EXPERIMENTAL AND THEORETICAL STUDY ON PULL-OUT CAPACITY OF JOINTS BETWEEN SELF-TAPPING SELF-DRILLING SCREWS AND DIFFERENT SUBSTRATES

The mechanical properties of screw-substrate joints have become one of the key issues in the research of long-span metal roofing systems. Reliable connections are one of the important components to ensure the wind resistance of metal roofs. The mechanical properties of carbon steel screw and aluminum substrate (CSA joint), of stainless steel composite screw and aluminum substrate (SSA joint), and of stainless steel composite screw and steel substrate (SSS joint) were tested in details through totally 360 experimental samples. The damage failure modes of these three joint types are analyzed, the influence law of parameters on the pull-out capacity of joints is revealed, and the design and calculation equation of joint bearing capacity is established. The study results show that: under the two drilling modes, the three joint types show the failure mode that the screw and the substrate are loaded together, the damage and deformation behavior occur at the occlusal place, and then the screw is pulled out from the substrate. The pull-out resistances of the joints are the SSS joint, CSA joint and SSA joint in turn. The substrate thickness, screw diameter and pitch have the influences on the pulled-out capacity of the joints, especially for the substrate thickness and pitch. Under the same conditions, the influence of changing pitch on the bearing capacity of three types of joints is up to 94.54%. The proposed equation for calculating the bearing capacity of the three types of joints are in a good agreement with the experimental results.