Head pull-through capacity of load-bearing timber screws – Influential parameters and shortcomings of European test procedure

Abstract

ABSTRACT Self-tapping screws have become elementary components in modern timber construction. Recently, the method of determination and the definition of the head pull-through parameter of self-tapping, partially threaded screws have been debated, e.g. in the draft version of the new Eurocode 5, Part 1-1. This paper aims to contribute to this discussion. First, today's normative regulations and assessment procedures for the head pull-through parameter, as well as an evaluation of manufacturer-declared values, are presented. Questionable details of the European test standard EN 1383 are then discussed and analyzed numerically. A comprehensive head pull-through campaign with 6 and 8 mm partially threaded screws using Norway spruce solid wood specimens reveals the significant influence of specimen thickness on the pull-through capacity in the case of flange-type screws. A much lower effect in the case of countersunk screws results from a different fracture pattern with rather constant pull-through friction. The head diameter is negatively correlated with the pull-through capacity. However, the best correlation with regard to pull-through strength is obtained with the newly introduced net screw head area. Screw alignment in the radial growth-ring direction yields the highest capacities; the density effect is linear. A fundamental revision of EN 1383 is considered inevitable to provide unambiguous results.