Cold-formed steel screwed connections with dimples: Experimental investigation and structural behaviour

In cold-formed steel (CFS) connections, self-tapping screws have increasingly been used with pre-punched dimples to improve assembly accuracy and installation efficiency. However, the structural behaviour of such connections remains poorly understood, and current international design standards provide no specific guidance for their design. This paper presents an experimental investigation on single-lap screwed connections, both with and without dimples, to examine their shear behaviour, ultimate strength, and failure modes. The test matrix comprised four steel sheet thicknesses and various plate arrangements relative to the screw heads. The dimpled holes were automatically pre-punched with an indent depth of 1.5 mm and hole diameters ranging from 4.9 mm to 5.3 mm, accommodating 14g-20×14 mm screws with a nominal diameter of 6.3 mm. Non-dimpled specimens typically failed by screw tilting or a combination of tilting and bearing, whereas dimpled specimens predominantly failed in bearing. The presence of dimples improved resistance to screw tilting due to the interlocking mechanism developed at the connection interface. Measured shear resistances were compared with predictions from EN 1993-1-3:2006, ANSI/SDI AISI S100:2024, and AS/NZS 4600:2018. For bearing failures, current design equations provided relatively accurate predictions for non-dimpled connections; however, no design provisions exist for dimpled-hole connections. These findings highlight the need for new design rules that explicitly account for the influence of dimples, particularly under bearing failure. A companion paper presents new design equations for shear-loaded CFS screwed connections with dimpled holes, calibrated against the experimental results reported herein and validated through nonlinear finite element analyses in an extended parametric study. • A single-lap shear test configuration was developed to evaluate screwed connections with dimples. • Screwed connections with and without dimples were tested using four different steel sheet thicknesses. • Non-dimpled specimens typically failed by screw tilting or a combination of tilting and bearing. • Dimpled specimens exhibited improved resistance to screw tilting due to an interlocking mechanism. • No design provisions currently exist in standards for the design of screwed connections with dimples in shear.