title: Bearing Behaviour and Design of Aluminium Sub-Heads in Window Wall Frames
Creators: name: Akbari, Masomeh
Publisher: name: Griffith University
Year: 2021
Subjects: subject: aluminium sub-heads
subject: concentrated load
subject: aluminium frames
subject: window walls

description: A unique combination of properties makes aluminium one of the most desirable materials in many construction sectors including façade industry. Aluminium window walls as a façade system provide resistance against wind load and are decisive elements in the performance of the building envelope. In considering their complex functions, they are subjected to numerous criteria and continuing research and improvement. Window walls are commonly made of glass supported by aluminium framing members, and occupy a considerable share of the building cost. The aluminium frames of window walls (comprised of heads, sills, and mullions) transfer the wind loads from glass panels to the aluminium sub-frames (comprised of sub-heads and sub-sills). The sub-frames then transfer the loads to the slab through the bolt connections. Under this loading condition, the aluminium sub-heads (at the top of the system) are the dominante wind load bearing elements, and are prone to bearing failure due to their long flange length. This phenomenon of bearing failure has never been researched in the past. To address this gap, the structural performance of aluminium sub-heads subjected to concentrated load was investigated in this study using comprehensive experimental and numerical studies. Furthermore, accurate design rules were developed to predict the bearing capacities of aluminium sub-heads. Two types of typical sub-head sections, known as C-shaped sub-heads and sub-heads with removable beads, were used in the experimental study. The main difference between these two sections is that the later included two parts (the base and the bead) which can facilitate effective installation and assembly of façade panels. Two series of experimental tests were conducted to investigate and evaluate the bearing behaviour of the aluminium C-shaped sub-heads and the sub-heads with removable beads. Four C-shaped sub-head sections and six sub-head with removable bead sections were tested subjected to bearing loads considering different loading and boundary conditions as well as different bearing widths. The governing modes of failure were found to be yielding and fracture at the web-to-flange junction, as a result of the bending of the cantilever flange. Following experimental tests, finite element models were developed to further investigate the bearing behaviour of the aluminium sub-heads. The general-purpose software ABAQUS, with implicit solver, was used to simulate the bearing behaviour of aluminium sub-heads. The models were validated using the experimental results and a good agreement was achieved in terms of the ultimate strengths, the load-deflection responses and the failure modes. Subsequently, parametric studies were performed using validated models to investigate a wide range of aluminium sub-head sections with varying thicknesses, flange widths, loading conditions, and bearing widths. Failure of aluminium sub-heads in the window walls under wind loading bear strong resemblance to the most prevalent failure mode in the cold-formed steel stud-to-track connection of a Light Gauge Steel (LSF) wall, which is the failure of the track under concentrated load. Since current aluminium standards do not have design criteria to predict the bearing strength of aluminium sub-head sections subjected to out-of-plane forces in window walls, the results acquired from this research were compared with the nominal bearing strengths predicted by the currently available cold-formed steel design rules (the North American Standard for Cold-Formed Steel Structural Framing (AISI S240, 2015), U.S. Army Corps of Engineers (TI 809-07, 1998), and Steel Stud Manufacturers Association (SSMA, 2000)) for the tracks in the stud walls. As a result of the comparisons, weaknesses in the current design standards were identified. Hence, based on the experimental and numerical results, new design rules were proposed which accurately predict the bearing capacities of aluminium sub-head sections. The findings of this research demonstrated that the proposed equations for estimating the ultimate bearing capacities of aluminium sub-head sections are reliable and in precise agreement with the experimental and numerical results.