Measurement of prestrain in trees: implications for the determination of safety factors

1. It has long been known that wood in trees is under internally generated tensile or compressive forces, known as ‘prestrains’. These prestrains are thought to limit compressive loading at points of high strain within the tree to counteract the mechanical anisotropy of wood. Prestrains can be relieved by making cuts in wood, allowing it to recover to an unstrained state.

2. Recently, electrical resistance strain gauges have been used to measure surface strains on the trunk, roots and branches of trees. We have found that, by making a shallow cut above and below gauges, the prestrain can be measured as an apparent residual negative strain. This negative strain, after prestrain relief, is indicative of a tensile prestrain having been present in the wood before cutting.

3. It is a simple procedure to determine prestrain magnitude at the site of gauge attachment. By knowing the prestrain state of wood at the measurement site of strain determination, a more reliable estimate of surface stress and safety factors in tree design can be made.