Influence of plate connection on nail plate moisture-induced backout within a dry-wet cycle

Abstract Using nail plates to connect timber structural elements is a way to reduce costs and time during assembly, but moisture-induced backout can compromise these connections in the medium and long term. Most nail plate backout models proposed are based on a single tooth backout, and therefore, do not consider the influence of the metal plates at their bases. This study aimed to verify the effect of the plate connection on the backout. The dimensional changes, including a holistic approach to nail plate behaviour were evaluated in two treatments: S-D cycle, where wood started saturated and then dried to 0% moisture content; and D-S cycle, where wood started at 0% moisture content and was then saturated with moisture. The hole made on saturated wood got tighter when wood dried, and enlarged when dry wood gained moisture, so that teeth squeeze may happen when wood shrinks. These results were confirmed in holes made with nails hammered into the wood using the same moisture regimens. The nail plates were bent in opposite directions with the wood dimensional changes: with the S-D cycle the concave side faced the wood while with the D-S cycle the convex side faced the wood. Although the moisture gradients assessed here were high, realistic moisture changes have the potential to generate high stresses (450–650 MPa) on the nail plates and the dry-wet cycles might promote nail plate bending in opposite directions resulting in nail plate backout. To minimize this effect, wide nail plates can be substituted by multiple narrower ones. The amount of stress transferred from the wood to the nail plate should be investigated prior to adding it to the backout models.