Keywords: timber, Norwegian spruce, aged wood, crack, knot, fibre reinforcement, bending test, finite element modelling.

Performance of historical timber structures can be negatively affected by the changes in the material properties and the integrity of the elements over its service life. Preservation of degraded timber elements is preferable over replacement, which can often be performed economically by retrofitting using fibre-reinforced composites. However, the efficiency of reinforcement is heavily influenced by the actual conditions, like the presence of cracks and knots. To assess the mechanical behaviour of aged timber beams, an experimental programme and a numerical analysis were performed on sixty-years-old Norwegian spruce beams which contained several cracks and knots. Load-deflection curves and load-bearing capacity were determined by four-point bending tests. Detailed finite element models were constructed, accurately reproducing the actual geometric features of the beams. The experiments have shown that knots in the tension zone significantly reduce the bending capacity while composite reinforcement proved efficient in enhancing the mechanical behaviour regardless the presence of knots. Moment--deflection curve from the finite element modelling showed good agreement with the the experiments. The stress distribution in the model accurately predicted the point of failure initiation in the beam.

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