Pro Ligno (Dec 2018)
Effect of Thermal Modification of Spruce Wood on Nails Embedment Strength
Abstract
Nails represent the easiest and most common method to connect timber members, but, since joints and connections frequently represent the weakest points in timber structures, they need special attention. The embedment strength of nails is one of the important properties of construction wood, since it determines the joint’s strength and required size and the number of nails used in the connection. The embedment strength of nails in spruce wood thermally modified at different temperatures (170, 190, 210, and 230°C) was measured. Nail embedment strength was determined according to EN 7383:2007 Timber Structures – Test methods – Determination of embedment strength and foundation values for dowel-type fasteners. The embedment strength was tested for nails, without pre-bored holes, in three ways: compression parallel to grain, tension parallel to grain and compression perpendicular to grain. The research showed that the embedment strength of nails slightly decreased with the temperature of the thermal treatment of wood, but the statistically significant differences were only between control samples and the most modified samples. The average nail embedment strength decreased with thermal modification at 230°C by 18% (compression parallel to the grain), 34% (compression perpendicular to the grain) and 33% (tension parallel to the grain). The highest decreases were recorded in nail embedment strength measured in compression perpendicular to the grain. This trend had been expected, since wood loses its strength with thermal modification. The measurements showed that thermally modified spruce has lower embedment strength and thus joints with nails need to be constructed differently and with a larger number of nails and larger spacing, in order to achieve the required strength comparable to non-modified wood.
