Ecological forest conversion is leading to an increasing supply of hardwood. Low-quality, weak hardwood has not yet been used for standardized applications in structural timber construction due to its trunk diameter and geometric shape. Innovative and new hardwood products are therefore in demand, as increased material use increases the wood product storage, reduces carbon dioxide and enables substitution with wood products.

These resources include small-diameter oaks, which are economically uninteresting for the rapidly growing construction sector due to their growth characteristics and long rotation period until harvest. By combining traditional processes and knowledge of the forestry and carpentry trades with the latest results of research, from field studies to non-destructive component testing with optical measurement value recognition to the validation of results from X-ray volume tomography, these previously unused hardwood assortments receive an "upgrade" to a sought-after renewable raw material via their digital twin and form an environmentally friendly, carbon dioxide storing and resource-saving alternative to nonorganic building materials.

Oak wood with a center diameter of 20-24 cm was used for the tests. The individual steps required were documented and recorded in the FOREST database. The initial physical assessment of the individual trunks was carried out using a mobile ultrasound transition time method in the forest. The suitability classifications of a total of 210 oak trunks with an average age of around 90 years and a trunk length of five meters took place in Freiburg and Mainz. The sample range from the Palatinate Forest was put through its paces: quality sorting according to the framework agreement for wood trade (external wood characteristics), computer tomography (internal wood structures, e.g. branches), 3D laser scanning (trunk geometry, e.g. curvature), series of wood samples (dry density, wood moisture), natural vibration measurement (dynamic elastic modulus), bending tests (static elastic modulus and modulus of rupture) and long-term storage tests to determine drying behavior. The results have been entered into the FOREST database too and the defined characteristics of the ranges have been added. In addition, the BUILDINGS database of the requested ranges of standardized load-bearing structures has been created and linked in order to generate optimized load-bearing structures using the FOREST+BUILDINGS database contents.

Objectives

• To make small-diameter oak logs available as structural timber members for various structures like columns, frameworks, and agricultural buildings,
• To determine the mechanical properties of such logs and the factors influencing them, as these have only been insufficiently investigated so far,
• To apply conventional Non-Destructive Testing (NDT) technology already in use for board strength grading, with which the strength of green oak logs can be inferred,
• To develop and examine solutions for the connections in true scale prototypes, avoiding complex traditional carpenter joints,
• To integrate these concepts into parametric design,
• To build a reference shed as a proof of concept.

Methods

• External Inspection according to the German framework agreement for timber trade (RVR),
• Assessment of the geometric features of the logs according to the Framework agreement for the factory measurement of logs (RVWV),
• Computed Tomography to detect knots in the logs by a conventional algorithm,
• Mechanical characterization of the mechanical properties, including dynamic and static modulus of elasticity and modulus of rupture,
• Comparison of test results with similar tests on softwood logs and other studies, comparing destructive and non-destructive mechanical characterization methods for roundwood, 
• Statistical modelling with more than 60 variables based on Generalized Linear Model (GLM) approaches,
• Wood density and moisture content measurement including their variation along the specimen length and over the cross-section,
• Assessment of internal and external wood defects,
• Capturing of the natural crack and annual ring pattern of each specimen.