Experimental investigation of timber samples under triaxial compression conditions

Timber is re-emerging as a prominent structural material in construction due to its sustainability and aesthetic features. Timber is increasingly used in composite elements made along with steel and concrete. In the composite elements, timber is subjected to a triaxial confined stress state. However, the characteristics of timber under triaxial stress state are not well understood. An experimental campaign was carried out to characterise the triaxial confined behaviour of timber samples. Two types of timbers (soft and hardwood) were used in the experimental campaign. The selected timber samples were subjected to confined and unconfined axial compression loading conditions. In total, 50 samples were tested with five different lateral confining pressures (unconfined, 3, 5, 8 and 12 MPa). The experimental results are presented in terms of compressive strengths and axial compression stress-strain curves. The experimental data obtained was used to define the critical parameters of confined timber, such as confined strength, strain at peak stress and stress-strain behaviour. Test results demonstrate that the confinement pressure improves the strength and ductility, and the conventional Mohr-Coulomb criterion can be used to represent the confined timber strength and strain evolution. Finally, based on the data gathered, a suitable constitutive model to characterise the stress-strain behaviour of timber under the triaxial confined stress-state is proposed. This model is similar to the constitutive models typically used for concrete, mortar and rock under confined stress state. However, differences are highlighted in timber in terms of strength gain and ductility.