Length effect on the pure compression behaviour of solid sawn timber boards

Timber pure compression behaviour is commonly characterised using short specimens, despite structural members being significantly longer and exhibiting marked material heterogeneity. The reliability of short specimens for characterising the strength and full stress-strain response of full-length members remains insufficiently established. To overcome this limitation, this study investigates both deterministic and statistical length effects on the compressive strength and full stress-strain response of Australian Radiata Pine timber boards. A total of 164 boards were examined using two independent sampling approaches. Compression tests were conducted on laterally restrained long and unrestrained short specimens. Deterministic length effect was evaluated through direct experimental comparison, while statistical length effect was quantified using Monte Carlo simulation. In addition, the interaction between short specimens was employed to numerically predict the behaviour of long specimens. The results indicate that reduction in compressive strength is attributable to the statistical length effect, while no measurable difference in strength distributions attributable to deterministic length effects was observed, supporting the use of short specimen testing for compressive strength characterisation. In contrast, specimen length was found to have a pronounced influence on peak strain, ultimate strain, stress-strain shape, and ductility. Long specimens exhibited lower peak strain, ultimate strain, and ductility, together with more pronounced softening response, compared with short specimens. Furthermore, long specimens demonstrated an approximately bilinear softening stress-strain relationship, whereas short specimens exhibited a nonlinear stress-strain relationship. Finally, numerical prediction of the stress-strain relationship of long specimens based on short specimens showed close agreement with experimental observations, indicating that the stress-strain response of full-length members can be inferred from short specimens.