Failure mechanisms, group effects and design method of powder-actuated fastener connections in thick existing steel plates with thin retrofit plates

Powder-actuated fastener (PAF) connections have been widely used in cold-formed steel structures owing to their advantages of eliminating pre-drilled holes, allowing single-sided installation, and providing high installation efficiency. However, the failure mechanisms, shear resistance, and group effects of PAF connections with thick existing plates (10–16 mm) and thin retrofit plates (4–8 mm) in primary load-bearing steel structures remain unclear, and corresponding design methods have not yet been established. To address this gap, this study is the first to systematically investigate the failure mechanisms, group effects, and design methods of PAF connections in thick base plate-thin retrofit plate assemblies through combined shear tests and finite element analyses. The results show that four typical failure modes can occur, including end tear-out, bearing failure, PAF failure (bending pull-out or shear fracture), and net-section fracture of the retrofit plate. The governing failure mode is controlled by the end distance, retrofit plate thickness, and steel strength. The peak resistance is determined by both the failure mode and the PAF embedment depth, with PAF failure providing the highest resistance and end tear-out failure the lowest. Existing PAF design provision (e.g., AS/NZS 4600: 2018) is unable to accurately predict the governing failure mode or the resistance of thick steel plate connections. Finite element analyses reveal a pronounced load redistribution effect (shear lag) in multi-row PAF arrangements, resulting in non-uniform load distribution among PAFs. Based on the experimental and numerical results, a group effect reduction factor and a resistance prediction model were developed for multi-PAF connections. The findings provide a theoretical basis for the engineering design and application of PAF connections in the rapid strengthening of steel structures.