Spiral grain of structural timber — an unresolved phenomenon affecting the twisting of timber and consequently the stability of timber constructions
Aim of the work:
The aim of the work is to compare the spiral grain between spruce and fir stands in different parts of the Czech Republic. Spiral grain will be determined using a non-invasive method based on the passage of elastic waves. Subsequently, it will be measured using a goniometer, ensuring the measurement of twistability on the surface. To determine spiral grain across the entire cross-section, a method using CT (computer tomography) will be employed to create an overall 3D model of structural beams. These measurements will allow for a comprehensive analysis of spiral grain on the surface as well as throughout the cross-section. The results will be compared and analyzed.
Specific objectives:
Additional goals include comparing the influence of site conditions on the occurrence of spiral grain — wind, humidity, sunlight, and altitude. Subsequently, the goal is to verify whether slightly right-twisted wood is suitable for structural purposes regardless of wood species and location.
Solution description:
Material:
The research material will consist of spruce and fir stands from various parts of the Czech Republic. Trees will be cut into 4m sections from a height of at least 1m above the ground to prevent distortion from root effects. These 4m sections will be sharply edged, and the resulting profile will be specified based on available material. Structural beams will be air-dried naturally for at least one year.
Variation 1 — After measuring surface twistability, 4m structural beams will be cut into meter-long samples, which will be measured using CT at FLD.
Variation 2 — After measuring surface twistability, 4m structural beams will be analyzed at the Center of Excellence for the Forest-Wood Complex — LignoSilva in Zvolen. A comprehensive CT scan for 4m sections can be conducted here.
Methodology:
Twistability measurement on trees:
Twistability on trees will be measured using the Fakopp Arborsonic Microsecond Timer device. Probes of this device will be placed in a special template in the corresponding right or left-twisting configurations. Individual points in the template are diagonally spaced at 0.5m and 0.05m from the template axis. Subsequently, the sensors will be excited using a rubber hammer, measuring the difference in the speed of elastic waves, determining the resulting direction of fiber deviation.
Measurement of twistability of 4m timber sections and structural beams on the surface:
For round timber, the measured area will first be debarked using a template of dimensions at least 300x100mm. The axis of the trunk is marked using a lath with a designated 200mm section. Using a template, auxiliary lines are marked at the ends of the 200mm long section. Along the applied template, the trunk axis is drawn using a pencil. Fiber deviation is measured using a goniometer, which is inserted into the starting point of the 200mm section. A groove is marked by pulling, following the actual direction of fiber deviation. Fiber deviation will be measured from opposite sides of the trunk and structural beams for higher measurement precision. The measured values will be averaged.
Measurement of environmental influence:
· Light will be measured in clear weather using a lux meter at certain locations in the stand in all seasons, examining the impact of sunlight intensity on twistability.
· Wind will be measured using an anemometer at certain locations in the stand in all seasons.
· Altitude will be determined using available geodetic data.
· Humidity will be determined on random trees using a calibrated resistance hygrometer.
Measurement of twistability in the entire cross-section:
Firstly, it is important to prepare and fix the wood sample to prevent movement during scanning. The sample is placed in a computer tomographic scanner, which records X-ray images from all angles around the sample. The images are then digitally processed to create a three-dimensional model of the sample. After obtaining data from computer tomography, segmentation is required, separating areas corresponding to wood fibers from other structures. This is done using special software that recognizes characteristic features of wood fibers. After segmentation, analysis of wood fiber twistability can be performed, including measurements of angles, lengths, or other parameters characterizing fiber twistability. Results can be visualized in the form of three-dimensional models, graphs, or other suitable formats.
