A sub-head refers to the horizontal member at the top of an aluminium window wall system. Under wind load, bearing failure of the aluminium sub-head (ASH) flange due to its long length is a common failure mode, being subjected to high concentrated load transferred from the vertical members (mullions). The bearing behaviour of ASH sections was previously investigated experimentally by the authors and suitable equations (DRAI) were proposed to predict the bearing capacities. These design rules were manufacturer and product specific and only applicable for sections considered in the test plan. To address these limitations, the present study further investigates the behaviour and design of ASH sections through non-linear static analyses using ABAQUS, based on implicit integration schemes. The effects of mesh size, element type, and loading plates on the bearing behaviour of ASH section under different loading and boundary conditions was addressed. The finite element models were validated against the test results in terms of ultimate loads, load–displacement curves and failure modes, and the validated model was then used to perform an extensive parametric study. A broad range of ASH sections covering flange widths ranging from 40 to 100 mm, thicknesses varying between 2 and 4 mm, bearing widths ranging from 50 to 150 mm, and six engagement lengths (5 to 30 mm) was considered in the parametric study. It should be noted that the ultimate loads obtained from the parametric study are not product nor manufacturer specific. Since no aluminium design rules exist to predict the bearing capacities of ASH sections, the accuracy of the current cold-formed steel design rules (DRSs) and that of DRAI were assessed. Both DRSs and DRAI were found not to be suitable for ASH sections in window walls, hence appropriate modifications were made in this study and the proposed design rules (DRAP) predicted accurate bearing capacities which agreed well with the numerical results.