Axial Rotation and Lateral Torsional Buckling of Extruded Aluminium Mullions in Curtain Wall Facades

Large modern buildings frequently are enclosed by lightweight, panelised, aluminium-framed facades, known as unitised curtain walls. This study shows that, in such wall systems, the established procedures for analysing the stability of structural extrusions ignore two of the three greatest causes of lateral movement in the main member, or mullion. One of these overlooked influences is the force caused by the pressurisation of the mullion's interior
cavity, and the other is the moment transferred to the mullion, through structural adhesive, from the wall's face material, which is usually glass.

A new, closed-form, algebraic expression is proposed to describe the lateral movement of a unitised mullion's interior flanges, and predictions obtained in this way are compared with results from a finite element model. It is suggested that the novel analytical approach might obviate the need for conventional lateral torsional buckling calculations, which are not only time-consuming to produce, but which are also of questionable accuracy. This simplification of the structural design process will make it easier for facade engineers to design extrusions in which metal is used efficiently, and because the production of aluminium is energy-intensive, material savings achieved in this way will bring both commercial and environmental benefits.