A new testing apparatus was designed and a test method developed to enable the application of axial loads, lateral loads, and controlled ratios of axial to lateral loads to nailed wood joints. The effect of axial load components on ultimate lateral load, joint deformations and several slip moduli were evaluated. These load-slip parameters were determined for three species of wood, three depths of nail penetration, and eight angles of load application. Several basic equations were examined to quantitatively describe the effect of combined axial and lateral loads. These included Hankinson's formula, a modified stress interaction formula, and a semi-log curve fitting process. Hankinson's formula proved to be a poor equation to describe the combined load effect, producing errors in excess of 500 percent in some cases. The stress interaction formula modified with a sine function produced an accurate method for describing ultimate load with errors below 15 percent. This is recommended as a design equation for nailed joints loaded in a combined manner. The use of a semi-log curve fitting process and a simple linear regression made it possible to describe the effect of axial load components on stiffness parameters of nailed joints.
Literature review
Current design practices with nailed joints are categorized into withdrawal and lateral load conditions. In past investigations, these two types of nailed joints have been evaluated differently. In the case of withdrawal resistance only ultimate load is determined [Perkins 1971; U.S. Dept. Agr. 1965]. However, for lateral shear resistance, the entire non-linear load-deformation curve is studied.
The joint displacement for lateral load is commonly called interlayer slip and results from the semi-rigid nature of the connection. Laterally loaded nailed joints fail by both yielding of the nail in bending and failure of the wood in bearing along the nail shank [Nor6n 1962]. Therefore, the non-linear effect is a function of the loadslip characteristics of the fastener-wood combination. Some investigators have attempted to describe theoretically the load-slip behavior