Can Fully Threaded ASSY Screws Be Used to Control the Shrinking and Swelling of Wood?

Differential shrinkage in wood products is a very important consideration that should always be accounted for. It is not recommended to use fully threaded fasteners for the purpose of restraining shrinkage and swelling, as the wood fibers can develop significant stresses. For pure wood applications, these stresses are usually dissipated by the inherent long-term creep effect of the material resulting in relaxation over time. However, when forces are restrained by hard materials (e.g. steel side plates), experience has shown that swelling pressures are able to exceed the tensile capacity of the fastener steel and result in snapping off of the fastener head. Designers should keep in mind that the use of steel plates also increases the likelihood of pre-tensioning, which contributes to tensile stresses in the fastener.

For a pure wood application, the use of fully threaded fasteners may show limited positive effects. Furthermore, it is particularly complex to quantify or predict the long-term effects and results.

MTC Solutions published a white paper regarding designing with high anticipated moisture changes and what to look out for in these cases. From a practical design standpoint, it is recommended to detail connections and structural timber members in a way that allows the wood to do what it does naturally – swell and shrink depending on the surrounding moisture content and air temperature. A common rule of thumb is the assumption of a 0.25% change in volume per 1% change in EMC. Meaning with a change in EMC from 10% to 14% roughly a 1% change in volume is expected perpendicular to the grain.