The influence of surface preparation and the lubricating effect of mill scale on the performance of slip-friction connectors

Slip-friction connectors have already seen implementation at the beam-column joints of numerous constructed multi-storey steel buildings, and are currently being researched for use as shear wall hold-downs, inter-wall energy dissipaters, column base hinges, and as the connectors between floor diaphragms and shear walls. Regardless of the context in which they are employed, the critical aspect of their seismic performance is the ability to provide stable sliding at a predictable structural load. If the connector strength spikes to a value significantly above the intended slip force, this would obviate the main purpose of these devices, which is to shield structural members from plasticisation. Researchers have recently found that connectors involving abrasion resistant steel in sliding against mild steel will typically exhibit stable sliding. The hardness differential of the two types of steel has been proposed as the main reason for this, and indeed stability of sliding does tend to increase with increasing difference in steel hardness. However, the experiments described in this article would suggest that other factors are also involved. Tests were carried out on slip-friction connectors in which the sliding surfaces were either in the clean mill scale condition, or had the mill scale completely removed through a process of either grit blasting or grinding. Aligning with previous research, those situations in which both the opposing surfaces in sliding are of clean mill scale, are invariably associated with highly stable sliding characteristics. However, where only one of the opposing sliding surfaces is of clean mill scale, and the other grit blasted or polished, stable sliding will eventuate only after a substantial amount of cumulative travel at low loads. Where both surfaces at a sliding interface have had mill scale completely removed, the sliding behaviour is erratic and characterised by frequent load spikes. During one test seizure occurred from friction welding. The results suggest that while dissimilar metallic hardnesses at the sliding surfaces is a necessary condition for stable sliding, it is an insufficient one, and the influence of surface preparation and the presence of mill scale are equally important. The experimental observations are then explained in the context of current tribology theory. To minimise the possibility of design loads being significantly exceeded during a seismic event, it is strongly recommended that the sliding surfaces of slip-friction connectors are in the clean mill scale condition prior to their installation.