Influence of Sorption Hysteresis and Multi-Phase Moisture Transport on Creep Behaviour of Timber Beams

The long-term mechanical behaviour of timber elements is strongly affected by environmental variations that induce moisture transport and time-dependent creep deformation. This study is conducted within the CRESTIMB project, a European collaborative research initiative funded under the Forest Value Research Programme, which aims to improve the service-life prediction of timber structural systems through advanced hygro-mechanical modelling and
experimental investigations. The work investigates the sequential coupling between moisture diffusion and creep behaviour of timber beams with rectangular cross-sections. The study considers three different wood species and focuses on the influence of sorption hysteresis under cyclic humidity conditions and varying temperature below the fibre saturation point. Moisture transport is modelled using a multi-Fickian formulation that distinguishes bound
water in the cell walls and water vapour in the lumens, including their interaction through a coupling term [1]. Sorption hysteresis is introduced by adopting different equilibrium relationships for adsorption and desorption, which significantly affects moisture gradients and the resulting internal stress development. The moisture distributions obtained from the transport simulations are subsequently used as input for the mechanical creep analysis. The creep behaviour is described using a rheological formulation developed for timber beams exposed to variable humidity conditions. The total strain response includes elastic, viscoelastic, mechano-sorptive, and shrinkage/swelling components, allowing evaluation of moisture-dependent creep during environmental cycling [2]. Experimental creep tests performed on the three timber species are used to calibrate and validate the numerical model. The results improve the understanding of species-dependent hygro-mechanical behaviour and contribute to the validation of reliable numerical tools for predicting the long-term performance of timber structural components under realistic climate variations.