Parallel-to-Grain Withdrawal Resistance of Structural Self-Tapping Screws in Canadian Timber

Abstract

This thesis investigates the withdrawal capacity of self-tapping screws (STS) inserted parallel-to-grain. An experimental study was conducted with 100 screws tested in total. The experimental program includes two main test series. First, withdrawal testing was conducted on screws with a diameter of 10 mm with varying embedment depths, timber density and moisture content. Strains in the screws were monitored to detect if yielding was occurring. The withdrawal resistance increase as the embedment depth increased until the screws began yielding and eventually reaching their ultimate tension capacity. The ultimate tensile capacity was consistently reached at an embedment depth of 240 mm in dry Douglas Fir, 280 mm in wet Douglas Fir and 320 mm in dry Eastern White Pine. Second, withdrawal tests were conducted on screws with a diameter of 12mm and a constant embedment depth of 280 mm with varying timber density and moisture content. Increases in moisture decreased the withdrawal resistance. The average withdrawal resistance of screws in Douglas Fir showed to more susceptible to changes in moisture content when compared to Eastern White Pine. For both screw diameters, control specimens were tested to determine the tensile capacity of the screws on their own. The data from all the withdrawal tests was compared to three withdrawal equations: the CSA O86 equation for lag screws, the CCMC 13677-R Report equation for STSs, and the ETA-11/0196 equation for STS. In its un-factored form, the CSA O86 equation for lag screws was the best at predicting withdrawal resistance of the STSs inserted parallel-to-grain.