Towards system reliability targeted design for flange-angle partially restrained steel moment connections

Partially or fully restrained steel moment connections are essential constituents of lateral force resisting moment frames. The constituents of the moment connection system – bolts, welds, plates and angles – are sized using their individual force demands by a limit state element-based load and resistance factor design procedure. The system reliability of the connection as a whole, which must be commensurate with reliabilities of adjoining frame components, has not been adequately investigated in the literature. This paper presents a methodology to determine the system reliability of commonly used steel moment connections that have been designed according to current element based procedures. A general expression is derived for a system factor that modifies element resistance factors to meet a specified system reliability target, as a function of the element’s relative importance within the system, and associated uncertainties. Uncertainties in element capacities (including their mutual dependence) and system demands are considered. The approach can be used to rationalize element design equations to achieve system reliability targets for the connection system. A detailed numerical example and a set of parametric studies on a partially restrained moment connection with angle supports at top and bottom beam flanges are presented. The results suggest the existence of beneficial system effects in current AISC provisions for the design of partially restrained moment connections.