Models for the calculation of the withdrawal capacity of self-tapping screws

Equations to calculate the withdrawal capacity of self-tapping screws, arranged in softwood, were determined. Based on results of 1850 withdrawal tests and by means of a regression analysis one equation for the withdrawal resistance and two further equations for the withdrawal parameter (resistance over nominal screw diameter and penetration depth) were derived. Nominal screw diameter, penetration depth and characteristic density of the softwood used are the independent variables in the equations, which are valid for an angle from 45° to 90° between the screw axis and the grain.
The equation for the withdrawal resistance delivers, also in comparison with code rules, the most favourable values. It enables, more accurately than until now, the estimation of the withdrawal resistance for wood screws with geometrical properties similar to those of the screws used for the tests. Hence, the benefit of this study is the reduction in testing, which is currently necessary for technical approvals.