of end-plate thickness design for extended stiffened moment connections with wide bolt layouts: A mechanically consistent yield-line formulation

Extended stiffened end-plate moment connections with wide (four-bolt-wide) bolt layouts are increasingly used in steel structural systems, including CFST composite frames, yet their end-plate thickness design is not explicitly addressed by current provisions such as ANSI/AISC 358–22. The widely used Sumner equation incompletely represents the geometric parameters governing the yield-line mechanism, potentially underestimating the required thickness under specific configurations. This study presents a mechanically consistent analytical formulation, derived from first principles via the virtual work method, that explicitly incorporates all governing geometric parameters of the yield-line pattern. Parametric analyses of W18–W40 beam sections with A992 steel and A490 bolts show that the proposed formulation consistently yields more conservative thickness predictions than Sumner's equation, with differences of 2%–7% within the investigated configurations, increasing for smaller beam sections and geometrically critical configurations. Validated finite element models further show that a thickness difference of only 2.6%–4.0% can result in a 4.8%–12.4% difference in post-yield rotational response, with a rotation-dependent amplification ratio of approximately 4.1 at a beam-end rotation level of 0.07 rad, revealing the significant influence of local connection design on nonlinear joint behavior. Sensitivity analysis identifies the geometric parameters governing this response. The proposed formulation offers a reproducible basis for specifying representative end-plate thickness in future finite element studies of similar connections and supports the refinement of current design provisions.