Effect of lamellas annual-ring orientation on cracking of glulam beams investigated with computer tomography and image processing

Abstract

Cracks in glulam beams can reduce their strength, depending on the crack's depth, length, and location and can also be a passage for the accumulation of water and dirt. To avoid cracks, the relationship between cracks (area of cracks, mm²) and annual-ring orientation in glulam beams of spruce (Picea abies), and pine (Pinus sylvestris) with different dimensions and surface treatments was investigated using RGB images of the surface and tomography images of the cross-sections. Image processing was used to measure characteristics visible in the photos such as crack area and lamella position in the beam. Combination of lamellas in lay up was measured from computer tomography images. Four types of combinations were defined; type 1 (pith side facing outward), types 2 and 3 (pith to the same side), and type 4 (pith sides meet pith side). It was found that the area close to the glue line in the lamellas in combination type 4 is the most subject to cracking, whereas combinations of type 1 show the highest resistance to cracking. This means that type 4 should not be used in glulam beams, because the lamellas shrink apart from each other causing stresses and resulting cracks.

Keywords:

• CT scanning
• cracks
• glulam beam
• image processing
• lay ups

Acknowledgements

The authors would like to express their thanks to TräCentrum Norr (Wood Center North) at Luleå University of Technology founded by the wood industry in northern Sweden, together with EU regional funding.