Experimental investigation on tensile failure mechanism of self-drilling screw connections

Abstract

Self-drilling screws are the most commonly used fastener in constructing joints of cold-formed steel sections. Because of its self-drilling feature and consequent advantages in construction procedures, this screw is widely used in steel construction. Accordingly, researchers conducted a wide range of experimental and numerical investigations to ensure optimum and safe design of these particular screw joints. Plenty of studies on screw joints are conducted by experts from North America and Australia in the last two decades. These existing studies are based on North American grades of cold form steel. However, in the Asian countries (e.g. Bangladesh) usually, Chinese grades of cold-form steel are used for construction purposes. It is necessary to evaluate the feasibility of the North-American guidelines while using them for the design of screw joints made of Chinese steel grades. In the present study, an experimental investigation has been conducted on self-drilling screw joints made of Chinese S280 and S350 grades of cold-formed steel. For this purpose, 55 test samples are prepared by overlapping two cold-formed steel sheets of similar thickness and connecting them with screw. Samples are tested in tensile loading till the failure point. Moreover, the 2D digital image correlation technique was applied to get the axial strain pattern of the whole sample surface. The influence of screw number, screw size, screw arrangement, and sheet thickness on joint behavior is evaluated. The response of the screw connections for varying geometric and material properties is examined in terms of load-elongation plot, connection strength, failure mode, stiffness, and ductility. It is observed, that with the increase of number of screws, failure mode changes from tilting-bearing type to net-section fracture for the cases studied. Usually increasing screw number, screw size, steel grade, and sheet thickness show a rise in joint capacity. Changing screw arrangement from transverse to longitudinal (with respect to loading direction) significantly increases connection capacity and stiffness. Group effect is observed in multi-screw joints due to the use of normal ductility steel and corresponding stress distribution. Current American design provisions are evaluated for these Chinese grade samples. For most of the studied samples, the American design provisions provided safe design strength prediction. Number of screw rows has been found to be an important parameter in influencing the joint behavior of multi-screw connections. The experimental data can be used in the future to develop more generalized design provisions and ensure an optimum and safe design.