Mechanical properties of ancient wood structural elements assessed by visual and machine strength grading

The maintenance of historic wooden structures requires the assessment of the timber elements that compose them. The accurate knowledge of their mechanical performance is a fundamental aspect to guarantee conservation, safety, and usability. In this work, the assessment of the mechanical properties of timber was done by visual and machine strength grading. The visual grading rule developed for new wooden products was applied, as well as that established by the Italian standard for historic load-bearing structures. Machine grading was also performed by simulating the procedures applicable to the dismantled or in situ elements. The strength and stiffness values expected according to the different strength grading procedures were compared with the values obtained from destructive tests. As a result of visual grading, the modulus of elasticity was in line with the provisions of the standardization, while the characteristic strength was lower. With machine grading the yields were lower, but the characteristic strength values of the classes were respected, especially if combined with a visual inspection rejecting decayed or damaged pieces. The modulus of elasticity, however, was higher than that of the class, thus being underestimated.