Evaluation of Screw Connections in Aluminum Members

Many façade systems are constructed with aluminum extrusions that are often fastened together with self-drilling and self-tapping screws. A failure of a cladding panel fabricated with this type of connection led us to evaluate the capacity of these connections for shear and tension loading, while also considering the effects of prying caused by eccentric loading. Thus, our evaluation consisted of performing laboratory testing of screw fasteners connecting two aluminum members in direct shear and tension, and comparing those values to capacities derived from equations found in the 2015 Aluminum Design Manual (ADM). Our laboratory testing also considered the installation method of the screws including drill speed (rpm), clutch settings, and effects of impact driving. We researched literature documenting prior testing on which the 2015 ADM equations were based and other industry literature to determine how screws should be installed into aluminum components to ensure proper connection performance. Our research found that there is limited guidance or recommendations for the method of screw installation. Thus, our limited laboratory testing consistently produced results lower than the ultimate capacities predicted by the equations provided in the 2015 ADM regardless of the installation method. However, we also found that the screw installation method has a significant effect on the capacity of the screw connection. Installation at higher rpm had the potential to strip the threads cut into the aluminum and screws installed using an impact driver typically resulted in fully stripped threads. Therefore, while further research on this topic is warranted, it does appear that the 2015 ADM equations may overestimate ultimate screw connection capacities and that screw installation methods have a significant effect on capacity.