Geometric representation of the irregular cross-section of old timber elements: Comparison of different approaches for mechanical characterisation

The assessment and the mechanical evaluation of the wooden elements in historical buildings are prerequisites to guarantee their conservation and safety at the same time. The availability of reliable reference values for the mechanical properties of old timber is crucial and they must refer to a well-defined cross-section. This could be a huge problem when the structural elements are irregular, as frequently happens with historical timber. A sample of 24 wooden elements dismantled from the roof of two buildings located in central Italy dated to the late nineteenth century and with very irregular cross-sections was used to investigate the geometry that best represents the actual cross-section for the mechanical characterization. Rectangular, elliptical and octagonal geometry patterns were defined and compared. Bending strength and stiffness were underestimated of 23% on average when calculated on rectangular cross-section and overestimated of 30% on average when the ellipse was used as a reference. The representation by an irregular octagon proved to be an excellent schematization of the actual cross-section, with an average overestimation of just 1%. For the tested elements, the value of 0.66 (2/3 of the side) of mean wane was identified as the threshold above which the elliptical representation should be preferred to the octagonal one. The data also suggested a sufficient representativeness of the rectangle for an average wane of less than 0.2 (1/5 of the side). The last conclusions, however, need further confirmation on a wider sample.