Wood is one of the basic building materials. It is a completely biodegradable raw industrial commodity, the resources of which, with proper forest management, are virtually inexhaustible. Additionally, its acquisition and processing does not require large inputs of fossil fuels. At the same time, forest areas which we obtain wood from neutralize the negative effects of producing and acquiring other raw materials, as one hectare of pine forest (the most popular in Poland) can absorb approx. 20–30 tons of CO2. Wood is characterised by low thermal and electrical conductivity, having simultaneously high sound insulation, which perfectly meets the requirements of the present market and its regulations. This study aimed at verifying the technical parameters of wood, i.e., its bending strength, with the use of an innovative method of the correlation between the bending strength measured along and across wood fibres. The procedure was envisaged as effective for testing the strength of beams in historic buildings, in which—due to their valuable structure—only a limited number of sample holes can be made. The aim of this experiment was to create tables and diagrams, from which, based on the correlation between the side and the head of the beam, using in situ tests and the sclerometric method, it will be possible to derive the bending strength of existing wooden beams. In the study of spruce and pine wood, a correlation between the recess from the side and the recess from the head was found, ranging from 0.64 to 0.76, with an average of 0.72 for spruce elements, and 0.66–0.84, with an average of 0.70 for pine elements. This means that when testing an element fixed in a building, measuring the parameters from the head of the beam with a Schmidt hammer (often such elements are more easily accessible, i.e., on the building facade), the obtained values should be multiplied by 0.72 for spruce elements and by 0.70 for pine elements to obtain the strength of the beam. The authors of this article indicate that the confirmation of this observation requires conducting further research on various types of wood. It should also be noted that the material collected from one batch of sawn timber had a different structure, which was proved by analysing it using SEM imaging. Modeling wood numerically is, to some extent, a simplified issue that assumes wood to be an orthotropic, homogeneous (homogeneous) material. In fact, wood is an anisotropic, very heterogeneous material. The analysis of wood (on the technical scale, construction wood) as an anisotropic material is practically impossible. Adopting wood as an isotropic material is too simplistic. Therefore, the most appropriate methods of strength testing are destructive methods, as all non-destructive methods should not be used without verifying the results with other methods. The results obtained by non-destructive testing pose great difficulties in their interpretation. Obtaining reliable results of experiments entails collecting a large number of research samples. The method described in this paper will allow for obtaining the necessary data for effective expertise assessment regarding the safety level of structural elements in historic wooden load-bearing structures, which is crucial for making conservation decisions.
