Withdrawal and combined load capacity of threaded fastener wood joints

Abstract

In this study, general models of the capacity of threaded fastener joints were developed from extensive experimental tests. One study objective was to develop a general model of threaded fastener withdrawal strength applicable to joints containing fasteners with widely varying thread geometries. A total of 419 tests of joints using six different fasteners and five species were tested. A multiplicative model containing wood specific gravity and the wood volume contained within the fastener threads provided very accurate predictions of withdrawal strength.

A second study objective was to assess the accuracy of existing design criteria for threaded fastener joints subject to combined axial withdrawal and lateral shear loading. A total of 321 joints using 3 different fasteners and two species were tested at five angles between 0° and 90° to fastener axis. Little to no interaction was found between the lateral and withdrawal force components on joint capacity. Current design philosophy in the National Design Specification generally yielded conservative predictions of actual joint capacity. Improved design criteria suggested by experimental results were derived.

A pilot study was also conducted to assess the influence of wood desorption on the withdrawal resistance of tapping screws. Forty joints of two moisture conditions, green and dry at insertion and two species were tested in withdrawal. In general, maximum and proportional limit loads were not affected by desorption whereas stiffness was significantly reduced for joints which desorbed after insertion.