Numerical investigation of split mullions subject to negative wind action

Abstract

In modern facade construction, unitized aluminium framed and glazed facades have gained increasing popularity. In these unitized systems, vertical load bearing member is a split mullion (or mullion couple) made of thin-walled male and female mullions of adjacent panels, thus facilitating pre-fabrication and site assembly. The cross-sectional shape of these mullions are complex, and their shear centre generally lies outside the section. During negative wind action (suction), the male and female mullions would rotate to lean on each other at their unrestrained compression flange, by giving lateral supports to each other, due to eccentricity between the load and their shear centre. Conducting full-scale tests to understand such complex behaviour is expensive and time consuming, thus numerical simulations are preferred in such applications. In addition, finite element (FE) models developed in the previous studies were unable to simulate the real behaviour of mullion couples. Therefore, advanced numerical models were developed in order to investigate the structural behaviour and capacity of mullion couples subject to negative wind action. Full and simplified FE models of three different mullion couples, comprising captive and structural glazing mullions, were developed and analysed. In the full model, mullion couple, glass panes and gaskets/silicone sealants were modelled, whereas, only the mullion couple was modelled in the simplified model. In addition, FE analyses were performed on individual mullion sections, and their capacity predictions were compared with the respective mullion couple analysis predictions. The results from FE analyses were then compared with the moment capacities predicted using the current design approach. Based on the comparisons, suitable recommendations are given for the design of mullion couples subject to negative wind action. Overall, this study has improved the knowledge and understanding of the structural behavior and capacity of mullion couples used in facades.