Abstract This study investigates the mechanical behaviour of preloaded beam‐to‐beam joints incorporating long slotted holes, which can be used to provide horizontal adaptability for the joint in the context of steel reuse. Finite element analyses, validated against experimental results, were performed to evaluate the impact of parameters such as plate thickness, bolt preload, bolt diameter, and bolt number on joint shear and slip performance. Results reveal that increasing bolt preload levels from 30% to 70% leads to a marked improvement in slip resistance and a modest increase in overall shear capacity. Force–displacement curves show that higher preload delays the onset of slip and enhances initial stiffness due to improved frictional engagement at the interfaces. Bolt slip was monitored at the individual bolt level and was found to initiate at significantly higher loads when greater preload was applied. The findings emphasize the critical role of preload in controlling interface behaviour and improving connection performance, particularly in the early stages of loading. In the shear resistance, number of rows and bolt diameter had a stronger influence, though gains diminished beyond M24 due to the adverse effects of larger clearance holes on bearing resistance.