Whether it is for renovation, repair or maintenance, the estimation of the properties of in situ timber elements is an essential part of the structural appraisal of many existing buildings and structures around the world.

Current methods of doing this are:

• inappropriate, as they utilise visual grading codes of practice intended for use on large batches of new timber and not for use on individual pieces of timber,
• inaccurate, as the visual grading parameters used are only weakly correlated with timber’s mechanical and physical properties, and finally,
• imprecise, as they utilise strength classification, which groups all timber into a small number of classes with associated characteristic properties.

This research summary looks at a PhD study, carried out to address these issues, using a sample of new UK grown structural sized timber joists of four lesser used species (n=527): noble fir, western hemlock, Norway spruce and western red cedar, grown in Scotland, England and Wales.

The study took a practical approach to develop models to estimate characteristic values of MoE (Modulus of Elasticity), MoR (Modulus of Rupture) and density, in a manner consistent with the Eurocodes and which accounts for the variability of in situ timber.

The most useful outcomes are a range of models for the estimation of the properties of individual situ structural timber elements in accordance with the Eurocodes, and an outline of what is required in the future to improve these models.

It concludes that, despite much authoritative advice to the contrary, structural engineers should stop using visual grading codes to assess in situ timber elements. This current practice is shown in the thesis to be inappropriate and ineffective.

It also shows the industry that there is a potential for using quantile regression in place of OLS regression in machine grading of new timber and this is worth exploring further.

I'm interested in:

• Durability
• Spruce
• Strength
• Western red cedar