Abstract

The load capacity of screw fixings supporting building services, such as sprinklers in Cross Laminated Timber (CLT) ceilings was tested before, during and after exposure to fire, to investigate the risk of detachment. The pull-out strength of screw fixing in ambient conditions, during and post-fire were investigated in relation to pull-out strength versus embedment depth. The relationships between pull-out strength have been reported as functions of screw fixing embedment depth, screw dimensions, and char formation. In the experimental study, samples of standard industrial CLT were tested with two distinct types of adhesives with typical construction industry screw fixings used for the suspension of Mechanical and Electrical (M&E) services. A purpose built fire-test rig was designed to expose screw fixings embedded into CLT to a fire in a ceiling mounted configuration. A series of eight fires were conducted, and the pull-out strength of each screw fixing was assessed during or after the fire. The reduction of load capacity can be conceptualised into two factors: The charring across the whole timber surface which was deeper close to the fixings leaving a fragile char which could be scraped off; and the weakening of the timber along the length of the screw thread, resulting from the higher thermal conductivity of the screw fixing. Both these effects increased as a function of the shank width of the screw. The outcome of this study is to inform guidance on the ability of screw fixings to support M&E services beneath timber ceilings in the event of fire.