Predicting moisture state of timber members in a continuously varying climate

A prerequisite for a sensible estimate of moisture induced stresses in timber members is an accurate prediction of the members’ moisture states during their service life. There are, however, an infinite number of possible moisture states for an arbitrary timber member in a natural varying climate. The naturally varying humidity is possible to describe as harmonic cycles, with different periods, superimposed. This work presents realizations of envelop curves over the possible moisture states in a timber member for some carefully chosen harmonic humidity variations. The calculations, on which the realizations were made, are based on a fully coupled transport model including a model for the influential sorption hysteresis of wood. A format containing required information suitable for assessing the “moisture” action on timber members is proposed. In addition it is illustrated how a model of high complexity and nonlinearity renders results with large degree of consistency. Large moisture gradients in a timber member are detected when variations of the humidity are frequent (daily period) and with large amplitude at high humidity levels. Overall and complete moisture changes of structural timber members with normal dimensions are found for members subjected to varying humidity with long (annual) periods. A timber member’s initial moisture state in relation to the ambient humidity can have a significant influence on both moisture gradients and overall moisture change of the member.

► Calculations of moisture state in wood in varying humidity.
► Multi-Fickian model including sorption hysteresis with the exploited sorption site model.
► The moisture response as a result to humidity changes abide the principle of superposition.
► A format for enveloping extremes of moisture states in wood is illustrated.