Seismic performance of stiffened end-plate steel joints with non-symmetric octagonal bolts arrangement

Extended end-plate connections are widely used in seismic prone regions since they could ensure a full strength performance allowing the activation of the plastic hinge within the connected beam. However, in recent years, particular attention was given to the stiffened joints focusing on the stress concentration within the beam at the rib toe section. With particular regard to the build-up section, some experimental tests already presented in literature have highlighted how the presence of the rib could compromise the connection performance, leading in some cases to brittle fracture for low level of rotation. This has led researchers to investigate an alternative unstiffened connection to those prequalified by the North-American standard. The unstiffened connection exploits a new octagonal bolts arrangement, to optimize the distribution of stresses within the bolts lines. Within this framework, the present work aims to adopt the new non-symmetric octagonal bolt arrangement also for stiffened hot-rolled I-shaped assemblies. Thus, two design procedures developed for both North American and European joints were proposed and numerically investigated. The effectiveness of the proposed design procedures was investigated by means of advanced numerical FE analyses, carried out on twenty assemblies designed varying beam-to-column assemblies and design approaches. The results show that the combination of the octagonal bolt arrangement with the rib stiffeners allows to obtain full strength joints reducing the connection dimensions with respect to the prequalified codes solutions.