Moment capacity of apex brackets of cold-formed steel portal frames

Abstract

The strength and stiffness of cold-formed steel portal frames are heavily related to the performance of their joints; these joints are normally formed through brackets, bolted to the webs of channels. They should have adequate strength to ensure the frame fails within the members, and the design moment capacity of a joint should be equal to or exceed the moment to which the joint is subjected. For apex brackets, based on the results of a non-linear elasto-plastic finite element study, the authors have recently proposed design guidance in the form of equations for predicting the moment capacity. However, this prediction was limited to fully restrained apex joints against lateral torsional buckling, which is an arrangement not commonly used in practice. Furthermore, the joints considered were composed of back-to-back brackets connecting back-to-back channels. The design recommendations were further limited to apex joints under bearing gravity load, equivalent to the opening moment applied to the bracket. This paper extends the previous work by now considering both single and double apex brackets, which are partially restrained, consistent with New Zealand practice, and considering both opening and closing moments on apex joints (or gravity and wind uplift load on portal frames). This study proposes unified design equations from a parametric study comprising 5,040 finite element models.