A simple unified model for withdrawal of lag screws and glued-in rods

Abstract

A mean stress failure criterion is applied to the Volkersen-model used for analysis of lag screws and glued-in rods subjected to withdrawal. Expressions are derived for pull-pull as well as pull-push loading conditions. The solutions for glued-in rods appear as special cases of the theory for lag screws, and the usual solutions for a maximum stress failure criterion appear as special cases of the solutions for a mean stress failure criterion. The theoretical solutions for lag screws are compared with test results, and a suggestion is made for taking into account initial shear stresses caused by the thread of the screw when driving the screw into the wood. The main advantage of considering initial shear stresses is that the number of necessary fitting parameters is limited to one, and that the usual values of shear strength and mode II fracture energy of wood can be applied.