Effect of reinforcement on shrinkage stresses in timber members

Reinforcement in timber members in form of fully threaded screws or threaded rods can restrict the free shrinkage or swelling of the wood material. This paper presents an evaluation of the influence of such reinforcement on the magnitude of moisture induced stresses with emphasis on shrinkage. For this purpose, experimental studies are presented in combination with analytical considerations on the basis of the Finite-Element method. Taking into account the influence of relaxation processes, the results indicate that a reduction of timber moisture content of 3% around threaded rods, positioned perpendicular to the grain, can already lead to critical stresses with respect to moisture induced cracks. Reduction of timber moisture content of 1% can already neutralize the proportionate stress transfer by the reinforcement in the uncracked member. The cracks appear as few but large cracks, a crack distribution, known from reinforced concrete, does not occur. This is explained by the much smaller ratio between stiffness and strength of timber members compared to concrete members. Guidance values for the equilibrium moisture content of reinforced timber members in different types of use are given.