Edinburgh Napier University

  It is well known that knots can reduce the strength of timber, and the selection of “critical section” for testing to EN 408 and EN 384 is typically based on the size and position of knots. In Ireland, the visual strength grading Standard IS 127 specifies the permissible limits of MKAR (margin knot area ratio) and TKAR (total knot area ratio) knot indexes for Irish softwoods. The limits are the same than those in the UK visual grading standard BS 4978.
A previous study in Ireland using density and the largest TKAR measured over a length of 150 mm for prediction of tension strength found a correlation with R2 = 0.50 (Gil-Moreno, 2019). Another study using TKAR as sole predictor found a correlation of R2 =0.39 (Raftery and Harte, 2014). The study presented here also explores the use of the MKAR index given in IS 127 for tension strength prediction. The research is ongoing, but there is already enough data to make some preliminary recommendations that could help to obtain better predictions of timber quality. Grading for tension strength is particularly important for glued laminated timber production.