Keywords: long term tests, heat and mass transfer, glulam, multi-Fickian moisture transfer.

In this paper the results of the experimental and numerical analysis of the long term behaviour of glued laminated timber in a sheltered outdoor environment are presented. The testing area was covered and shaded, so by these means the investigated beams were protected against the direct impact of rain, snow and sun. The tests on six loaded and five unloaded glulam beams were initiated in the summer of 2003 and finished in autumn of 2006. The temperature, relative humidity, displacements and strains were measured at hourly intervals.

The coupled problem of moisture and heat transfer over the beam has been solved numerically by an enhanced finite element based calculating procedure. The model used for calculation is a so-called multi-Fickian or multi-phase model. This model describes the complex moisture transport system in the wood, which consists of separated water-vapour and bound-water diffusion interacting through sorption [1,2]. The water-vapour transport in pores and bound water transport in wood tissue are modelled using two individual transport equations following Fick's law. Since the moisture state history influences the relationship between the moisture state of the wood and the air humidity, it must therefore be taken into account.

The multi-Fickian moisture transport model is coupled to the heat transport by the Soret and Dufour effect. It is assumed that moisture transport depends on temperature gradients, and conversely, mass transport involves transport of heat. Moisture diffuses from warmer to colder regions, what is known as the Soret effect, transport of heat by mass transport of molecules with given enthalpy is known as the Dufour effect. A model for temperature dependent diffusion of water vapour has been used based in the empirical model which assumes that a higher temperature would increase the mobility of water molecules. Considering the temperature dependency of the Fickian model, a fully coupled heat and mass transfer model is formulated. Temperature dependence of the boundary curves has been taken into account in the formulation of the sorption hysteresis model. The temperature dependency of the boundary curves was described by the Anderson and McCarthy model.

In a numerical example the spatial and time distributions of temperature and moisture content over the beam were determined and compared to those measured during the experiment. The test results and numerical simulation in general confirm that at the stress level applied the influence of shrinkage, creep and mechano-sorptive effect on real timber structures is important in the early phase of loading. However, these effects become almost negligible after a relatively short period of time. For the case of one beam this time period was around fifty days.

1

H.L. Frandsen, S. Svensson, L. Damkilde, "A hysteresis model suitable for numerical simulations of moisture content in wood", Holzforschung, 61, 172-181, 2007.

2

H.L. Frandsen, S. Svensson, "Implementation of sorption hysteresis in multi-Fickian moisture transport", Holzforschung, 61(6), 693-701, 2007.

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