The inﬂuence of surface preparation and the lubricating effect of mill scale on the performance of slip-friction connectors Wei Y. Loo a,⇑ , Pierre Quenneville b , Nawawi Chouw b a Department of Civil Engineering, Unitec Institute of Technology, New Zealand b Department of Civil and Environmental Engineering, Faculty of Engineering, The University of Auckland, New Zealand highlights  Abrasion resistant steel on mild steel produces stable sliding when surfaces are in the clean mill-scale condition.  Where faying surfaces are polished so that bright steel is visible, severe frictional behaviour invariably arises.  In one instance of polished on polished sliding, the test was terminated due to the surfaces friction welding together.  The observed sliding behaviours of various surface types can be explained in the context of the adhesion theory of friction.  The clean mill-scale condition should be speciﬁed in the design and implementation of slip-friction connectors. article info Article history: Received 12 August 2016 Received in revised form 14 August 2017 Accepted 17 August 2017 Available online 23 September 2017 Keywords: Slip-friction connectors Energy dissipation Seismic design Adhesion theory of friction Mill scale abstract 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 dia- phragms 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 sur- faces 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 fric- tion welding. The results suggest that while dissimilar metallic hardnesses at the sliding surfaces is a nec- essary 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 con- nectors are in the clean mill scale condition prior to their installation. Ó 2017 Elsevier Ltd. All rights reserved. 1. Background Damage avoidance in seismic design, is essentially about enabling non-linear behaviour at speciﬁc locations in a structure, http://dx.doi.org/10.1016/j.conbuildmat.2017.08.100 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: wloo@unitec.ac.nz (W.Y. Loo). Construction and Building Materials 155 (2017) 1025–1038 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat