Full-scale tests of aluminium mullion couples in unitized facades under wind actions

Abstract

Unitized facade systems made of aluminium frame and glass infills are widely used, particularly in high-rise buildings. The vertical load bearing member of the aluminium frame is a mullion couple (or split mullion) made of thin-walled male and female mullions. The main function of the mullion couple is to transfer the lateral wind load on the glass panes to the main structural frame. During negative wind action (or suction), the male and female mullions lean on each other, and may achieve a capacity higher than the sum of their individual moment capacities. Under positive wind action (or pressure), the glass panes are assumed to provide full lateral restraint to the compression flanges of mullions, which is questionable. In addition, clips are sometimes used to increase the capacities of mullion couples without any evidence. Therefore, currently used mullion design methods could result in either conservative or unsafe designs. This study investigates the structural behaviour and capacity of mullion couples through full-scale wind simulation tests of a real facade assembly made of two unitized facade panels with and without clips under both negative and positive wind actions. This paper presents the full-scale test results and compares them with the current design rule predictions. Based on the comparisons, suitable conclusions and recommendations are given for the structural design of mullion couples under wind actions. Overall, this study has significantly advanced the current knowledge and understanding of the structural behaviour and capacity of unitized façade mullion couples subject to wind actions.